Abstracts

Session I | Session II | Session III | Session IV | Session V | Session VI | Session VII | Session VIII | Session IX | Poster Session A | Poster Session B | Poster Session C

Session I: Paleoclimate and Cryosphere

Intermediate water dynamics and origins during Heinrich Events in the North
Atlantic

Tom Chalk, University of Southampton, UK
The response of the Atlantic Ocean to rapid shifts in climate, and in particular the potential role of Atlantic Meridional Overturning Circulation (AMOC) weakening, is the subject of much debate. Assessing the sensitivity of this overturning to climate change is important because it is potentially a major amplifier of global change. A recent example of such behaviour is during the last glacial cycle where during Heinrich events (H-events) fresh water was added to the North Atlantic by melting armadas of icebergs. This is thought to have impeded deep water convection causing dramatic oscillations in northern hemisphere and global temperature. A commonly quoted response to the rapid influx of fresh water during H-events is a complete shutdown of the northern component of the overturning AMOC, filling the Atlantic basin with water of entirely southern origin. Here we present a new, highly resolved and well dated B/Ca-[CO32-] proxy dataset from a core from the mid-depth (2168 m) North Atlantic spanning the last 40ka. We show that similar to other tracers (e.g. δ13C) carbonate ion varies as a result of deep water circulation changes associated with H-events. Importantly, this added constraint aids in the identification of the origin of mid-depth water masses during H-events. In light of these new results, we suggest the standard model of northern deep water formation cessation during H-events is too simplified and does not fully reflect the complex response of AMOC to fresh water addition related to Heinrich events.
Basal Hydrology and the Stability of Marine-Terminating Ice Streams
Alex Robel, Harvard University
The Marine Ice Sheet Instability (MISI) hypothesis predicts that ice sheets or glaciers which terminate on retrograde (upward-sloping in the direction of flow) beds are unstable to perturbations in the position of the grounding line. MISI has long been investigated (i.e. Mercer 1978) as a potential mechanism for destabilization of the West Antarctic Ice Sheet, which rests on a bed below sea level and so is at risk for unstable retreat given perturbations to climate or sea level. However, a fundamental shortcoming of the theory underlying MISI is its assumption that ice slides on a hard bed according to a power law. In reality, almost all the mass flux in West Antarctica occurs through a number of fast-flowing ice streams, which lay on soft beds with dynamic basal hydrology. Expanding on earlier results from a simple model of ice stream hydrology (Robel et al. 2013), we will show where the MISI hypothesis fails and succeeds in describing West Antarctic Ice Streams. This will include both dynamical systems analysis of ice stream stability and numerical investigations using a high-resolution flowline model with hydrology and a migrating grounding line boundary condition.
Constraining ice sheet visco-elastic response to supraglacial lake drainage events
Laura Stevens, MIT-WHOI Joint Program
Across much of the ablation region of the Greenland Ice Sheet (GrIS), hydro-fracture events related to supraglacial lake drainages create large meltwater conduits to the bed (moulins), providing efficient surface-to-bed drainage networks for the remainder of the summer melt season. Uncertainty remains around the mechanisms of hydro-fracture crack initiation; explanations for the differing supraglacial lake drainage styles (“catastrophic” vs. “slow”) observed; as well as the potential for hydrofracture events to propagate inland under future warming scenarios. We use inverse and forward numerical modeling efforts, constrained by GPS observations of ice surface motion during summer hydro-fracture events at a specific GrIS supraglacial lake, to investigate (1) the fracture opening and propagation geometry, (2) the stress field in the ice-sheet prior to a hydro-fracture event, and (3) acceleration of ice due to lubrication of the bed with surface meltwater. Results will increase prediction accuracy of future ice-sheet contributions to sea level rise by improving scientific knowledge on the mechanics of ice-sheet hydro-fracture and the influence of surface meltwater on ice-sheet flow.
Last Millennium Climate Change in CMIP5 Models
Alyssa Atwood, University of Washington
The Little Ice Age (LIA) is broadly defined as a time of cool Northern Hemisphere (NH) temperatures ~ 1350 – 1850 AD. Paleoclimate data suggest that changes in tropical rainfall patterns, including a southward shift of the Intertropical Convergence Zone (ITCZ), also occurred during this time. I am investigating climate change during the LIA through the use of simulations performed by the latest Coupled Model Intercomparison Project (CMIP5) General Circulation Models (GCMs). By comparing last millennium simulations of eight GCMs, which are driven by the ‘observed’ record of natural and anthropogenic forcing to unforced 1000 year control runs, I am investigating whether LIA conditions occur in the last millennium runs and whether they can be attributed to solar and volcanic forcing. Initial results indicate that although the models generally capture the colder European and Northern Hemisphere temperatures and the expansion of Arctic sea ice ~ 1500 – 1850 AD, there is little agreement on the behavior of the ITCZ. This may suggest that the Northern Hemisphere cooling was due to solar and volcanic forcing, while the southward shift of the ITCZ was unforced. However, an alternative hypothesis is that the models fail to capture some aspect(s) of last millennium climate change important for the southward shift of the ITCZ. There is evidence to suggest that the models’ use of different volcanic data sets may be related to the differing behavior of Arctic sea ice and the position of the ITCZ.
Back to top

Session II: Hydroclimate

The effect of greenhouse-gas-induced changes in SST on the seasonality of tropical precipitation
John Dwyer, Columbia University
“CMIP5 models project changes to the seasonality of both tropical SST and precipitation when forced by an increase in greenhouse gases. Nearly all models project an amplification and a phase delay of the annual cycle for both quantities, indicating a greater annual range and extrema reached later in the year. I perform two sets of AGCM simulations to study this phenomenon. The first, in which I have uniformly increased the annual mean SST, shows a strengthened annual cycle of precipitation due to enhanced vertical moisture advection, and a delay to the timing of peak precipitation, consistent with a delay to the timing of the circulation. A budget analysis of this simulation indicates a large degree of similarity with the CMIP5 results. In the second experiment, I change only the seasonal characteristics of SST. For an amplified annual cycle of SST I find an amplified annual cycle of precipitation, while for a delayed SST I find a delayed annual cycle of precipitation. Additionally, there are cross-effects: the phase of SST affects the amplitude of precipitation and the amplitude of SST affects the phase of precipitation. Assuming that the seasonal changes of precipitation in the CMIP5 models are entirely due to SST effects and that ocean feedbacks are not important, the AGCM simulations suggest that the annual mean SST warming can explain around 90% of the amplitude increase and 60% of the phase delay of precipitation in the CMIP5 models with the remainder due to seasonal changes in SST.”
Impacts of climate change on the surface water balance of the central United States
Bo Dong, University at Albany – State University of New York
Observations over the central U.S. in recent decades show numerous changes in climatic variables. This includes decreases in cloud cover and wind speed, increases in air temperature, and seasonal shifts in precipitation rate and rain/snow fraction. To assess the impacts of these variations in climate on the regional water cycle, a terrestrial ecosystem/land surface hydrologic model (Agro-IBIS) is employed in this study, forced by observed climatic inputs for the period 1984-2007. The results generally show an acceleration of the water cycle in the Upper Mississippi, Missouri, Ohio, and Great Lakes basins, but with significant seasonal and spatial complexity. Over the past 24 years, evapotranspiration has increased in most regions and most seasons, particularly during the fall, which is also a time of pronounced solar brightening. Trends in runoff are characterized by distinct spatial and seasonal variations. Since recent warming has led to a greater fraction of winter precipitation falling as rain rather than snow, spring runoff in some snow-dominated regions (such as the northern Great Lakes) has declined significantly since 1984. Other regions, however, such as the northern Missouri basin, show large increases in runoff throughout all seasons, primarily as a result of increased precipitation. Sensitivity experiments show that the water balance is most linearly sensitive to solar radiation and relative humidity, followed by precipitation, air temperature and wind speed. Because of the interdependencies among the climate factors, the hydrological responses of climate change are highly non-linear. Seasonal hydrological responses are notably dependent on regional water and energy availability, and are affected by seasonal conditions of soil moisture and snow cover. Furthermore, precipitation is characterized as the predominant factor that affects the decadal scale hydroclimatic changes in the central U.S..
Analysis of Changing Hydroclimatic Systems: Correlated Climate and Vegetation Parameter Impacts to Evapotranspiration Modeling
Morgan Levy, University of California – Berkeley
“This study performs an analysis of the sensitivity of the radiation-based Penman-Monteith (PM) equation for reference evapotranspiration (ET) to climate and vegetation parameters. This study quantifies variability in reference ET estimates due to equation structure using data- and simulation-based methods. The objective of this research is to: 1) measure the first order and total sensitivity effects of both climate and vegetation parameters according to a standard Sobol’ sensitivity analysis (Saltelli, 2002), building on previous studies that exclude vegetation parameter effects; and 2) evaluate the effects of parameter correlation on sensitivity results according to an improved approach for models with dependent variables (Kucherenko et al., 2012). A Sobol’ sensitivity analysis decomposes (numerically simulated) total output variance of a model function into estimates of the variance attributable to individual model parameters and parameter interactions. It is a “global” sensitivity analysis that measures the relative influence of individual parameters and subsets of parameters on a model function, not individual solutions (e.g. derivatives at exact points). Motivations for this study include addressing gaps in the analysis of ET sensitivity to vegetation parameters in addition to climate parameters; lack of accounting for dependence in model input parameters when performing a sensitivity analysis; and consideration of ET model sensitivities relative to new uses of (spatial grid) satellite data for ET estimation in the place of, or in combination with, (point) weather station data.”
Influence of synoptic weather events on the isotopic composition of atmospheric moisture in San Diego, California
James Farlin, University of California – Davis
We present a month long data set of continuously-calibrated water vapor isotopologue data to examine the dominant processes controlling vapor isotopic composition on a synoptic scale and evaluate an isotope enabled GCM’s ability to capture observed changes in vapor and precipitation isotope values. Observations were collected with new high temporal resolution laser spectroscopy instrumentation to describe the rapid changes in δD, δ18O, and deuterium excess (d) of water vapor and precipitation during an extreme drying and wetting event in San Diego, California. The IsoGSM was able to predict the dynamic changes in observed vapor isotopologues, with greatest agreement at relatively low ambient vapor mixing ratios. During these extremely dry events the vapor follows Rayleigh or super-Rayleigh two-source mixing. Isotopically depleted air from the upper atmosphere subsided during these times rapidly depleting vapor δD and δ18O. Patterns are less clear during precipitation, especially following evaporative enrichment of the precipitation on the surface vapor. Quality, hourly vapor data allowed us to investigate the changing isotopic composition on an event-basis. Using a daily keeling plot approach whenever we had good agreement (R2>0.8) allowed us to show that two-source mixing was dominant. Using the intercept allows us to infer the isotopic value of the source water of the vapor and infer the trajectory of the ambient air parcel. Continuous ground based measurements provide validation for model and remote sensing data sets, as well as providing insight into the dominant atmospheric controllers of vapor isotopic composition.
Back to top

Session III: Aerosols, Clouds, and Atmospheric Chemistry

Semi-direct dynamical and radiative effect of North African dust transport on lower tropospheric clouds over the subtropical North Atlantic in CESM 1.0
Michael DeFlorio, Scripps Institution of Oceanography
This study uses a century length pre-industrial climate simulation by the Community Earth System Model (CESM 1.0) to explore statistical relationships between dust, clouds and atmospheric circulation, and to suggest a semi-direct dynamical, rather than microphysical, mechanism linking subtropical North Atlantic lower tropospheric cloud cover with North African dust transport. The length of the run allows us to account for interannual variability of dust emissions and transport downstream of North Africa in the model. CESM’s mean climatology and probability distribution of aerosol optical depth in this region agrees well with available AERONET observations. In addition, CESM shows strong seasonal cycles of dust burden and lower tropospheric cloud fraction, with maximum values occurring during boreal summer, when a strong correlation between these two variables exists downstream of North Africa over the subtropical North Atlantic. Calculations of Estimated Inversion Strength (EIS) and composites of EIS on high and low downstream North Africa dust months during boreal summer reveal that dust is likely increasing inversion strength over this region due to both solar absorption and reflection. We find no evidence for a microphysical link between dust and lower tropospheric clouds in this region. These results yield new insight over an extensive period of time into the complex relationship between North African dust and lower tropospheric clouds over the open ocean, which has previously been hindered by spatiotemporal constraints of observations. Our findings lay a framework for future analyses using different climate models and sub-monthly data over regions with different underlying dynamics.
The Role of Aerosols in Shallow Tropical Convection
Alison Nugent, Yale University
The Dominica Experiment (DOMEX) is an ongoing project to understand the clouds and airflow dynamics over the Commonwealth of Dominica. Twenty flights on repetitive tracks measured upstream and downstream airflow properties as well as the convective clouds and precipitation over Dominica. This flight data along with a simple numerical model have been used to understand the relationship between wind speed, convective type, and aerosols over this small island in the Caribbean. When the wind speed is low, thermally driven convection is present and aerosols from the island surface are lofted into the cloud. When the wind speed is strong, terrain driven (orographic) uplift and plunging flow dominate, and fewer aerosols are found in the convection above the island. The quantity of aerosols lofted into the convection plays an important role in the cloud microphysics and precipitation processes. Strong evidence from observations and confirming results in a numerical model connect aerosol content, cloud droplet size, cloud droplet number, and rain amount showing an ideal example of how aerosols affect clouds and rain. While this study over Dominica is one simple case study, it highlights the important role of aerosols in the climate system. The indirect effect of aerosols on convective clouds is one of the major unknowns in climate change. In a warming world, how will the aerosol properties modify, and how will these modifications affect the clouds and precipitation? A better understanding of the role of aerosols in convection in today’s world will help us to prepare for and understand future potential changes.
The effect of atmospheric properties and processes on aerosol indirect effects in a trade cumulus regime
Kristina Pistone, Scripps Institution of Oceanography
An assumption inherent in the expression of the Twomey effect is that the liquid water path of a cloud remains constant between the clean and aerosol-perturbed cases. However, natural variability in the liquid water path due to meteorological conditions may be greater than changes in the liquid water content of a cloud due to perturbations from changes in aerosol. In the recent CARDEX (Cloud, Aerosol, Radiative forcing, Dynamics EXperiment) field campaign conducted in March 2012 in the northern Indian Ocean, continuous measurements of precipitable water vapor (PWV) and column liquid water path (LWP) of the trade cumulus clouds present were made from a microwave radiometer (MWR), concurrent with water vapor flux, cloud and aerosol vertical profiles, meteorological data, and surface and total-column aerosol measurements. Our results show a positive relationship between aerosol and cloud LWP when filtered for low atmospheric PWV only; we show that this is due to specific atmospheric conditions and air mass history. Using aircraft measurements of atmosphere and cloud properties and vapor flux as well as model comparisons, we examine the effects of this aerosol-cloud relationship and explore how atmospheric processes and dynamical mechanisms govern this observed correlation.
Quantification of Atmospheric Aging and Properties of Ambient Black Carbon Aerosols
Misti Levy, Texas A&M University
Because of the strong absorption over a broad range of the electromagnetic spectra, black carbon (BC) is a key short-lived climate forcer, which contributes significantly to climate change by direct radiative forcing and is the second most important component causing global warming after carbon dioxide. The impact of BC on the radiative forcing of the Earth-Atmosphere system is highly dependent of the particle properties. In this presentation, emphasis will be placed on characterizing BC containing aerosols in Houston, Texas and at the California-Mexico border to obtain a greater understanding of the atmospheric aging and properties of ambient BC aerosols. A comprehensive set of directly measured aerosol properties, including the particle size distribution, effective density, hygroscopicity, and light extinction and scattering coefficients, will be discussed to quantify the mixing state and composition of ambient particles. In Houston, Texas, the average BC concentration is 0.31 ± 0.22 µg m-3 accounting for approximately 3% of the total submicron aerosol concentration, which is lower than many US urban cities. Our results show that BC noticeably influences the overall effective density and optical properties. In Tijuana, Mexico, submicron aerosols are heavily influenced by vehicle emissions; subsequently, the BC concentration in Tijuana is considerably higher than most US cities with an average BC concentration of 2.71 ± 2.65 g cm-3. BC accounts for 24.75 % ± 9.44 of the total submicron concentration on average, but periodically accounts for over 50%. This high concentration of BC strongly influences many observed aerosol properties.
Back to top

Session IV: Humans, Climate, and Policy

How grammatical choice shapes mass media representations of climate (un)certainty
Adriana Bailey, University of Colorado – Boulder
When it comes to complex issues like climate change, mass media play a key role in translating, and sometimes constructing, scientific (un)certainties for the public. To examine how grammatical and word choices modify the certainty of newspaper statements about climate science, this study analyzes four newspapers, two from the US (New York Times and Wall Street Journal) and two from Spain (El País and El Mundo). While the US has shown considerable reluctance in embracing international policy on climate change, Spain has ratified the Kyoto Protocol and committed to national climate action. We expect reporting from these countries to reflect their contrasting ideological views on the topic. Moreover, to evaluate whether news reports reflect the increasing certainty of climate scientists with time, articles from both 2001 and 2007 are analyzed, the years in which the Intergovernmental Panel on Climate Change (IPCC) released its third and fourth Assessment Reports, respectively. Across 92 news articles we identify 1193 epistemic markers—words and grammatical constructions that create room for doubt about the scientific understanding of or consensus around climate change. As expected, the US papers use more epistemic markers than the Spanish, however lexical choices in both languages are remarkably similar. Surprisingly, the density of epistemic markers does not decrease with time but remains constant or increases, depending on the newspaper. The US papers also exhibit a tendency to construct uncertainty by highlighting differences between IPCC reports, or between predictions and observations, without providing appropriate context for why such differences occurred.
An Earth-system perspective to fisheries and fisheries economics
David Carozza, McGill University, Canada
Interactions between marine biogeochemistry, marine ecosystem processes, and fisheries economic have been extensively analyzed in isolation. However, the majority of this research has focussed on particular regions and species. We instead model these processes from the viewpoint of a globally coupled system based on fundamental physics, ecology, and economics. This Earth-system perspective to global fisheries and fisheries economics will illuminate general patterns of biomass, harvest, and profit that are not possible with more specialized studies, and is ideally suited to study interactions with climate. We use CM2Mc+BLING, a coupled Earth system model that consists of an ocean-atmosphere general circulation model, coupled to BLING, a simple representation of marine biogeochemistry with light, iron, and macronutrient limitation of growth. We then modeled fish biomass based on primary production by coupling CM2Mc+BLING to a dynamical size-based ecological model, and then coupled this fisheries model to the classical Schaefer-Gordon economic fishery model. We compared our results to experimental compilations and to other models and found that our simulations represented the main features of global fish biomass and the total biomass, with peaks of biomass in high-nutrient coastal areas and upwelling regions. Since fish are iron-rich, we found that they can have a localized impact on biogeochemistry in high-biomass regions. Moreover, by tuning a single parameter of our economic model, the catchability of fish for a given amount of fishing effort, we accurately represented global totals of present-day global harvest and revenue.
Inter-Tribal Meeting on Climate Variability and Change
Paulette Blanchard, University of Oklahoma
Oklahoma tribal representatives convened for a meeting on December 12, 2011 to 1) enhance and foster dialogue between tribal representatives and climate scientists that was previously initiated through two statewide meetings in which tribal representatives were invited and some attended, 2) educate tribal representatives about climate science and climate change, and 3) develop recommendations for material to be included in the 2013 National Climate Assessment. Hosted by the Haskell Environmental Research Studies Center at Haskell Indian Nations University, in collaboration with the Oklahoma Climatological Survey and Southern Climate Impacts Planning Program, the meeting brought together representatives of 22 tribal nations and three tribal colleges.
This presentation will include findings from the meeting’s breakout session in which tribal representatives discussed the impacts of climate variability and change on tribal sectors and cultures, and long term recommendations for implementing climate change adaptation and mitigation strategies. Currently, drought and heat constitute tremendous climate stressors and impact water resources, ecology, human health, agricultural practices, and energy supply and use. In fact, water resources were seen as the most important sector since water is vital for agriculture and many ceremonial practices. The ability to gather certain plants and food was described as very important for ceremonies, and potentially vulnerable to a changing climate. Moving forward, representatives were interested in working with state and federal partners to use more climate change data and information, considering indigenous perspectives, and increasing their capacity through formal and informal education to make adaptation and mitigation decisions.
Climate Science, Social Science, and the Humanities: A Role for Critique?
Jessica Lehman, University of Minnesota
Recently, scholars in the social sciences and humanities have taken a keen interest in climate change. The social, political, and economic effects of warmer temperatures, rising seas, extreme and variable weather, long-term drought, sea-ice melt, and more have become impossible to ignore. But many critical scholars go further, asking how reactions to climatic changes at all levels of society might also engender new desirable or undesirable ways of understanding and governing life. In this way, critical scholars examine not only the negative impacts of climate change and how to prepare for and reduce them, but also the possibilities for doing more than returning to a stable climate and ‘business as usual.’ Yet, climate science and critical social sciences and humanities scholarship is rarely in direct conversation or collaboration. I look at confluences and tensions between climate science and critical scholarship in the social sciences and the humanities, to address questions of critical climate change scholarship, in all senses of the word critical. Why is critical scholarship needed? How can scholars be critical without being climate deniers? What barriers exist to more productive collaborations between critical social sciences and humanities scholars and climate scientists? I draw on my experience as co-organizer of the 2013 Workshop on Critical Climate Change Scholarship at the University of Minnesota. Rather than provide answers, I address these questions from a graduate student perspective, examining how these debates might prove fertile grounds for transdisciplinary research.
Back to top

Session V: Atmospheric Dynamics – Tropics

The temperature dependence of moist convection
Martin Singh, MIT
Tropical convection is important for the energy and water budgets of the Earth, but, because of the small spatial scales of convective clouds, is not well represented in global climate models. Our understanding of basic questions such as what sets the updraft velocity and precipitation intensity in tropical convective clouds, and how these quantities change as the temperature is changed are not well settled. Recent modeling studies have shown that a measure of convective instability, the convective available potential energy (CAPE), increases with warming in the tropics. The implications of this result for precipitation intensity or updraft velocity are, however, unclear. Here we investigate the CAPE and precipitation intensity in an idealized cloud-system resolving model in which moist convection is simulated over a range of surface temperatures characteristic of the current tropics, as well as the tropics as it may have been in the distant past. A striking result is that the amount of CAPE increases rapidly with warming over the range of surface temperatures studied. A conceptual model is developed to explain the CAPE increase in terms of cloud entrainment and mixing, and some observational evidence is presented in support of it. The mechanisms governing the precipitation intensity, however, are distinct, with the fall speed of hydrometeors playing an important role in setting the precipitation intensity and its changes with temperature.
Examining the frequency, intensity, and sensitivity to SST of observed and model-generated tropical cyclones
Sarah Strazzo, Florida State University
Of broad scientific interest is the ability of global climate models (GCMs) to accurately simulate tropical cyclones (TCs) in the context of climate change. Improvements to resolution and physics provide promising results, however uncertainty about the fidelity of model-generated to actual TCs remains. We present a spatial lattice approach for comparing observed and model-produced TC tracks. The methodology employs a spatial tessellation of TC basins using equal-area hexagons to create a common framework to compare per-hexagon TC statistics for observed and modeled data. We present comparisons for two atmospheric GCMs (GFDL-HiRAM and FSU-COAPS) as well as for synthetic tracks generated through a downscaling approach. Multiple performance metrics show that the HiRAM compares well with observations at a global scale; however, both models fail to reproduce the observed distribution of storms over the Gulf of Mexico. Additionally, this approach allows us to spatially match TC and covariate data. We demonstrate this using observed and model-generated track data with sea surface temperature (SST). We use extreme value theory to estimate the limiting intensity (LI) of North Atlantic TCs for observations and model data. We use a weighted regression of per-hexagon LI on SST to obtain an estimate of the sensitivity of LI to SST. Results indicate high sensitivities for observations (7.9 ± 1.19 ms^-1K^-1) and synthetic data (5.1 ± 1.38 ms^-1K^-1), but significantly lower sensitivities for the GCMs (1.8 ± 0.42 ms^-1K^-1). Future work will expand this model to investigate the relative roles of upper tropospheric temperatures, vertical wind shear, and relative SST.
Synoptic scale monsoon vortices in idealized aquaplanet simulations
Varun Murthy, Yale University
The Indian, Australian and West Pacific monsoon circulations contain embedded synoptic scale low pressure systems. These systems, also called Monsoon Depressions, occur in baroclinic zones with large vertical wind shear and their governing dynamics differ from Tropical Cyclones . Monsoon Depressions are characterized by surface wind velocities of 8-17 m/s, spatial scales in excess of 1000km and produce precipitation rates of 200-400 mm/day. Monsoon depressions propagate westward at approximately 6m/s and exhibit strong cyclonic vorticity, convergence and heavy precipitation to the west and south west of the depression center. They play a significant role in the meteorology of the tropics by causing a major fraction of monsoon precipitation and by taking part in tropical cyclogenesis. Despite their significance, definitive theories explaining their formation, structure and propagation do not exist. In this study, we analyse the dynamics governing the growth and propagation of monsoon depressions in aquaplanet simulations. The simulations are forced by an off-equatorial SST maxima, which produces an off-equatorial ITCZ, as observed in monsoon zones. The depressions are identified as surface pressure anomalies and are required to possess bimodal vertical structures of PV and temperature anomalies, as described by previous observation studies. We analyse the energetics of depressions during various stages of their growth and propagation and state the deficiencies of previously proposed theories explaining monsoon depressions. We also present a preliminary analysis of the sensitivity of monsoon depressions to different governing factors such as SST gradients, wind shear and moisture.
Contrasting effects of aerosols and greenhouse gases on Hadley Cell energy transport: simulations and theory
Spencer Hill, Princeton University
Anthropogenic aerosol and greenhouse gas radiative perturbations affect both the mean and spatial pattern of surface temperatures differently. These surface temperature components, in turn, are known to affect tropical circulations via different mechanisms, some well understood and others less so. We explore this behavior by imposing SST anomalies representative of either historical aerosol forcing, its tropical mean value at all ocean gridpoints (i.e. a uniform component), or its tropical mean-subtracted values (i.e. a spatially varying component) and an analogous triplet for historical well-mixed greenhouse gas forcing in two AGCMs. The gross moist stability (GMS) indicates the efficiency with which the Hadley Cell transports moist static energy poleward as it overturns and has been assumed not to change in prior studies of the tropical precipitation response to climate change (e.g. Kang et al. 2009, J. Climate). We find that whereas greenhouse gas SST anomalies affect GMS weakly, aerosol SST anomalies cause GMS to decrease in boreal summer and increase in boreal winter, both of which contribute to anomalous northward moist static energy transport that counteracts the hemispheric energy imbalance introduced by the NH-concentrated aerosol cooling. We provide a qualitative theoretical explanation of the GMS behavior in both greenhouse gas and aerosol cases based on arguments from Held 2001 (J. Climate) that GMS varies with the meridional surface moist static energy gradient. Work is ongoing to understand these GMS changes more quantitatively and their relationship to the ITCZ response, which will help constrain 21st century tropical precipitation changes.
Back to top

Session VI: Atmospheric Dynamics – Extratropics

Observational Trends and the Dynamics of Summer Temperature Extremes
Andrew Rhines, Harvard University
The frequency of extremely hot summers — as defined by seasonal mean surface temperature — has increased markedly in many parts of the world since the beginning of the instrumental era. This shift implies greater heat stress on agriculture, natural ecosystems, and human populations. However, most heat-related damages stem from short-duration events whose frequencies may not be a simple function of seasonal averages. Heat waves may respond differently than the mean because they operate on different time scales and occupy the tails of the distribution where nonlinearities are more prevalent. It is therefore important to characterize changes in the tails of the distribution of maximum temperatures independent of changes in the mean, and on a time scale that resolves brief events. Using quality-controlled daily observations from a global network of surface stations, we estimate trends in the 5th, 50th, and 95th percentiles at each station between 1960 and 2012. Coherent patterns emerge from this analysis, showing that large swathes of the world are now experiencing more variable summer temperatures. This contrasts with a similar analysis of monthly averages which fails detect any changing variability about the mean, illustrating the importance of resolving synoptic time scales. Gradients in the trends are then linked to different regimes of synoptic conditions. We decompose the pattern of changes in variability into local responses to both large-scale trends and the frequency of transient synoptic conditions favorable to producing extreme events.
Stratospheric Final Warming Events and their Surface Impact
Aditi Sheshadri, MIT
In both hemispheres, stratospheric polar vortices form in the fall, reach maximum strength in midwinter, and decay in late winter-spring. The breakdown of these vortices is known as the Stratospheric Final Warming (SFW). In the first part of this study, we investigate stratospheric final warming events in a simplified GCM forced by specified equilibrium temperature distributions. We use a dry, hydrostatic, primitive equation model with T42 resolution in the horizontal and 40 levels in the vertical. The bottom boundary includes topography, which causes variability in the timing of final warming events. Seasonal variations in the equilibrium temperature distribution are included in the model stratosphere only, so as to demonstrate explicitly the effect of the stratospheric seasonal cycle on the model troposphere. The model produces qualitatively realistic final warming events whose influence extends down to the surface, much like what has been reported in previous observational analyses. In the Southern Hemisphere, a tendency towards an increase in the positive phase of the Southern Annular Mode has been reported, which is associated with a poleward shift of the midlatitude jet and storm tracks. It has been suggested that these changes are primarily due to the effects of stratospheric ozone depletion. Stratospheric ozone depletion cools the polar stratosphere, causing the polar vortex to persist longer, thus delaying stratospheric final warming events. In the second part of this study, we examine stratospheric final warming events in the Southern Hemisphere using data from the NCEP/NCAR Reanalysis. Our results confirm a statistically significant trend towards later final warming events over the last several decades. We also test the hypothesis that the observed trends in surface winds are directly consequent on this trend towards later final warming events. To do this, we examine composite differences between years of large ozone depletion and the pre-ozone-hole years. Our results indicate that the observed changes in surface winds cannot be attributed to the trend towards later final warming events.
On oceanic influence and patterns of temperature change
Karen McKinnon, Harvard University
Climate change is often discussed in terms of increases in global mean surface temperature, yet there is substantial variability in the rate of temperature change across space. Understanding the currently observed spatial pattern of surface temperature change is important, because of the inherently local nature of human impacts, and the influence of temperature gradients on other climate variables, such as precipitation. Here, we analyze the spatial pattern of surface temperature variability between 10 and 60 degrees latitude in both hemispheres on three different timescales — seasonal, inter-annual, and multi-decadal — and show a similarity across timescales. Using the seasonal cycle as a laboratory to examine the mechanism controlling this pattern, we find that it can be well-explained by a single, scalar parameter that measures oceanic influence, itself regulated by dominant patterns of atmospheric circulation. Calculations with a simple, zero-dimensional energy balance model suggest that transient warming should scale linearly with oceanic influence. This scaling is evident in the atmosphere-only version of the GFDL general circulation model, AM2.1, and in the HadCRUT4 surface temperature data over the previous half-century, where oceanic damping can explain almost half of the observed spatial variability. After removing the expected temperature signal of ocean heat uptake, the significant remaining variability can now be attributed to non-uniform forcing, feedbacks, or natural variability. Broad regions of cooling in the Southern Ocean, North Pacific, and South America stand out as regions that must be primarily controlled by other mechanisms.
What will happen to the general circulation of the atmosphere if the West Antarctic Ice Sheet collapses?
Kat Huybers, University of Washington
Much of the ice that comprises the West Antarctic Ice Sheet (WAIS) is grounded below sea level, and because its underlying bedrock deepens toward the interior, the ice sheet’s stability in the face of future climate change remains uncertain. The imminent collapse of the WAIS is improbable, but would be high-impact, with the conspicuous risk of a ~4.3 meter rise in global sea-level. However, the presence of the WAIS also affects patterns of winds, temperature, and precipitation, and the loss of the ice sheet’s topography may additionally have profound local and global consequences on the general circulation of the atmosphere. In this study, we use two global circulation models (GCMS) to explore the atmospheric effects of a collapse of the WAIS: an idealized moist GCM and a more comprehensive GCM that includes feedbacks. We run our simulations under a present-day climate, imposing the orography of Antarctica, both with and without the WAIS. Initial findings indicate that the loss of the WAIS leads to a weakening and equatorward shift of of the zonal winds, a distinct change in the meridional wind wave pattern, and a northward migration of the Intertropical Convergence Zone.
Back to top

Session VII: Ocean Dynamics

How sensitive is ocean model utility to resolution?
Maike Sonnewald, University of Southampton, UK
One of the most intriguing problems in recent ocean modelling research is the impact of varying model resolution on model accuracy. Increasing model resolution is encouraged by organizations such as the IPCC, and thought to increase accuracy. However, the increase in accuracy with resolution is unlikely to be linear. Thus, as computational cost increases with resolution, a critical assessment of achieved benefits is prudent. Here we analyze a suite of compatible global OGCM NEMO model runs from coarse (1o, ORCA1), eddy-permitting (1/4o, ORCA025) and eddy resolving (1/12o, ORCA12) resolutions. Comparisons of steric height variability (varSH) highlight changes in ocean density structure, revealing impacts on mechanisms such as downwelling and eddy energy dissipation. We assess vertical variability using the covariace of the deep and shallow varSH. Together with assessing isopycnal movements, we demonstrate the influence of deep baroclinic modes and regions where the barotropic flow sheds eddies. Significant changes in the deepwater formation and dispersion both in the Arctic and Antarctic are found between resolutions. The varSH increased from ORCA1 to ORCA025 and ORCA12, particularly in the Southern Ocean and Western Boundary Currents. However, there is no significant covariance between the surface and deep in ORCA1, while ORCA025 and ORCA12 show significant covariance, implying an important missing energy pathway in ORCA1. Comparing ORCA025 and ORCA12 we see significant differences in eddy energy dissipation. We assess the impact of varying model resolution on the mean flow, discussing implications to dissipation pathways on model accuracy, with reference to stochastic parametrization schemes.
Observed strengthening of the Pacific Equatorial Undercurrent: coupled mechanisms, ocean dynamics, and implications
Elizabeth Drenkard, WHOI
The Pacific equatorial undercurrent (EUC) plays a crucial role in global climate and geochemical cycles. It delivers cold, CO2- and nutrient-rich water to the eastern Pacific where it feeds the cold tongue. Here, EUC water contributes to the largest oceanic source of atmospheric CO2 and to maintaining the zonal sea surface temperature (SST) gradient across the Pacific. This thermal gradient is one of the primary drivers of tropical Pacific atmospheric circulation, which affects global weather patterns and climate. Thus, changes in EUC intensity could have dramatic geochemical and climatic repercussions. We show that long-term trends in the SODA reanalysis indicate that the EUC has strengthened by as much as 16% since the mid-nineteenth century. Analysis of the zonal momentum budget in the equatorial Pacific explicitly diagnose the mechanisms responsible for this intensification and identify two dynamically distinct seasonal periods (Boreal spring and summer) that account for the majority of the EUC’s strengthening. We propose that EUC intensification is key to reconciling the controversial and paradoxical co-occurrence of Walker circulation slow-down and zonal SST-gradient strengthening in the equatorial Pacific. Additionally, this provides support for the proposed ocean “thermostat” control on CO2-forcing of tropical Pacific circulation and reemphasizes that it is imperative for global climate models to rigorously incorporate ocean dynamics in order to produce accurate forecasts.
Observational evidence of the abundance of mixed layer eddy induced stratification
Leah Johnson, University of Washington
Classical theory predicts that upper ocean spring stratification results from one-dimensional processes dictated by wind and solar heating. More recent theory and models suggest that small submesocale baroclinic instabilities (mixed layer eddies or MLEs), that arise at strong lateral density gradients, are a prevalent leading order process acting to restratify the mixed layer (ML) by advection. This process would hasten ML spring stratification and therefore influence processes that have large climatological impacts, such as the phenology of phytoplankton blooms, surface fluxes, mode water formation/ventilation and the strength of the meridional overturning circulation (MOC). MLEs have been studied in models, yet few observations have been able to validate the abundance of these phenomena. Therefore, this work takes a global perspective to evaluate the importance of these lateral processes on the timing of spring restratification. MLEs are parameterized as a heat flux proportional to the horizontal density gradient of the upper ocean and the depth of the mixed layer. This parameterization is applied to the Monthly Isopycnal / Mixed-layer Ocean Climatology (MIMOC) to identify where these instabilities are thought to be a dominant process in the ocean. Additionally, springtime restratification events are identified using observations by the global ARGO database and are compared with results predicted by 1-D mixed layer models to identify where one-dimensional dynamics fail to describe observed ML features. Combined, these analyses will suggest the abundance and geography of MLE induced spring stratification and this effect on ocean biogeochemistry and heat budgets.
Fjord circulation and heat transport at the terminus of a major outlet glacier: the relative importance of glacial versus shelf forcing
Rebecca Jackson, MIT-WHOI Joint Program
Greenland’s fjords form a key link in the climate system between the open ocean and outlet glaciers of the ice sheet. Circulation within these fjords transports oceanic heat to the glaciers, driving submarine melting and potentially affecting ice dynamics and the mass balance of the Greenland Ice Sheet. While recent studies link ocean variability with glacier behavior, little is known about the processes by which oceanic changes on the shelf can reach outlet glaciers. Resolving fjord dynamics, therefore, is crucial for understanding the controls on submarine melting and glacier retreat. Yet, we know little about what controls ocean properties at the edge of Greenland’s outlet glaciers or how these properties vary. Here, we present the most comprehensive water property and velocity records in a major glacial fjord in Greenland to date – from year-long mooring deployments, mid-fjord in Sermilik Fjord, near the terminus of Helheim Glacier. We find that the circulation is dominated by shelf-forced velocity pulses, that these flows change water properties and fjord heat content, and that the flushing power of external driving from the shelf is much greater than the buoyancy-driven circulation from glacial inputs of freshwater. Our results suggests that ocean variability on the shelf can be rapidly translated to the glacier.
Back to top

Session VIII: Terrestrial Ecosystems

Quantifying the range in variation in climate response and stand dynamics within range-margin jack pine (Pinus banksiana) populations in north-central Minnesota, USA
Kyle Gill, University of Minnesota
Ecotones, the transition zone from one biome to another, exist where groups of biota meet their climatic and abiotic limits. Small changes in these limiting factors can generate significant impacts on ecosystem structure and function in relatively short time periods. Species in these transition zones often display behaviors and dynamic patterns that differ from the non-marginal portions of their range posing challenges for their conservation and management. Jack pine (Pinus banksiana) forests meet the southwestern edge of their native range in the prairie-forest ecotone in north central Minnesota. Much of the science concerning the dynamics and management of this species, which often grows in nearly monospecific stands, is based upon research performed in portions of this species’ range where large, lethal fires predominate and create even-aged populations. However, management guidelines based upon this research have had limited success for this species along its range margin, especially within the past two decades. This has raised questions as to whether the forest dynamics (stand establishment, regeneration, age structure, disturbance regime) and climate responses of these populations display deviations from the assumed patterns. Correspondingly, this project aims to quantify the range in variability in forest dynamics and climate responses for these range-margin populations using dendrochronological techniques. Preliminary results show multi-aged stands on the landscape. These indicate dynamic patterns which may include low-intensity surface fires, canopy gaps created by wind events, or afforestation and woodland infilling during favorable climate conditions.
The times they are a-changin’: observation and modeling of vegetation phenology under changing climate
Xi Yang, Brown University
Changes of vegetation phenology in response to climate change in the temperate forests have been well documented and have important implications on the regional and global carbon and water cycles. Predicting the impact of changing phenology on terrestrial ecosystems requires both accurate phenological observations and modeling. In this study, we compared three major methods for phenological observation (manual, digital camera and remote sensing), which span from individual scale to global scale. At the local scale, we used weekly measurement of leaf biochemical, biophysical and spectroscopic properties to interpret the phenological metrics calculated from remote sensing and recently developed continuous digital camera observations. At the regional scale, we used phenological models to estimate the change of start of season in New England in the past 5 decades. We found that 1) Generally the three phenological observation methods are comparable, though discrepancies exist; 2) We found that there is a “decoupling” between camera-based observations and the plant biochemistry, especially leaves’ chlorophyll concentration; 3) Since 1960, the start of season in New England on average advanced about 1 week.
Seasonality of Tropical Dry Forests and its Sensitivity to Climate Change
Xiangtao Xu, Princeton University
Tropical dry forests, accounting for nearly 42% by area of total tropical vegetation, are characterized by 3-6 months’ dry season annually. Many species in tropical dry forests undergo a characteristic seasonal cycle (“phenology”), in which they gradually shed some or all their leaves in the dry season and then re-grow leaves in the wet season. Current dynamic vegetation models have only coarse representations of this process and have rarely been challenged by ground-based observations. To evaluate our understanding of tropical dry forest phenology, we integrated recent field measurements of leaf litterfall from 18 forested stands in Costa Rica starting from 2007, into a state-of-art dynamic vegetation model, Ecosystem Demography Model version 2 (ED2). We first used the data to construct a new phenology scheme that predicts how rates of leaf litterfall change in response to changing soil moisture conditions and photoperiod. The resulting model was then evaluated with additional leaf litterfall data and MODIS-derived Leaf Area Index (LAI). We found that the stand-average output simulated time series of LAI and leaf litterfall correlated well with MODIS-derived LAI (r = 0.84) and field leaf litter data (r = 0.66). We are currently carrying out regional-scale simulations forced with CMIP5 future climate projections. We expect that these simulations will improve our understanding of the phenology and its sensitivity to future climate change in tropical dry forest ecosystems, which is important to enhance our knowledge on carbon dynamics, energy budget and hydrological cycles there.
A global assessment of climate-driven vegetation change from the last glacial period to present
Connor Nolan, University of Arizona
Climate projections for the next 100 years include major increases in average global temperature that are likely to cause changes in plant community composition and structure across the globe. Characterizing the magnitude of impending climate-driven vegetation changes is important for conservation planning and adaptation but difficult because climate-driven vegetation change is the result of interacting processes operating on multiple spatial and temporal scales. While we do not understand all of the processes involved, we do have paleoecological records from all vegetated continents that offer a proxy record of the vegetation during the last glacial period (defined here as 14,000 to 21,000 years before present) which we can use to assess past vegetation change. Panels of regional experts compiled all available pollen and plant macrofossil records with coverage during the last glacial period. Then, for each site the experts compared glacial period vegetation to modern (or late Holocene) vegetation and assessed the magnitude of vegetational change, in terms of species composition and physiognomy (e.g. grassland vs. deciduous woodland vs. coniferous forest), and the role of the climate change since the last glacial period in driving the observed changes. Our results illustrate the spatial variability of vegetation response to this magnitude of climate change, including whether the plant communities of any area have remained unchanged since the last glacial period. This project represents the first global-scale analysis and synthesis of climate-driven vegetation change since the last glacial period and will provide an important framework for future conservation efforts and decision-making.
Back to top

Session IX: Ocean Biology and Chemistry

Evaluation of Coral records of equatorial Pacific Sea Surface Temperature
Alice Alpert, MIT-WHOI Joint Program
Global climate models project large changes in ocean circulation within the Central Equatorial Pacific as a consequence of global warming. The significance of these projected changes and the proposed mechanisms involved must be considered in the context of the magnitude and drivers of natural inter-annual and multi-decadal variability and trends. Real time data from moored ocean buoys capture only the past decade of variability. Thus, we investigated the potential for massive long-lived corals to provide continuous, multi-decade long proxy records of equatorial circulation that would allow us to extend the short instrumental record back through time.
We quantified both structural and geochemical changes in the aragonitic skeletons of Porites corals strategically located to index circulation changes. Data from in situ temperature loggers reveals the differential impact of circulation-driven topographic upwelling across our study sites. Where upwelling brings cool, nutrient-rich water to the coral habitat, mean annual temperatures are relatively low (26.5 °C) with a large annual range (3.6°C). Conversely, at sites not directly impacted by topographic upwelling, mean annual temperatures are higher, with a smaller annual temperature range. This signature of equatorial circulation is also evident as temporal changes in water temperatures across study sites. In this presentation we report results of a multi-proxy approach to evaluate the ability of corals to accurately record these differences both across space and through time, and thus provide a means to reconstruct changes in ocean circulation patterns on multi-decadal timescales.
How does plankton distribution and activity influence the variability of carbon dioxide uptake in the North Atlantic?
Claire Ostle, University of East Anglia, UK
The North Atlantic is an important sink for atmospheric CO2, and with the continued increase in anthropogenic CO2 and subsequent climate change, understanding the processes affecting this CO2 sink is vital. I am investigating the influence of plankton on the drawdown of atmospheric CO2. Surface measurements of pCO2 have been collected since 2002 between the UK and the Caribbean alongside measurements of plankton abundance and diversity derived from the Continuous Plankton Recorder. During four seasons in 2012, we have made measurements of dissolved inorganic carbon, total alkalinity and oxygen along this transect to further enhance this dataset. I have completed the laboratory analysis of these samples and I am currently writing a paper on deriving net community production in dynamic biomes across the North Atlantic for 2012. Ultimately I aim to show how the varying abundance, composition, and distribution of phytoplankton influence the North Atlantic CO2 sink, based on numerical and multivariate statistical analysis of this combined dataset. Due to the interdisciplinary nature of my project I have been able to set-up a number of collaborations with scientists at Plymouth Marine Laboratory and the National Oceanography Centre in Southampton. I have also been teaching on a number of undergraduate degree modules including the module “Global Environmental Change” which looks at the science, policy and social issues surrounding key topics such as climate change.
Numerical Simulation of Seafloor Weathering as a Climate Stabilization Mechanism
Navah Farahat, University of Chicago
It has been suggested that the oceanic crust of warm mid-ocean ridges flanks is an important carbon dioxide sink, with a possible carbon dioxide uptake rate that is controlled by climate. Such a dependence would suggest seafloor weathering can act as a climate-weathering feedback much like the continental silicate thermostat. Though continental silicate weathering dominates silicate-weathering climate feedback on present-day Earth, seafloor weathering may be more pronounced on waterworld planets, in oceans with significant porous crust, and on Earth during a snowball state. A significant difference between continental silicate weathering and seafloor weathering is the water that interacts with the rock; on continents fresh water is involved in weathering, while in oceanic basalts seawater, whose exact constituents depend on climate and other controls, is involved in the weathering reactions. To quantify seafloor weathering’s role in climate, I am constructing numerical model of hydrothermal alteration and carbonate precipitation that corresponds to the upper layers of porous oceanic basalt, where seafloor weathering occurs. In this model, hydrothermal reactions and carbon dioxide uptake are simulated through a two-dimensional porous media flow simulation. I am using this model to design a suite of experiments that explore the effect of geologic controls and climate controls on seafloor weathering as a climate feedback and a carbon sink.
Back to top

Poster Session A

.

Jane Baldwin Princeton Interactions Between Precipitation and Temperature in Determining the Equilibrium of Glaciers
A glacier’s response to climate is commonly characterized by a change in the position of its equilibrium-line altitude (ELA) over time— ELA being the elevation at which the accumulation and ablation rates of a glacier are equal. Mark et al. (2005) compiled a dataset of worldwide mountain glacier ELAs at the present and Last Glacial Maximum (LGM). These observations indicate a linear trend of lower ELAs rising more since the LGM than higher ELAs. Assuming that lapse rates were steeper at the LGM (Sun and Lindzen 1993), this glacier trend contradicts expectations from basic ELA models that focus on ablation. In this study, I show that a more complete understanding of the physical controls on glacier ELA can reconcile the trend in the ELA observations with understanding of LGM climate. I model glacier ELA on a mountain by coupling a positive-degree-day ablation model with Roe and Baker (2006)’s orographic precipitation model for accumulation. Parameters in this model are varied to simulate the LGM to present climate transition, and determine LGM ELA and change in ELA under various climate and mountain constraints. It is found that precipitation variations with mountain environment play a key role in determining the range of ELAs at the LGM and present. Our model results suggest that a spatial warming trend can account for the LGM to present inverse relationship between ELA and change in ELA. These results confirm understanding of LGM climate conditions, and provide suggestions for better interpretation of past glacier data and current glacier change.

.

Jase Bernhardt Penn State The Synoptic Climatology and Surface Temperature Impacts of Jet Contrail Outbreaks and Their Forecasting Applications
The artificial cloudiness, or “contrail cirrus,” that results from multiple jet contrail outbreaks can last for several hours and alter the radiation budget and, thereby, surface temperatures. An extensive database of satellite-derived “clear-sky” contrail outbreaks over two regions of the continental United States– the South and the Midwest –for two mid-season months of 2008 and 2009 was used to determine the potential climatic impact of contrail outbreaks on surface temperature. The aggregated impact of outbreaks on maximum and minimum temperatures and on the diurnal temperature range (DTR) was determined by comparing the departures at stations overlain by outbreaks with those at adjacent stations having similar synoptic and land surface conditions, but not experiencing contrail cloudiness. The results at the South stations during the month of January and the Midwest stations during the month of April both indicate a statistically significant suppression of DTR at outbreak stations versus adjacent non-outbreak stations. Meanwhile, a synoptic climatology (i.e., composite average) of upper troposphere (UT) variables (temperatures, specific humidity, horizontal winds, and vertical lapse rate) for these longer-lasting jet contrail outbreaks was also developed to link the impacts of contrail outbreaks with conditions favorable for their formation. This climatology was also utilized as the basis for successful prediction and retrodiction of contrail outbreaks through the analysis of reanalysis and forecast model data.

.

Shaena Berlin MIT Determining the conditions necessary for cloud formation on Mars: Simulating ice nucleation in an electrodynamic balance
The Martian atmosphere contains water ice clouds similar to Earth’s cirrus clouds. These clouds influence the atmospheric temperature profile, alter the balance of incoming and outgoing radiation, and vertically redistribute water and dust. Extrapolations of classical heterogeneous nucleation theory to colder temperature and pressure regimes present in extraterrestrial atmospheres may be inaccurate, and thus hydrological models describing these regimes could lack physical meaning. In this project, an electrodynamic balance (EDB) is used to levitate individual aerosol particles. Aerosols tested include previously characterized dusts and more novel Mojave Mars Simulant (MMS) dust. Relative humidity, temperature, and pressure are altered; first, experiments take place at terrestrial regimes to validate the setup of this EDB, and then the instrument is brought to more extreme conditions. These projects should better determine the conditions under which ice nucleation occurs.

.

Yassir Eddebbar Scripps Natural Climate Variability and Air-Sea Heat Exchange Influences on the Atmospheric Potential Oxygen
The Atmospheric Potential Oxygen (APO) record (calculated using the combined long term measurements of atmospheric CO2 and O2 to remove the terrestrial carbon cycle influence) opens a unique observational window into ocean biogeochemical and physical processes at various time scales. The last two decades of the APO record exhibit spatial and temporal variability features at seasonal, interannual, and decadal timescales. One such interesting decadal feature is the acceleration of the APO downward trend since 2000 as compared to the 1991-1999 trend, coinciding with the observed warming “hiatus” associated by recent studies with changes in ocean heat uptake and decadal to multidecadal climate variability. Here we explore the “natural” (or unforced) variability in the APO, and investigate the influence of air-sea heat exchange and internal modes of climate variability on the APO fluxes in an unforced 1000-year control model run using the Community Earth System Model (CESM). Understanding these relationships, and the influence of internal climate variability, provides context for recent APO observations, and insight into the mechanisms driving anomalies, including processes behind the warming “pause” observed since 2000.

.

Sarvesh Garimella MIT Characterizing a new ice nuclei counter: The SPectrometer for Ice Nuclei (SPIN)
The majority of cloud formation in Earth’s atmosphere occurs via nucleation of water droplets or ice crystals on particulate matter. In this study, the interaction of clay mineral dust particles and water vapor is examined to determine the conditions required for cloud droplet and ice crystal formation. Activation conditions for size-selected Arizona Test Dust, illite, and sodium-rich montmorillonite are determined in the laboratory using continuous flow chambers, in which the relative humidity and temperature are carefully controlled. In previous studies, particle size selection is performed using a Differential Mobility Analyzer (DMA), which sorts particles based on their electrical mobility after they have been imparted with a Boltzmann charge distribution. However, the efficiency of this process is dependent on the original size distribution of the aerosol, a consideration that is often neglected in the literature. In this study, the activation experiments are performed with extra attention to the shape of the pseudo-mondisperse size distribution outputted by the DMA. These results are compared to literature values that do not consider the actual size of the post-DMA particles.

.

Daniel Gilford MIT Changes in the radiation balance of the tropical tropopause layer
The transition region between the troposphere and stratosphere in the tropics, known as the tropical tropopause layer (TTL), is a critical part of the climate system. Most air transport into the stratosphere occurs across the TTL. This transport injects water vapor into the stratosphere and modifies stratospheric trace greenhouse gas species concentrations, including ozone. Outside of rare deep convective events, TTL transport is driven by primarily radiative heating or cooling, which makes understanding of the TTL radiative balance critical. Temperatures have declined in the TTL region since 1979, which has impacted large-scale atmospheric circulation and the strength of tropical cyclones. These trends imply that the radiative balance of the TTL has likely changed since 1979. This study uses available radiosonde and satellite observations of TTL water vapor, ozone, and aerosols to determine TTL radiation balance climatology, seasonality, and trends. We use an offline narrow-band Column Radiation Model from the Community Atmosphere Model version 3 (CRM-CAM3) to calculate shortwave and longwave heating/cooling rates attributed primarily to water vapor, ozone, and aerosols from 1979 to the present. Shortwave and longwave heating/cooling rates are evaluated with trend analysis. Results indicate that decreases in ozone concentration and increases in water vapor concentration have increased TTL cooling rates since 1979. Aerosols generally have a transient effect and do not significantly impact trends. It is hypothesized that cooling TTL temperature trends can be partially attributed to these changes in water vapor and ozone TTL profiles.

.

Laura Guzman Simon Fraser Beyond the Carbon Tax: Personal Carbon Trading and British Columbia’s Climate Policy
Personal carbon trading (PCT) is a scheme under which all individuals are allocated a number of free carbon allowances forming a carbon budget. Persons whose carbon emissions are lower than their carbon budget can sell their surplus to persons who have exceeded theirs. As distributed allowances are reduced annually, consumers are encouraged to modify their behaviour and reduce carbon emitting activities in order to not exceed their carbon budget. PCT and carbon tax are both carbon pricing instruments that, using different policy framings, aim to reduce greenhouse gas emissions. In 2011, a comparative experiment was done in the UK to test the hypothesis that “due to economic, pro-environmental and mental accounting drivers PCT would have greater potential to deliver emissions reduction than taxation” (Parag & Capstick, 2011). The results showed that “it may be possible to encourage people to save further emissions, given a low price signal, by altering the [policy] framing” (Parag & Capstick, 2011).

A comparative analysis between the British Columbia (BC) carbon tax and PCT policy frames provides valuable input for policy makers in North America. Key opinion leaders in the low carbon economy, climate policy and sustainable energy sectors have been interviewed about the potential of a PCT scheme to increase people’s willingness to shift carbon emitting behaviour in BC. The results provide a comprehensive analysis of the two carbon pricing policy frames and their effectiveness in modifying behaviour. Policy recommendations are also made regarding personal engagement, carbon budgeting and collective action in the province.

.

Toni Klemm Oklahoma Developing Short-Term, Seasonal Climate Projections For Growers Using a High-Resolution Global Climate Model
Livestock producers and warm-season crop growers in the United States can be severely affected by dry and wet periods, as seen during the 2011 and 2012 drought and this year’s strong rain fall in some regions of the U.S. Great Plains. This study seeks to answer the question whether a high-resolution global climate model can be used to make reliable climate projections for the current growing season. The goal is to create a decision-support tool to help agricultural and livestock producers better prepare for potential dry and wet conditions which are projected to occur more frequently according to the 2009 National Climate Assessment and the 4th Assessment Report of the Intergovernmental Panel on Climate Change. Similar projection tools have already been developed by the National Weather Service’s Climate Prediction Center, but are spatially coarse, have few meteorological variables provided, and have relatively high uncertainty, even in the short term.

To conduct this study, qualitative methods will explore grower’s preferred prediction variables, while comparison of station measurements with climate model output will evaluate the model’s uncertainty for those variables in the south-central U.S. The final desired product, a map- or GIS-based decision-support tool, will combine model output and uncertainty indicators to inform about atmospheric and soil conditions and the certainty of their occurrence.

.

Lisa Tabor Kansas State Stakeholder Adaptation to Climate Change in Kansas: What have we learned?
Earth-system variations have combined with transformations induced by an expanding global society to shape a dynamic planet where change has happened and changes will happen in the future. Agricultural stakeholders in Kansas have reacted to a transforming business environment by adopting new strategies to maximize production while experiencing the climatic vagaries of floods and major periods of drought.
This paper combines information from 42 stakeholder interviews with ideas from the scholarly literature on past adaptation in the central Great Plains to address the question: are Kansas farmers going to be able to adapt to climate change? There is clear evidence that autonomous adaptation has happened over that last century as new land management practices and technological innovations have been put into play. Interviews in north-central Kansas in 2010 with farmers and ranchers suggest that they are observing a changing environment and that they desire credible information on how future climate change might impact their production practices.

.

Liam Brannigan Oxford Do submesoscale flows shallow or deepen the ocean mixed layer?
The depth of the ocean mixed layer plays a key role in the climate system by regulating exchanges of heat and carbon between the ocean and atmosphere. It also effects the supply of nutrients which drive marine ecosystems.
The role of submesoscale flows in setting the mixed layer depth and transition layer stratification in the upper ocean over the annual cycle is investigated. It is found that simulations which resolve submescales have a deeper mixed layer and sharper transition layer. This destratification is primarily diffusive and happens despite the presence of submesoscale restratification. The diffusive flux does not have a submeoscale spectral signature and so it suggests that the presence of submeoscales has altered the larger-scale flow field to generate the stronger diffusive changes in stratification. The results imply that implementing submesoscale parameterisations which only act to restratify global climate models could accentuate existing
biases in the mixed layer depth compared to observations.

.

Nicole Couto Rutgers Characteristics of eddies shed from the Antarctic Circumpolar Current
Winter temperatures in the western Antarctic Peninsula (WAP) region are the fastest-warming temperatures on Earth, however understanding the heat sources driving the rise in temperatures remains an open research question. The only heat source which is able to account for the atmospheric warming is found in the deep waters of the Circumpolar Deep Current (CDW). This offshore source is thought to deliver heat to the coastal regions in the form of eddies shed off the Antarctic Circumpolar Current (ACC). It has been suggested that Marguerite Trough provides a path for CDW intrusions to follow from the shelf break to Marguerite Bay. Data from moorings located on both the eastern and western boundaries of Marguerite Trough indicate that warm water moves along the trough as distinct bursts occurring an average of four times per month between about 200 m and 400 m depth. In this study, autonomous Webb glider observations along the WAP continental shelf region are used to examine the spatial extent of eddies shed from the ACC. In each of two deployments (austral springs 2010/11 and 2011/12), gliders traveled southward along the peninsula and encountered distinct water masses with properties typical of CDW, allowing estimates of the spatial scale of these features. During the second deployment (Nov. 2011 – Jan. 2012), the glider encountered two of these features more than once, allowing estimates of the transport speed and rate of decay of the eddies. These high-resolution observations may fill the gap in our understanding of the heat budget of this important region.

.

Landon Burgener University of Washington New insights from “clumped” isotopes on how the timing of soil carbonate formation affects paleoclimate reconstructions
Reconstruction of continental paleoclimates has long been hampered by a lack of suitable proxies. To address this need, carbon and oxygen isotope analysis of soil carbonates have long been used as climate proxies, since the isotopic composition of soil carbonates is influenced by the temperature of the soil at the time of carbonate precipitation. Early studies proposed that past changes in the soil formation temperature could be reconstructed by measuring the isotopic ratio of the carbonates using conventional isotopic analysis. Unfortunately, the relationship between the carbon and oxygen isotopic composition of a carbonate sample and temperature is complicated by a variety of factors which make distinguishing between changes to temperature, precipitation, and other variables very difficult. This study uses a new geochemical tool known as clumped isotope geothermometry to analyze a new suit of soil carbonate samples that were collected in central Chile. This new method, based on the abundance of 13C-18O bonds, is appealing because the temperatures reconstructed using clumped isotope geothermometry depend only on the formation temperature of the soil carbonate. This work further refines the use of soil carbonates as a paleoclimate proxy, and provides better constraints on the suitability of this technique for reconstructing past climates. The results of the clumped isotope analysis are used to better understand the seasonality of soil carbonate precipitation, and how the formation temperatures relate to actual air temperatures at the earth surface.

.

Yavor Kostov MIT Using climate response functions to quantify ocean heat uptake
We analyze the climate response functions of CMIP5 coupled general circulation models and show that the ocean reacts to radiative forcing on two inherent timescales, fast and slow. We interpret these fast and slow timescales in terms of heat uptake by the mixed layer and heat uptake by the interior ocean. By analyzing and fitting an idealized box model to the climate response functions, we find that there is a notable diversity among the rates of vertical heat transport across CMIP5 models. We relate the ocean’s effective heat capacity to properties of the ocean circulation.

.

Isabela Le Bras MIT/WHOI A PV Budget in the Western North Atlantic
As an intense western boundary current, the Gulf Stream transports heat to the poles, and is hence an important regulator of climate. Our understanding of western boundary current dynamics center around simple analytical theories that can be framed in terms of the conservation of potential vorticity (PV).

In this study, we focus on observations in the surface of the Western North Atlantic and quantify contributions to the PV balance in a layer averaged sense. Advection of PV out of the layer is quantified using ADCP velocity and CTD density data from the Clivar section A22. Wind stress input of PV is quantified using satellite data. The remaining PV is likely dissipated in frictional boundary layers, which are difficult to observe on a large scale.

We therefore turn to ECCO, a state estimate that parameterizes friction, to quantify this particular term and compare its budget of PV to the observed one.

Although it does not represent the ocean exactly, ECCO also provides insight into the time variation of the calculated terms; and large seasonal variations of PV advection caution against the use of single hydrographic section to represent the mean state.

The analysis of ECCO output confirms the overall balance that we calculated in the observations; that the advection of PV out of the layer is balanced primarily by frictional dissipation of PV along the coast. The question remains what phenomena dissipate PV, or if the importance of friction is a function of the approximations made.

.

Tom DeCarlo WHOI Climate signals recorded in coral skeletons: from proxy development to paleoclimate interpretation
Knowledge of climate variability prior to the beginning of the instrumental record around the mid-19th century must come from paleoclimate proxies. Paleoclimate reconstructions are critical for (1) improving our understanding of climate processes and feedbacks acting under different climate states and (2) placing instrumental record observations of modern climate change in the context of natural variability. Annually resolved climate signals preserved in the chemistry of coral aragonite skeletons have provided information about sea surface temperature (SST) variability on annual to centennial timescales throughout much of the past millennium. Extracting SST information from corals has typically relied upon calibration of a proxy with modern instrumental records before application of the proxy back in time. Recently, it has been recognized that ‘vital’ effects due to the biologically mediated precipitation of coral aragonite are convolved with the desired oceanographic signals. We have conducted abiogenic aragonite precipitation experiments in the laboratory, combined with efforts to model the coral biomineralization process, in order to account for vital effects and resolve the SST variability recorded in coral skeletons. Here we present the application of these techniques to modern coral skeletons for which instrumental temperature records serve as a test of the accuracy of our approach. Our novel techniques offer the ability to improve the accuracy of coral-based paleoclimate SST reconstructions compared to conventional methods. Many applications exist for these techniques, such as resolving El Nino Southern Oscillation (ENSO) variability in the context of the mean Pacific climate state and cross-basin SST gradients.

.

Greg de Wet UMass Amherst Using organic geochemical methods to investigate paleotemperature and paleoprecipitation during “super interglacials” from Lake El’gygytgyn sediments
In light of predicted climate change, high-resolution paleoclimate records are essential to accurately contextualize future warming. The Arctic region in particular is currently lacking highly-resolved, terrestrial paleoclimate reconstructions. A sediment core from Lake El’gygytgyn in northeastern Russia provides a continuous record of Arctic climate spanning the past 2.8million years (Melles et al., 2012). Preliminary work has identified numerous “super interglacials” during the Quaternary, such as Marine Isotope Stages (MIS) 11c and 31. Pollen analysis suggests that maximum summer temperatures were ~4-5°C warmer and annual precipitation was ~300 millimeters higher than MIS 1 and 5e (Melles et al., 2012). The goal of this research will be to use organic lipid biomarkers to reconstruct temperature and precipitation at high resolution through these super glacial periods.

The Methylation of Branched Tetraethers/Cyclization of Branched Tetraethers (MBT/CBT) Index (Weijers et al., 2007) will be used to reconstruct past temperatures. Previous work has found these compounds to be abundant in Lake El’gygytgyn sediments and that the MBT/CBT derived temperature record captures glacial-interglacial climate variability found in global climate records like the LR04 benthic δ18O stack (Lisiecki and Raymo, 2005). Paleoprecipitation will be estimated using hydrogen isotopic ratios of leaf waxes preserved in the sedimentary record (Hou et al., 2008).

Recently, the Lake El’gygytgyn sediment record was sampled at centimeter scale resolution through these super interglacial periods, which will allow for the creation of a highly resolved paleotemperature and paleoprecipitation record from the Arctic. When complete, this data will provide an important indication of how the terrestrial high latitudes will respond to a warming climate.

Hou J., D’Andrea W., Huang, Y. 2008. Geochim. Cosmochim. Acta 72, 3503-3517.

Lisiecki L. E. and Raymo M. E. 2005. Paleoceanography 20.

Melles M. et al. 2012. Science 337, 315-320.

Weijers J. W. H., Schouten S., van den Donker J. C., Hopmans E. C., and Sinninghe Damsté J. S. 2007. Geochim. Cosmochim. Acta 71, 703-713.

.

Andrea Dubin MIT A magmatic trigger for the Paleocene-Eocene Thermal Maximum?
Fifty-six million years ago Earth experienced rapid global warming (~6°C) that was caused by the release of large amounts of carbon into the ocean-atmosphere system. This Paleocene-Eocene Thermal Maximum (PETM) is often cited as an analogue of anthropogenic climate change. Many trigger mechanisms for the carbon release at the PETM have been proposed. Common to all scenarios is rapid release of isotopically light carbon (<13C/12C values) from methane hydrates, terrestrial or marine organic matter, as indicated by a pronounced excursion to light carbon isotope values across the PETM. I am testing the hypothesis that the PETM warming and isotope excursion were caused by an intrusion of a magmatic sill complex into organic-rich sediments. The intrusion of hot magma into sedimentary rocks will cause heating and metamorphic reactions in a thermal aureole around the intrusion. If these sediments are rich in organic matter, large volumes of isotopically light carbon dioxide could be released quickly. I am examining both organic and inorganic changes to organic-rich sediments that occur when these sediments are intruded by a magmatic dike. This is accomplished through examination and chemical analysis of samples at varying distances from an intrusion at a field site in New Mexico. Additionally, I am scrutinizing inorganic evidence for magma-sediment interaction in marine PETM sediment sections. Analyses of changes in osmium and lead isotopes during the PETM may give additional evidence as to the potential magnitude of carbon release by this mechanism during the PETM.

.

Lynn Brennan UMass Amherst Modeling stream temperature for climate change analysis in the Connecticut River Basin
Stream temperature is a critical factor in the persistence of aquatic ecosystems. In the Northeast, increasing stream temperatures due to climate change pose a significant threat to cool and coldwater fish communities. Climate projections indicate warmer conditions in the region, particularly during the summer months when fish are more susceptible to extreme temperatures. For this study, two different coupled hydrology-stream temperature models were implemented to predict stream temperatures in select Connecticut River subbasins. The VIC-RBM model and SWAT-stream temperature model were implemented for the Westfield basin in western Massachusetts (1,338 km2) and the Passumpsic basin in central Vermont (1,129km2), respectively. These basins were selected for analysis due to the significant amount of historical stream temperature data available, the presence of USGS HCDN reference gages, and their largely unaltered flow patterns. For the climate change analysis, future projections of climate were obtained from the Coupled Model Intercomparison Project Phase 5 archives (CMIP5). Two emission scenarios were chosen: a low emission scenario (RCP 4.5) and a high emission scenario (RCP 8.5). Monthly precipitation and maximum and minimum temperature were statistically downscaled and bias-corrected (BCSD) at a resolution of 1/8 degree (~12.5km) for the period 1950-2099. Daily values were obtained by resampling from the observed records. Stream temperature simulations for the 21st century will be used to assess the vulnerability of aquatic ecosystems under warmer conditions and will help local wildlife managers develop adaptation plans for fish communities.

.

Tim Cronin MIT A Sensitivity Theory for the Equilibrium Boundary Layer over Land

Real land surfaces are highly heterogeneous, and the exchange of energy between the surface and atmosphere can be strongly and nonlinearly influenced by the diurnal cycle. Due to these intrinsic complexities associated with modeling land-atmosphere interactions, global models typically use elaborate land surface and boundary layer physics parameterizations. Unfortunately, it is difficult to arrive at deep understanding by using elaborate models alone. At the same time, it is also increasingly important to gain a deeper understanding of the role of changes in land cover, land use, and ecosystem function — induced by e.g., urbanization, deforestation, drought, or CO_2-induced stomatal closure — in past and future climate change. To improve the foundation of our understanding, we suggest a simple framework of boundary layer climate sensitivity based on surface energy balance; just as global climate sensitivity is based on top-of-atmosphere energy balance. We develop an analytic theory for the boundary layer climate sensitivity of an idealized model of a diurnally-averaged well-mixed boundary layer over land (Betts, 2000). This analytic sensitivity theory identifies changes in the properties of the land surface — including moisture availability, albedo, and aerodynamic roughness — as forcings, and identifies strong negative feedbacks associated with the surface fluxes of latent and sensible heat. We show that our theory can explain nearly all of the sensitivity of the Betts (2000) system of equations. To evaluate the theory, we also perform simulations with a two-column radiative convective model.

.

Karl Lapo University of Washington Impact of forcing data uncertainty on simulating snow: understanding the limitations of simulating the cryosphere response to climate
Snow-covered surfaces play an important role in climate, ecosystems, and society through their influence on the surface energy balance, controlling the distribution of mountain ecosystems, and providing water for one sixth of the world’s population. Projecting how snow-covered surfaces will vary under future climates is therefore of great importance. Predicting and understanding those changes requires a detailed understanding of snowmelt physics. There have been numerous studies examining how uncertainties caused by the structure and parameters of snow models impact simulations of snow. However, very little work has been done examining how errors in the forcing data propagate into these simulations.

In this study we examined how uncertainties in surface irradiances, typical of those found in climate models, propagate through simulations of snow. To isolate the impact of forcing uncertainty from errors in model parameters we performed an idealized sensitivity study using three commonly used energy balance models at four sites representative of different montane environments of the Western United States. We perturbed the input radiation data for each model-site combination using pre-defined error type and magnitude based on evaluations of GCM irradiances compared to mountain observations. We find that the biases GCM surface irradiances are of sufficient size to substantially impact simulations of snow, providing a strong control on accumulated snow, melt rates, and snow surface temperature. Finally, we show that uncertainties in the model parameter space can mask these errors through compensating errors.

.

Keyvan Malek Washington State Impacts of climate change and change in irrigation management strategies on Evapotranspiration and agricultural water availability
Evapotranspiration (ET) is one of the key components of the terrestrial water and energy cycles. It’s tightly coupled and potentially affected by climate change and shifting agricultural practices. Variations in the magnitude of ET changes the partitioning among components of water and energy cycles and impacts water availability and agricultural productivity. This investigation aims at better understanding the impacts of climate change and irrigation management (i.e. different types of irrigation systems, and different irrigation frequencies) on ET and irrigation water availability in the Yakima River Basin (YRB) in central Washington State. This basin is an important component of the state’s agricultural economy. Warming will exacerbate current water shortages in this snowmelt-dominated basin as streamflow seasonality shifts away from the summer season of peak irrigation demand and into the winter season, and as irrigation demands increase. Our research approach involves tightly coupling two process-based models: a spatially-explicit macroscale hydrologic model, the Variable Infiltration Capacity (VIC) model, and a cropping system model, CropSyst. The VIC model simulates all the hydrological phenomena including soil moisture, while CropSyst simulates transpiration from vegetation and crop growth and phenology. ET includes 4 components: crop transpiration (Tc), Soil water evaporation (Es), evaporation of canopy-intercepted water (Ec), and evaporation from droplets during irrigation (Ed). The analysis suggested an important change in the YRB water and energy balance as a result of climate change, and showed a high sensitivity of ET to irrigation management, with implications to water use policies to adapt to increasing water shortages in the region.

Back to top

Poster Session B

.

Sagar Parajuli Texas Improving dust emission characterization in climate models
Dust is known to affect the earth radiation budget, bio-geochemical cycle, precipitation, human health and visibility. Despite the increased research effort, dust emission modeling remains challenging because dust emission is affected by complicated geomorphological processes. Existing dust models overestimate dust emission and rely on tuning and static erodibility factor so as to make simulated results comparable to remote sensing and ground-based observations. In those models, dust emission is expressed in terms of threshold friction speed which ultimately depends upon percentage clay content and soil moisture. Unfortunately, due to the unavailability of accurate and high resolution input data of percentage clay content and soil moisture, estimated threshold friction speed doesn’t represent the variability in field condition.
In this work, we attempt to improve dust emission characterization by developing high resolution geomorphological map of the Middle East and North Africa (MENA) which is responsible for more than 50% of global dust emission. We classify the study area into several key geomorphological categories and use unique parameterization in each category to represent the varying dust emission potential of different surfaces. We also propose a dynamic erodibility map based on the correlation between wind speed and satellite retrieved aerosol optical depth (AOD) to address the seasonality of erodibility. The reference dust scheme used in this study is the Dust Entrainment and Deposition (DEAD) model which is also a component of community land model (CLM). Proposed improvements in the dust emission representation will help to better understand the accurate effect of dust on climate processes.

.

Ethan Butler Harvard Variable temperature sensitivity of US maize yield across development stages
The sensitivity of maize to high temperatures has been widely demonstrated. Furthermore, field work has indicated that reproductive development stages are particularly sensitive to stress, but this relationship has not been quantified across a wide geographic region. Here, the relationship between maize yield and temperature variations is examined as a function of developmental stage. Development data are aggregated into four distinct phases: vegetative, silking, grain-filling and mature. Temperatures that correspond to each developmental stage are then inferred from a network of weather station observations interpolated to the county level, and a multiple linear regression technique is employed to estimate the sensitivity of county yield outcomes moderate and high temperatures referred to as growing and killing-degree days respectively. Early results indicate that the silking and grain-filling stage are generally the most sensitive to killing degree days, while the grain-filliing stage is also quite sensitive to growing degree days. The period of greatest sensitivity is also spatially correlated with greatest sensitivity during silking in the southern corn belt and during grain-filling in the north. These variable spatial and temporal sensitivities point toward a much more complicated picture of how crops will respond to a warmer environment.

.

Josue Delgado Balbuena Instituto Potosino de Investigacion Cientifica y Tecnologica A. C. Effects on the net ecosystem carbon exchange of temperature and soil moisture variability in a semiarid tropical grassland ecosystem.
Productivity in semiarid grasslands is mainly controlled by precipitation, which comes as stochastic events varying in amount, intensity and frequency. High seasonal and interannual variability of precipitation could be enhanced by future changes in climate, imposing new challenges for ecologists to elucidate the role of arid lands as carbon (C) sinks or C sources as well as the mechanisms controlling C and water fluxes at short and long temporal scales, in particular to predict how these ecosystems will respond to future scenarios of climate change. In this study, net ecosystem carbon exchange (NEE) measurements for two years were carried out in a semiarid grassland in Central Mexico, with the goal to elucidate the environmental controls of NEE and its components (gross ecosystem exchange, GEE, and ecosystem respiration, Re), and their indirect effect by ecosystem functional changes, to finally quantify their effect on the annual C balance of this grassland ecosystem. Photosynthetic photon flux density and air temperature were the main drivers of GEE and Re, respectively, at diel time scale, but both of them were modulated seasonally by soil water availability. Even though precipitation was the main factor explaining interannual NEE variability, differences in amount and frequency between years modified the response of the grassland to precipitation. An extreme winter precipitation event stimulated C uptake by grasses, but also the activity of soil microorganisms, resulting in just a slight net C uptake. This study shows some advances in the knowledge of semiarid grasslands functioning and its response to climate variability.

.

Cristina McKernan Colorado State The effects of glacial loss on high mountain riparian vegetation
In 1850 at the end of the Little Ice Age, 150 glaciers existed in Glacier National Park (GNP), MT. In 2010, only 26 remained. Climate warming in mid-high latitudes and mountain regions, like GNP, is occurring more rapidly than any other place on Earth. This warming is causing extensive loss of glaciers and snowpack and in high elevation watersheds, glacier melt water exerts substantial influence on hydrogeomorphic processes producing floods, landslides, and debris flows. Riparian wetlands are formed and maintained by the hydrologic and geomorphic processes of streams. They occupy a relatively small percentage of mountain landscapes but are important and highly sensitive ecosystems worldwide. Any changes in stream flow and geomorphic activity caused by glacial loss could impact riparian vegetation. The goal of my study is to assess the current vegetation composition and status of high elevation riparian wetlands. I will evaluate how riparian vegetation may respond to changes in stream flow and geomorphic processes caused by glacial recession. Understanding the potential influences of climate induced changes in the hydrologic drivers of riparian wetlands is a critical topic with implications for channel stability, flood control, water chemistry, and biodiversity, all high priority concerns for GNP.

.

Angelica Patterson Columbia Physiological response to temperature across nine tree species in a northeastern temperate forest
Extensive botanical surveys and long term plots at Black Rock Forest in southeastern New York have shown that since the early 1930’s, three northern-ranged tree species were extirpated and eleven tree species were introduced or had migrated from the southern USA. These observations are consistent with a warming climate and suggest the Hudson River Valley may be an important location to study the effects of climate change on Northeastern forests. In this study, we compared a suite of physiological and leaf traits across nine tree species that have one of four distribution ranges (northern, central, southern, introduced) and one of two leaf types (broad-leaf and coniferous). Carbon to nitrogen ratio, and nitrogen content (N) differed significantly among species and range category for both broad-leaved and conifer trees. Specific leaf area (SLA) of broad-leaved trees differed significantly between species and range category. Photosynthetic rate at light-saturation (Amax), maximal rate of carboxylation (Vcmax), electron transport limitation (Jmax), triose phosphate limitation (TPU), and dark respiration (Rd) differed significantly between species and range category where southern ranged species had the highest photosynthetic and respiration rates for broad-leaved species.
Pinus strobus, a centrally ranged conifer species had higher photosynthetic and lower respiration rates than Pinus resinosa, a northern ranged conifer species. There were strong correlations between leaf traits and physiological traits for both broad-leaved and conifer species revealing that trees with high N content and high SLA have higher photosynthetic rates and is consistent with the literature. These results highlight the physiological advantages southern and introduced species have over their northern ranged counterparts with influential effects on forest community composition and energy flows.

.

Stefanie Mack Old Dominion Tidally forced mesoscale variability in the Ross Sea from a regional ocean model
The Ross Sea is subject to large amplitude diurnal tidal currents, particularly along the shelf break, that affect sea ice concentration, surface heat fluxes, and the formation of Antarctic Bottom Water. On shelf, these tidal currents can play a key role in the resuspension and upward mixing of iron from bottom sediments. As the Ross Sea is highly iron limited, it is important to investigate processes that can provide iron to the euphotic zone. A study of these mesoscale processes is crucial to not only better understand the dynamics of the Ross Sea system, but also to improve global climate models. Satellite observations have shown that ice concentration along the shelf break can vary from ~60% to 100% on tidal time scales. Using a fine resolution (5-km or less) ocean circulation model with realistic atmospheric forcing coupled with a sea-ice sub-model, we identify the contribution of tides to sea ice concentration by comparing runs with and without tidal forcing. The model is also used to estimate the net contribution of tide-forced changes in sea ice to the exchange of heat between the ocean and atmosphere. In conjunction with the Processes Regulating Iron Supply at the Mesoscale (PRISM) project, we further investigate the contribution of tides to the resuspension and upward mixing of iron over Ross Bank.

.

Dana Mastropole MIT/WHOI The Variability of Denmark Strait Overflow Water
The Meridional Overturning Circulation (MOC) is driven by deep, ocean convection north of the Denmark Strait. The Denmark Strait overflow water (DSOW) is a cold, fresh water mass which comprises the densest portion of North Atlantic Deep Water (NADW) and sustains the lower branch of the MOC. However, the origins, average size and location, and variability of this water mass are still poorly constrained. This study presents a compilation of ten years of conductivity-temperature-depth and acoustic Doppler current profiler data taken from ships. These observations are used to construct average potential temperature, salinity, and absolute geostrophic velocity sections across the strait. Such mean sections are useful to characterize variations in the flow and identify the origins of this water. They can also tell us how often fully developed eddies are present at the sill. Such an investigation can provide significant insight into the mechanisms driving formation of DSOW and how the strength of the MOC varies in a changing climate.

.

Benjamin Scheifele University of British Columbia Double Diffusion in Powell Lake: New insights from a Natural Laboratory
Double diffusion is an oceanic transport mechanism that is observed globally and is especially prominent in the Canada Basin of the Arctic, where it transports heat from the Atlantic Water layer to the Surface Layer. We study double diffusion in Powell Lake, which is an ex-fjord on the south coast of British Columbia, containing a layer of relic seawater between 250 and 350 m depth. The salt gradient together with a known geothermal heat flux to the bottom of the lake provides the necessary conditions to induce the diffusive layering mode of double diffusion. The process is naturally isolated in the quiescent waters of the deepest lake basin, and because the boundary conditions are well described, the constraints on the system can be defined more clearly than in previous observational studies of double diffusion. We have taken a series of high-resolution temperature and conductivity measurements, resulting in an extensive dataset which we use to describe double diffusive layers in the lake, calibrate traditional heat flux parameterizations, and characterize details regarding the nature of the interface between successive layers. We find that layers are coherent on the basin scale and that their characteristics have a well defined vertical structure, that parameterizations correctly estimate heat fluxes within a factor of approximately 3, and that transport through the interface between layers becomes dominated by molecular diffusion only as the density ratio becomes large.

.

Elizabeth Asher University of British Columbia Concentrations, turnover rates and sea-air fluxes of dimethylsulfide (DMS) in coastal and offshore waters of the SubArctic Pacific during summer 2010- 2011.
The Subarctic Pacific Ocean (SA PAC) is an important source of the climate-active gas dimethyl sulfide (DMS), yet the spatial coverage of DMS concentration data in this region remains sparse, and few measurements exist of the underlying DMS production and consumption rates. The SA PAC contains two distinct ecological provinces, coastal British Columbia (BC) waters, characterized by summertime upwelling and high biomass, and offshore Fe-limited high nutrient, low chlorophyll (HNLC) waters of the Alaskan gyre.
We present results from two surveys of DMS concentrations and turnover rates in coastal and offshore waters of the SA PAC during the summer of 2010 and 2011. We measured rates of DMS production and consumption through various pathways (i.e. via DMSP and DMSO), and produced high-resolution maps of surface water concentrations and sea-air fluxes using ship-board membrane inlet mass spectrometry (MIMS). Our results show that the rates of DMS production and consumption were significantly higher near shore (0.47 ± 0.11 d-1) than offshore (0.19 ± 0.08 d-1) waters, and that DMSP cleavage (0.69±0.2 d-1) exceeds DMSO reduction (0.17±0.11 d-1) as a source of DMS production, suggesting that DMSP is the main proximate precursor for DMS. Beyond a strong coastal – oceanic gradient, we also observed significant DMS variability within both the coastal and oceanic regions, with concentrations ranging between (<1nM – >30nM) in both provinces. Our findings corroborate previous relationships between physical and biogeochemical parameters and DMS, suggesting relationships between phytoplankton biomass and DMS in coastal waters and between mixing layer depths and DMS offshore.

.

Pamela Barrett University of Washington Input of aerosol Fe to the North Atlantic Ocean: Decadal-scale trends
Understanding factors that partition CO2 between atmospheric and oceanic reservoirs is critical to understanding the fate of contemporary CO2 emissions as well as glacial-interglacial climate change. Carbon fixation by phytoplankton in the surface ocean transfers CO2 from the atmosphere to the deep ocean via the biological pump. It has been demonstrated that the availability of trace metals such as Fe and Zn limits the ability of phytoplankton to utilize macronutrients and thus controls their ability to fix CO2. Hence, it is important to investigate and quantify processes delivering biologically-important trace metals to the ocean. Deposition of atmospheric aerosols is a major source of trace metals to open-ocean surface waters. We participated in the CLIVAR/CO2 Repeat Hydrography Program 2003 occupation of line A16N in the North Atlantic and have generated large-scale, high-resolution datasets for aerosol, dissolved, and particulate trace metals. These distributions have been used to calculate residence times for dissolved and particulate phases of Fe, Al, and Mn in diverse oceanic regimes with varying dust inputs and biological activity. These tracers can then be used to estimate mineral dust inputs over different timescales. This dataset also indicates that anthropogenically-sourced aerosols, primarily emitted as products of fossil fuel combustion, are significant and correlated with increased delivery of labile (bioavailable) Fe, which could have large impacts on ocean productivity. We will also present results from the re-occupation of A16N in 2013 to assess how ocean trace metal distributions reflect both short-term variability and decadal trends in dust delivery to the surface ocean.

.

Zhixuan Feng Miami Impact of climate change, hydrology, and beach characteristics on microbial water quality throughout the Florida coast
Many subtropical beaches suffer from high enterococci levels, resulting in beach closures to comply with the Environmental Protection Agency (EPA) guidelines that recommend enterococci as fecal indicators. Changes in water temperature, rainfall, and waves, as well as occurrence of episodic events such as tropical storms, can substantially influence coastal water quality. We compiled and analyzed a 10-year record of weekly measurements of enterococci levels at 300 recreational beaches in Florida. The objectives were to examine spatial patterns of microbial dynamics and microbial responses to climatic and hydrological fluctuations in the intra- to inter-annual time scales. Results showed that ocean-facing beaches exceed the EPA threshold much less frequently than enclosed beaches, suggesting that local mixing is an important factor in determining beach water quality. In addition, we found opposite correlations in log-normalized seasonal mean enterococci levels and environmental variables (i.e., offshore wave height, water temperature, and precipitation) between Atlantic coast beaches versus Gulf coast beaches. We hypothesize that microbial variations within each of the Florida coasts are likely controlled by different mechanisms. Finally, when evaluating the impacts of major climatic events, patterns of sudden decreases in enterococci levels were observed following hurricane landfalls. These patterns suggest that hurricanes may clean and destroy the microbial communities in the beaches, which then take months to recover to pre-storm conditions.

.

Jordon Hemingway MIT/WHOI Typhoon-Mediated Organic Carbon Export in the Western Pacific – The Role of Steep Mountainous Rivers
Steep, mountainous rivers (SMRs) have been recognized as globally significant sources of carbon to the world’s oceans, and have been postulated to have strong control on atmospheric carbon feedbacks over many timescales. By exposing a range of carbon end-members (e.g. plant-derived, soil-derived, petrogenic) during various stages of the hydrograph, SMRs are shown to export a unique suite of carbon during large storm events. If such events correspond to an increase in plant-derived carbon export and eventual burial in marine sediments, SMRs could act as a negative feedback loop to increased intensity and frequency of typhoons in the future.

Here we explore the temporal evolution of exported particulate organic carbon (POC) by the LiWu River draining the east coast of Taiwan over the course of three typhoon events in 2008. Total suspended sediment concentration is shown to vary by over an order of magnitude between storm and non-storm periods, with a corresponding shift in carbon source. By using a suite of stable- and radioisotope measurements we are able to constrain the fraction of each carbon end-member exported. We build upon a historical discharge database and previous export measurements to estimate an annual flux of plant- and rock-derived carbon by the LiWu River and to describe the global significance of SMRs in response to changes in atmospheric CO2.

.

Malee Jinuntuya Old Dominion Impacts of rising CO2 on carbon and nitrogen metabolism of eelgrass, Zostera marina L.
Rising CO2 levels predicted under future climate scenarios will enhance photosynthesis and promote growth of seagrasses. Despite evidence for positive feedback on photosynthetic rates, the effects of higher CO2 on the assimilation of inorganic nitrogen (N) and N demand for growth in seagrass are still unclear. This study examined the direct influence of CO2 enrichment on the plant C and N budget in addition to the N uptake and assimilation pattern in eelgrass Zostera marina L. Experimental CO2 and nutrient enrichment caused increases in both %N (1 to 1.5) and %C (33 to 38), and a decrease of C:N in leaves. The sugar content in leaf tissues increased by ~60% (250 to 403 mol Sucrose gDW-1) after 24 h grown under CO2 replete conditions. Thus, the present study provides some evidence for the direct influence of long-term CO2 enrichment on changes in the metabolic responses of eelgrass. Increases in C assimilation from elevated [CO2] should increase the N demand, resulting in higher tissue N content. Resulting shifts in C and N content of leaves may have significant impacts on higher trophic levels that rely on eelgrass by altering the quality status of the food source.

.

Rebecca Saari MIT The US market for cool, clear skies: exploring key drivers of air quality impacts under climate change policy
A national climate or clean energy policy will significantly affect US air quality. Dozens of studies now suggest that air quality co-benefits alone may be large enough to “pay for” the costs of a climate or clean energy policy. However, most authors use a valuation approach that is not consistent with the economy that implements these policies. In this work, we explore what it means for these co-benefits to “pay for” these policies in terms of real economic resources. We compare the air quality impacts of two national climate policies: a Cap-and-Trade (CAT) program and a Clean Energy Standard (CES). We track the effect of each policy on pollutant emissions, concentrations, health impacts and welfare through a novel integrated assessment framework that couples a computable general equilibrium economic model to a regulatory air quality model. In this study, we extend that framework to capture the economy-wide impacts of air pollution in order to compare economy-wide policy costs to economy-wide air quality co-benefits for both polices. We ask what fraction of costs are offset by these policies, and how those net co-benefits are shared across regions and income groups in the US. We find, even using our conservative approach tracking market-based outcomes, that the air quality co-benefits do “pay for” the costs of the CAT on a national scale; however, this is due to large net gains in the eastern states. The CES reduces more pollution but at a higher policy cost, so that about 10% of its costs are offset by air quality gains. This work generally agrees with previous findings that air quality co-benefits alone can offset the costs of a climate policy, but this is the first comparison of economy-wide, resource constrained benefits to their comparable economy-wide costs. Further, it highlights regional disparities in the distribution of costs and co- benefits. Finally, this work explores only a single, incidental impact of climate change policy; future work must seek to quantify its effectiveness regarding its primary purpose – mitigating climate change.

.

Azusa Takeishi Yale On the Development of Deep Convective Clouds with Varying Aerosol Loading
One of the largest uncertainties in future climate projections lies in the climatic effects of aerosols. It has been shown that the cooling effect of aerosols could partially offset the current global warming induced by increased greenhouse gas concentration. Among the effects of aerosols, the interaction between aerosols and deep convective clouds is especially difficult to quantify, due to the complex interaction and limited field measurements available. Although the radiative effect of deep convective clouds on climate is small, they could affect the local, regional, and global climate by altering the large-scale circulations. Thus, it is of importance to understand how deep convection changes its development and evolution with aerosol loading.
This study aims to understand the effects of varying aerosol properties and number concentrations on deep convective clouds, using the Weather Research and Forecasting (WRF) model and WRF coupled with Chemistry (WRF-Chem) as a cloud resolving model. Simulations with two different microphysics schemes show quite different sensitivities to varying aerosol number concentration, particularly in the simulated amount of precipitation and updraft velocity. This highlights uncertainties in the complex microphysical interactions in convective clouds. In addition to this microphysical effect of aerosols, the radiative effect of aerosols and its sensitivity to aerosol composition will be examined by both WRF and WRF-Chem. This will enable us to analyze the effect of absorbing and scattering aerosols, as well as assess the total effects of aerosols on deep convective clouds.

.

Justin Bandoro MIT Influence of the Antarctic Ozone Hole on Seasonal Changes in Climate in the Southern Hemisphere
Antarctic surface climate has undergone pronounced changes in the past three decades that have been linked to stratospheric ozone depletion. In this study we use the strong seasonal character of the springtime ozone hole to identify linkages between ozone loss, the accompanying circulation changes, and changes in the seasonal cycle of Southern Hemisphere surface temperatures outside of the polar regions. At the surface, the seasonal amplitude (summer to winter temperature difference) has significantly decreased over southeast and south-central Australia and inland areas of southern Africa as a result of changes in the southern annular mode driven by the Antarctic ozone hole. Remarkably strong year-to-year correlations between the surface seasonal temperature amplitude and total column polar ozone of the preceding spring were found, with the most robust and strongest signals emerging in south-central and eastern Australia. With the pending recovery of the ozone hole over the next several decades, we can therefore expect an increasing frequency of extreme hot summers in parts of Australia and Africa, such as the Australian summer of 2012/13.

.

Adam Bowerman Texas The role of Pacific and Southern Hemisphere variability on the southwest extension of the North Atlantic Subtropical High
This study demonstrates the causal relationship between South Pacific midlatitude waves, anomalous Amazon convection, and southwestward extension of the North Atlantic Subtropical High (NASH) during boreal summer. Because variability of the southwestern ridge of the NASH has previously been associated with changes in the flow of moisture from the Gulf of Mexico into the central and southeastern U.S., determining the mechanisms that control the NASH are important for North American drought prediction and future climate projections. Our proposed pathway begins in the South Pacific with an eastward propagating midlatitude wave train, such as that produced by the Pacific South American pattern. As the waves approach South America, high latitude South Pacific blocking causes a shift in the waveguide, deflecting the wave train into the tropical Amazon. As noted in previous studies, the subsequent frontal boundary and tropical cold incursion kicks off anomalous Amazon convection. This large scale convection overturns onto the Intra-Americas region, inducing subsidence and a southwestward extension of the NASH. Recent authors have highlighted the role of tropical Pacific variability, such as the El Nino – Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO), in the generation of Southern Hemisphere midlatitude waves. Our proposed teleconnection offers an intra-hemispheric pathway through which the Pacific Ocean can influence the hydrology of the central and southeastern U.S. during boreal summer when Northern Hemispheric wave activity is weakest.

.

Sihan Li Oregon State Superensemble of Regional Climate Model simulations for the western US using Climateprediction.net
For over a decade, a citizen science experiment called climateprediction.net organized by Oxford University has used computer time contributed by tens of thousands of volunteers around the world to create superensembles (many thousands) of global climate simulations.  A new climateprediction.net experiment, a partnership between Oregon State University, Oxford University, and the UK Meteorological Office brings these computing resources to look into regional climate modeling for the Western US and other regions.  For the western US, the spatial resolution of 25 km permits important topographical features — mountain ranges, valleys, and inland waters — to be resolved and to influence simulated climate, which consequently includes many important observed features of climate. Perturbations of initial conditions, land and sea surface boundary conditions, and physics parameter values provides estimates of uncertainty and a measure of the importance of model formulation to regional climate. Furthermore, the thousands of simulations (for 1960-2009 based on observed sea surface temperatures and, soon, 2030-2049) offer a new approach to envisioning future regional climate by way of the unique combination of spatial detail and statistical richness.By quantifying uncertainty more carefully as a function of initial conditions, boundary conditions, and model formulation, this superensemble of regional model results will provide an unprecedented amount of guidance in dealing with climate change on a regional level. In the process it will also advance the statistical analysis of large ensembles.

.

Elizabeth Maroon University of Washington The tropical precipitation response to idealized mountain ranges and ocean heat fluxes
In the zonal mean, there is greater precipitation in the northern hemisphere tropics. This study aims to understand the causes of this climate asymmetry by adding simple forcings that move precipitation in two aquaplanet climate models. The first model (GRaM) is a gray radiation atmosphere that has no water vapor or clouds feedbacks, but does include the dynamical impact of moisture. The second model (AM2) has more complex parameterizations for clouds, convection and radiation. Two types of forcings on the climate are employed: mountain ranges and idealized surface heat fluxes (q-fluxes). In both models, it is easier to produce a large precipitation response with a “real world” zonally-averaged q-flux than with a “real world” mountain range. For both types of forcings, the response of the climate in the gray model is much less than in AM2 because of the gray model’s lack of feedbacks. Mountain ranges affect the zonal structure of the precipitation response, with an especially strong response to their west. When examining q-flux simulations, the top-of-atmosphere radiation response in both models is similar in the extratropics. It is the overactive response in the tropics and subtropics of the AM2 simulations that distinguishes it from the gray model, despite the fact that the largest q-flux forcings originated in the extratropics, not the tropics.

.

Vince Agard MIT Diagnosis of physical contributors to CAPE buildup in severe thunderstorm environments
The feasibility of using simplified numerical models to diagnose the contributions of various physical processes to the climatological preconditioning of the atmosphere for severe convective storms is examined, focusing on climatological and transient buildup of convective available potential energy (CAPE). A minimal numerical model is used to examine the effects of gradients in surface properties in equilibrium levels of atmospheric CAPE. Experiments using a two-dimensional model in both 2-column and 40-column configurations demonstrate a link between discrete gradients of surface moisture and high levels of equilibrium CAPE. Furthermore, a CAPE tendency budget is constructed, and used to determine that gradients in boundary layer entropy are the leading order term responsible for generating CAPE maxima in the model. Additional studies are proposed to extend this framework to further examine the effects of climate change on severe local storms.

Back to top

Poster Session C

.

Christopher Kinsley MIT/WHOI Constraining rates of sea-level change at MIS5e using 230Thxs normalisation of Red Sea sediments
Current sea-level rise is estimated at 0.3cm/yr due to melting of the polar ice caps, thermal expansion of oceanic water masses, and exchange of water between oceans and terrestrial reservoirs, all attributed to anthropogenic climate forcing. A constraint on potential future rates of sea-level rise is important to inform governmental planning decisions related to sea level. The Red Sea is sensitive to sea-level change and has been used to build the most comprehensive relative sea-level curve for the past 500,000 years by using oxygen isotope records combined with a hydraulic model of the Red Sea’s water exchange with the world ocean as a proxy for sea level [Siddall  et al. 2003]. Previous work [Rohling et al. 2008] focussed on rates of sea-level rise during the last full interglacial (MIS-5e), where sea level is thought to have been at the same height as today and ice sheet configuration similar, using an age model for central Red Sea core GeoTü-KL11 constructed assuming a constant sedimentation rate leading to uncertainties in the robustness of the age model. This study utilises 230Thxs as a constant flux proxy to calculate sedimentation rates over MIS-5e on samples from Red Sea core GeoTü-KL11, which dropped below the average rate of 4cm/kyr to about 3±0.15cm/kyr over the interval of MIS-5e,  and thus construct a robust age model. This age model is then used with the sea-level proxy data to calculate sea-level rise above the base value of 0m (current sea height) during MIS-5e of 2.15±0.1cm/yr. The response of the climate system to increasing global temperatures caused by anthropogenic forcing is not well constrained, but if the system responds in a similar manner as in MIS-5e due to increased global temperatures, rates of sea-level rise of 2.15±0.1cm/yr in the future are plausible and could have very significant impacts in the near future.

.

Zachary Schuster Wisconsin Assessing Climate Change Impacts on Precipitation and Flood Damage in Wisconsin
Studies on the impacts of anthropogenic climate change have found that the magnitude and frequency of intense precipitation events are expected to increase over the next century for the Midwestern United States. The goal of our study was to use statistically-downscaled and de-biased precipitation projections for the state of Wisconsin derived from 14 General Circulation Models (GCMs) to assess the projected precipitation changes for the mid-21st century in a way that is relevant to water resource decision-making. We did this by analyzing metrics such as the 100-year, 24-hour quantile (event) that are frequently used in urban stormwater design.

The results of our design-metric analysis shows that the 100-year, 24-hour quantiles for Wisconsin are projected to have significant but modest increases of about 11% over the next 50 years. However, we also found a large amount of inter-model variability that may make the direct application of the climate model data difficult in practice.

In addition to our climate research, we also held workshops with public works directors, engineers from the Wisconsin Department of Transportation, and consulting engineers to present our research and get feedback on the strengths and weaknesses of incorporating our findings into design practice.

.

Naresh Neupane UT Austin Identification of a Congo Basin Walker Circulation and its Interaction with the West African Monsoon
The Gulf of Guinea (GoG) in the tropical Atlantic is characterized by the presence of strong subsidence. This subsidence, which suppresses convection over the GoG, appears in June and becomes well established from July–September. Since the GoG is where the monsoon flow originates, this subsidence is important for determining the moisture sources for the West African monsoon (WAM). Here we contribute to a physical understanding of what causes this subsidence, and how it relates to the WAM and precipitation distributions across West Africa. We use reanalysis products and climate model simulations.
We identify a Walker circulation between the Congo basin and the GoG from July-September in the reanalyses. Winds rise from the Congo basin, flow westward above 700 hPa, and subside over the GoG. The subsiding branch diverges at 2°W, and winds to the east of 2°W flow towards the Congo basin creating an complete overturning circulation. This circulation is identified in the ERA Interim, ERA40, NCEP1, and NCEP2 reanalyses. Two state-of-the-art regional model (WRF) simulations at 90km and 30km resolutions accurately represent the circulation. There are, however, discrepancies among the CMIP5 AOGCMs.
We hypothesize that when rising motion over the Congo basin is anomalously strong, the GOG subsidence, the monsoon flow, and precipitation across West Africa are also strong.This hypothesis is supported by most of the reanalyses, the WRF simulations, and the rainfall observation (CRU). Most of the AOGCMs, however, do not have a similar relationship.

.

Matthew Niznik Rutgers Circulation, moisture, and precipitation relationships along the South Pacific Convergence Zone in reanalyses and CMIP5 models
One theorized control on the position of the South Pacific Convergence Zone (SPCZ) is the amount of low-level inflow from the relatively dry southeastern Pacific basin. Building on the analysis of observed SPCZ-region synoptic scale variability by Lintner and Neelin (2008), composite analysis is performed here on two reanalysis products as well as output from 17 models in phase five of the Coupled Model Intercomparison Project (CMIP5). Using low-level zonal wind as a compositing index, it is shown that the CMIP5 ensemble mean, as well as many of the individual models, captures patterns of wind, specific humidity, and precipitation anomalies resembling those obtained for reanalysis fields between strong- and weak-inflow phases. Lead-lag analysis of both the reanalyses and models is used to develop a conceptual model for the formation of each composite phase. This analysis indicates that the strength and zonal extent of the subtropical jet north of New Zealand five days prior to an extremum of the composite index are at least partially responsible for driving these changes. Despite the presence of well-known biases in the CMIP5 simulations of SPCZ region climate, the models appear to have some fidelity in simulating synoptic scale relationships between low-level winds, moisture, and precipitation, consistent with observations and simple theoretical understanding of interactions of dry air inflow with deep convection.

.

Diana Thatcher University of Michigan Comparison of a Moist Idealized Test Case and Aquaplanet Simulations in an Atmospheric General Circulation Model
The vast array of dynamical and physical processes within atmospheric general circulation models (GCMs) makes it difficult to correctly isolate sources of model errors. Simplified test cases are important in testing the accuracy of individual model components, such as the fluid flow component in the dynamical core. Typically, dynamical cores are coupled to complex subgrid-scale physical parameterization packages, and nonlinear interactions between various components of the model mask the causes and effects of atmospheric phenomena. Idealized tests are a computationally efficient method for analyzing the underlying numerical techniques of dynamical cores. The proposed test case is based on the widely-used test for dry dynamical cores by Held and Suarez, which replaces the full physical parameterization package with temperature relaxation and damping of low-level winds on an idealized planet. The impact of moisture, a crucial physics-dynamics coupling process, is missing from this test.

Here we present an idealized test case of intermediate complexity with moisture feedbacks. It uses simplified physical processes to model large-scale condensation, boundary layer turbulence, and surface fluxes of horizontal momentum, latent heat, and sensible heat between the atmosphere and an ocean-covered planet. We apply this test to the Spectral Element (SE) dynamical core within the NSF/DoE Community Atmosphere Model (CAM) version 5.3 and compare the results to aqua-planet experiments with complex physical parameterizations. The moist idealized test case successfully reproduces many features of the general circulation seen in the aqua-planet simulations, such as the Hadley cell, precipitation patterns, and atmospheric waves.

.

Allison Wing MIT Physical Mechanisms Controlling Self-Organization of Convection in Idealized Numerical Modeling Simulations
Convective cloud clusters are responsible for much of the rainfall and cloudiness over the tropics, allowing them to modulate the radiative heating and cooling rates of the surface and atmosphere and influence the large-scale circulation and moisture distribution. Therefore, understanding how and why tropical convection organizes is important for understanding both tropical and global climate variability. In this study, the problem is approached through the context of idealized modeling of convective organization in radiative convective equilibrium using a cloud system resolving model. Previous studies have investigated interactions between the environment and the convection that allow convection to self-aggregate into a single cluster, and have found this self-aggregation to be dependent on a sea surface temperature threshold. In this study, the System for Atmospheric Modeling is used to perform 3-d cloud system resolving simulations with a doubly periodic horizontal domain, interactive radiation and surface fluxes, and no rotation or external forcing other than solar insolation. Simulations are run at fixed SST.

We quantify the magnitudes of the various feedbacks that control self-aggregation within the framework of the budget for the spatial variance of column frozen moist static energy. The absorption of shortwave radiation by atmospheric water vapor is found to be a key positive feedback in the early stages of aggregation, while the longwave radiation feedback term can be either positive or negative. The role of horizontal advection is also explored. In addition, we find a positive wind speed-surface flux feedback whose role is to counteract a negative feedback due to the effect of air-sea enthalpy disequilibrium on surface fluxes. Finally, we note that the self-aggregation processes begins as a dry patch that expands, eventually forcing all the convection into a single clump. Thus, when investigating the temperature dependence of self-aggregation we focus on processes that can amplify this initial dry patch. One approach is to insert a localized negative relative humidity anomaly into a simulation that does not aggregate on its own. The consequent decay or amplification of that anomaly is then analyzed, with the goal of determining both why self-aggregation occurs above a certain temperature and whether or not there is a lower temperature threshold below which it never occurs.

.

Ryan Zamora Texas A&M Extratropical Lapse Rates in Very Hot Climates
The interplay between convective processes and the stabilizing effects of large-scale systems remains debated, especially for warmer climates. We study sets of simulations of past and present climates in which carbon dioxide (CO2) concentrations vary over a wide range: from preindustrial-era levels of 280 ppm to an exceedingly high value of 8960 ppm. This allows us to assess the importance of convective processes relative to middle latitude thermal stratification and changes to the general circulation in progressively warmer climate states. As a tool to assess the stability of the atmosphere, we calculate a thermodynamic variable called saturation potential vorticity (P*), which has the property of being identically zero wherever lapse rates are neutral with respect to moist convection, and large where lapse rates are stable.

Tropical regions are neutral with respect to moist convection while higher latitudes most often have stable lapse rates, especially during the winter months. In the warmer climate simulations, the frequency of convectively neutral air masses increases in both middle latitude and polar regions, especially during the summer months. These simulations also show expansion of the Hadley Cell and shifting of middle latitude storm tracks. Using Maximum Potential Intensity (MPI) as a tool to assess the upper bound of hurricane strength, we show sustainment of intense tropical cyclones in regions they cannot in our present climate.

.

Evan Howard MIT/WHOI Primary production in salt marsh tidal creeks and the effect of nutrient enrichment
Salt marshes are widely distributed, highly productive, and likely represent an important sink for atmospheric carbon dioxide. These environments also mediate nutrient cycling between terrestrial sources and the coastal ocean, and have been proposed to act as buffers against coastal eutrophication. Nutrient loading increases BMA biomass and likely stimulates production rates, but the magnitude of nutrient effects on photosynthesis and respiration is not well constrained. We measure the effect of nutrient loading in fertilized and unamended salt marsh tidal creeks (part of the TIDE experiment at PIE-LTER, Massachusetts) by quantifying two metabolic fluxes: gross oxygen production, an indicator of total photosynthesis, and net oxygen production, which is related to the balance of autotrophic and heterotrophic processes. Metabolic rates for the creek communities are derived using in situ triple oxygen isotope ratios and noble gas based gas exchange estimates. We find that photosynthetic rates are significantly higher in the fertilized creek vs. the unamended control. Respiration rates respond even more strongly to the elevated nutrients, resulting in increased rates of heterotrophy in the fertilized creek versus the control. Enhanced respiration with fertilization may explain observed creek bank and marsh platform losses over the fertilization period, with important implications for salt marsh carbon storage and export with eutrophication.

.

Shirley Leung University of Washington A Zonally-Banded Phytoplankton Response to 21st Century Climate Change in the Southern Ocean across the IPCC AR5 Earth System Model Suite
The Southern Ocean plays a key role in the global climate system via its vast ability to store anthropogenic CO2. A key mechanism that drives the influx of CO2 here is the biological pump, in which phytoplankton take up CO2 and export it to the deep ocean for long-term storage as they sink after dying. Previous studies have predicted that this Southern Ocean biological pump will change dramatically in response to rising atmospheric CO2 concentrations, thereby potentially altering the entire global carbon cycle. It is therefore important to quantify predicted Southern Ocean phytoplankton productivity changes and understand the mechanisms responsible for them. Despite their importance, few studies have looked at these changes across fully-coupled global climate models. To address this gap in the literature, we study the response of Southern Ocean phytoplankton to 21st Century climate change across the CMIP5 earth system model suite using correlation and regression analysis. On average, the models predict a zonally-banded pattern of phytoplankton abundance and production changes. Consistent with classical theory and previous studies, iron supply and light availability (controlled by cloud cover, minimum yearly mixed layer depth during blooms, and sea ice) are the most important limiting factors in the subpolar and polar Southern Ocean, while nitrate is most important in the subtropical Southern Ocean. We find that shifts in these limiting variables drive changes in phytoplankton abundance and production on not only interannual, but also decadal and 100-year timescales: the timescales most relevant to 21st Century climate change.

.

Anto Kajajian Old Dominion Otolith chemistry allows the discrimination of the estuarine nurseries of summer flounder along the US east coast.
Chemical analysis of otoliths is a technique that uses specific signatures within the earbones of fishes to answer a plethora of biological and ecological questions. The summer flounder is an important species from the perspectives of both the east coast ecosystems and fisheries therein. Our broad-scale study aims to investigate its population structure and movement patterns around the estuarine nursery areas along the east coast. We tested the hypothesis of spatially distinct otolith chemistries in juvenile flounder by using LA-ICP-MS to analyze trace elements 7Li, 25Mg, 48Ca, 55Mn, 85Rb, 88Sr, 89Y, and 138Ba, and an IRMS for stable isotopes δ13C and δ18O. We processed left otoliths exclusively as we had previously shown that the left and right otoliths were not chemically equivalent. A MANOVA on the overall elemental signatures showed strong differences between the sampled areas, while univariate ANOVA’s showed specific differences in every one of the studied elements. To build classification functions distinguishing between areas, we used discriminant function analyses on all possible combinations of elements in conjunction with jackknife cross-validation. A five-variable model utilizing 7Li, 25Mg, 89Y, δ13C, and δ18O provides the highest classification accuracy of 93%. Therefore, otolith chemistry is a powerful tool to distinguish between flounder from different nurseries and is a means of ultimately quantifying the contributions of the different estuaries to the adult population. This approach would also allow scientists to monitor changes in fish distributions both spatially and temporally in ecological time, and tie those changes to the pervasive climatic patterns.

.

Kimberley Pyle Cardiff University Silicate, sea ice, sediments, and mixing: What controls barium cycling in the Southern Ocean?
Due to its associations with biological cycling, meltwater fluxes, and alkalinity, the biogeochemical cycle of barium (Ba) offers a unique insight into past and present oceanic conditions. Barium is currently utilised in various forms as a palaeproxy for components of organic and inorganic carbon storage, and as a conservative tracer in some polar regions of the modern ocean. However, many questions still remain about the nature of barium cycling in the ocean; the interactions between biological activity and the dissolved/particulate barium pools are unclear, and little is understood about the influence of coastal processes such as upwelling, sea ice, and glacial inputs.
By investigating the distribution of dissolved barium in the modern Southern Ocean the comparative influences of these biological and physical processes can be better established. Presented here is a new dataset of high precision dissolved barium measurements from the coast of the west Antarctic Peninsula. Combining these data with nutrient and oxygen isotope measurements from the Palmer Long Term Ecological Research Grid offers new insights into the major controls of the barium cycle in this climatically sensitive region. These insights can be utilised to improve applications of barium palaeoproxies in this region, thus increasing our understanding of past and present carbon storage in the Southern Ocean.

.

Kendra McKoy Rutgers Evaluating Paleo-sea level Proxies and Their Application to Statistical Ice Sheet Modeling
In a changing climate, understanding the spatial relationship between global ice sheet volume and global sea level is became increasingly important. Due to the complexity of this problem, one of those best methods of prediction is analyzing paleo-sea level from the Last Interglacial (LIG) stage (~125 kyr) when polar temperatures were ~3-5 °C warmer than today, making it an excellent analog. Research on Acropora palamata, a coral species well-known as a sea level proxy, has yielded many records but its poorly constrained indicative range leads to too broadly defined paleo-sea level estimates including, but not limited to the poor resolution it yields during periods of strong oscillations.
Kopp et al. (2009) employed a Bayesian statistical framework to couple a physical model of the sea level response to ice sheet melt with varied LIG sea level records across the globe, allowing the assessment of LIG global mean sea level and its associated uncertainty. Building on that approach, I seek to answer the questions:
• Does the colloquial use of uniform distributions for facies deposition ranges or ecological growth ranges (e.g. Acropora palamata grows strictly from 0-5 m below mean low tide level) represent reality?
• How biased are interpretations made with that assumption and will further refinement of sample quality and quantity establish a more robust model?

.

Sarah Rosengard MIT/WHOI Organic carbon export across the Great Calcite Belt: Probing the surface ocean ballast mechanism in calcareous communities
Observed correlations between particulate organic carbon (POC) and both calcite (PIC) and biogenic silica (BSi) in deep sediment traps have led to the hypothesis that mineral ballast, predominantly PIC, facilitates organic carbon export from the productive euphotic zone to the deep sea. However, evidence for the ballast hypothesis at the ocean surface is equivocal, suggesting that this mechanism is not uniform throughout the water column. The Great Calcite Belt, a calcite-rich band across the Southern Ocean resulting from massive coccolithophore blooms, is an ideal site to test for an upper ocean ballast effect. In January 2011 and February 2012, size-fractionated (<51 μm “suspended” and >51 μm “sinking”) POC, PIC and BSi samples were taken from the upper 1000m of the Atlantic and Indian sectors of the Belt. 234Th-238U deficit measurements were used to convert sinking POC and mineral measurements into export flux estimates. Correlations between the fluxes suggested a possible ballast association between BSi and POC, but not between PIC and POC, contrary to deep sediment-trap data. Comparisons between 234Th fluxes and relative calcification rates suggest that ecosystems with higher relative calcification activity export smaller and denser particles, which may sustain a high PIC flux, but not necessarily a high POC flux. Moreover, compound-specific biomarkers in “suspended” POC indicate that the reactivity of POC produced by different phytoplankton communities may be more influential to carbon export. Altogether, these data highlight the need for more vigorous mechanistic models of POC flux to resolve observed inconsistencies between ballast correlations at different depths.

.

Sally Wood University of Bristol Climate change and coral connectivity
Coral reefs, a vital cultural and economic resource, are under significant threat from the combined effects of direct anthropogenic stresses and climate change. Predicting reef responses to climate change requires an understanding of the factors and processes controlling reef biogeography on a global scale. Of these processes, dispersal of coral larvae by ocean currents and subsequent recruitment into a population, termed ‘connectivity’, may have particular significance to the future of reef ecosystems; affecting resilience, potential for adaptation and distribution shifts in response to changing environments. However, connectivity patterns are also predicted to change under future climate, via changes to adult populations, larval physiology and reef habitats.
I present a global model of coral larval dispersal developed to investigate the factors controlling connectivity on scales relevant to biogeography as well as the effects of climate change on patterns of connectivity, and discuss potential applications of the model in aiding predictions of future reef distributions

.

Andrew Parker Texas A&M Tropical North Atlantic temperature anomalies linked to AMOC variability across Dansgaard-Oscheger events
A common mechanism often proposed to explain the abrupt changes in climate that characterized Marine Isotope Stage 3 (MIS 3) invokes variability in the strength of the Atlantic Meridional Overturning Circulation (AMOC). Coupled ocean-atmosphere models demonstrate that AMOC variability is linked to abrupt change in the tropical North Atlantic (TNA) through both oceanic and atmospheric processes. During AMOC slowdown, surface cooling is countered by a subsurface warming as a result of rapid reorganizations in ocean circulation at intermediate depths. Recently, Schmidt et al. (2012) identified an abrupt subsurface warming at the onset of AMOC slow down during both Heinrich 1 and the Younger Dryas, suggesting this signal may be a robust feature of AMOC variability in the TNA. Here, we present new, high-resolution Mg/Ca and δ18O records from the near-surface dwelling planktonic foraminifera G. ruber and the lower thermocline dwelling foraminifera G. crassaformis across MIS 3 from southern Caribbean core VM12-107 in order to investigate whether AMOC variability is the driver of Dansgaard-Oscheger cycles. Initial results from G. ruber indicate abrupt millennial-scale temperature changes of 1.5-2oC during MIS 3. Our age model, based on 8 radiocarbon dates, indicates the onset of abrupt surface warming in the TNA coincides with the onset of stadial conditions in the North Atlantic. We also present a Mg/Ca record from G. crassaformis in the same core, reflecting lower thermocline temperature evolution through MIS 3. So far, our records support the hypothesis that millennial-scale AMOC variability during MIS 3 had a significant impact on TNA climate.

.

Craig Rye National Oceanography Centre Southampton Evidence of increased glacial melt in Antarctic coastal sea level rise
The Antarctic shelf seas are of great climatic importance due to their vigorous interactions with the atmosphere and cryosphere, which influence continental deglaciation, global sea level, and the production of dense bottom waters. However, our understanding of these interactions and their impacts is confounded by sea ice, which covers these regions for much of the year. Here, we use satellite measurements of sea surface height (SSH) during ice-free months to determine the evolution of regional sea level over the last two decades (1992-2011). We show that Antarctic coastal sea level has risen by ~1 mm yr-1 more rapidly than the global mean, peaking in the Ross Sea and eastern Indian sector. In situ ocean measurements and an ocean circulation model reveal that the steric expansion of the water column associated with upper-ocean freshening can account for the bulk of the signal, where this freshening arises largely from the observed mass loss from the Antarctic Ice Sheet. Our findings demonstrate the strength of the sea level response to the accelerating discharge from Antarctica over the last two decades, and expose a major climatic perturbation to the cryospheric forcing of the Southern Ocean.

.

Jianghao Wang University of Southern California Temperature reconstructions of the Common Era: Impact of methods and source data
Climate field reconstructions (CFRs) of the past millennium can provide insight into dynamical causes of low-frequency climate variability. However, large discrepancies among existing reconstructions [Solomon et al., 2007, Chap 6] preclude robust inference about past climates. Here we disentangle methodological and source data uncertainties with focused experiments.

First we examine the effects of different methodological choices. Starting with the network of Mann et al. [2008], we perform a 2000-year temperature reconstruction using four different CFR techniques: RegEM-TTLS [Schneider, 2001], the Mann et al. [2009] implementation of RegEM-TTLS, Canonical Correlation Analysis [Smerdon et al., 2010, CCA] and GraphEM [Guillot et al., in revision].

Next we explore the impacts of pre-processing of input source. We investigate the effects of: (1) screening for divergence [D’Arrigo et al., 2008] in tree ring series, (2) controlling for skewness in the source data via a power transform [Emile-Geay and Tingley, submitted] and (3) controlling for spurious feature selection by multiple correlation tests [Ventura et al., 2004]. In each of these cases, we perform reconstructions with the four CFR techniques, and compare our reconstructions with existing ones.

Preliminary results show that reconstructed patterns of temperature change are highly sensitive to procedural choices. Results are greatly method-dependent even with identical inputs. For instance, the reconstructed pattern of sea-surface temperature difference between the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA) is La Niña-like with RegEM-TTLS, El Niño-like with GraphEM, and neutral with CCA. The magnitude of the globally averaged MCA warmth is also greatly method-dependent.

.

Mike Byrne MIT Land-ocean contrasts in the response of the hydrological cycle and surface temperature to climate change
It is widely predicted that radiative forcing resulting from rising levels of greenhouse gases in the Earth’s atmosphere will lead to important changes in the hydrological cycle and surface
temperature distribution. Observations and model simulations indicate that this climate response is substantially different over land and ocean regions.

We examine land-ocean contrasts in the projected response of precipitation minus evaporation, and the near-surface temperature and humidity fields to climate change.
Using a combination of theory, idealized simulations, and CMIP5 model output, we investigate the physical mechanisms relating changes in temperature over land to changes
over ocean, and how these temperature changes impact the hydrological cycle. We find that near-surface temperature changes over land and ocean are strongly coupled to low-level
humidity changes, and outline a theory based on convective quasi-equilibrium to estimate the ratio of land to ocean warming as a function of relative humidity changes.

Changes in the hydrological cycle over ocean are closely tied to the local temperature changes via a simple thermodynamic scaling; the so-called “rich-get-richer” mechanism. Over land, however, where there is limited moisture availability, the behavior is more complex, with the thermodynamic scaling failing to capture important features of the precipitation minus evaporation response. We discuss prospects for developing a theory to estimate the response of the hydrological cycle to climate change over both land and ocean
regions.

.

Mary Moore Harvard Clarifying the Amount Effect
A stable water isotopologue-enabled cloud-resolving model was used to investigate the cause of the amount effect. Results indicate that, as convection becomes stronger, the δD of precipitation (δDp) depends on the isotopic composition of the converged vapor more than that of surface evaporation. Tests with disabled fractionation from rain evaporation demonstrate that this mechanism does not account for the amount effect as has been previously suggested. If the isotopic content of converged vapor is made uniform with height, the amount effect largely disappears, further supporting the dominance of converged vapor in changes to the δDp signal with increasing precipitation. δDp values were compared to the water budget term (P-E)/E, where P is precipitation and E is evaporation. Results from this comparison support the overall conclusion that moisture convergence is central in determining value of δDp and the strength of the amount effect.

.

Jacob Nienhuis MIT Effect of fluvial sediment supply fluctuations on wave dominated deltas
River deltas and individual delta lobes frequently face changes of fluvial sediment supply, either from climate change in the hinterland, autogenic deltaic processes or, more recently, due to human activities such as river damming. Using a process-based one-contour-line model of plan-view shoreline evolution, we investigate the adjustment of deltaic shorelines after change of fluvial input.
Model results suggest fluvial input elimination can result in four characteristic modes of wave reworking, ranging from diffusional smoothing of the delta (or delta lobe) to the development of recurved spits. Assuming constant marine conditions during growth and abandonment, we find that the pre-abandonment delta shape, along with the directional characteristics of the wave climate, determine the mode of reworking. Overall, development of alongshore-extending spits tends to occur on abandoned delta lobes with initially high ratios of offshore versus alongshore extent—i.e., sharply protruding spits tend to form when a delta is close to fluvial dominance before sediment supply is eliminated.
Regular variation in fluvial input, such as the time-periodic megadroughts that influence the Godavari delta in India, influences downdrift migrating sandwaves. Unstable downdrift shorelines have a highly non-linear response on input signals. The coupling of input signals with updrift deposition is much simpler. Regular updrift erosion creates convex beach ridges up to a distance that is determined by the riverine variation.
Simple analysis of delta shape and wave characteristics provides a framework for predicting the mode of delta reworking, providing insight into the potential evolution of active delta environments in case of future change of fluvial sediment input.

.

Jack Scheff University of Washington Does terrestrial water availability fundamentally depend on planetary temperature? An idealized modeling study
The geologic and paleobotanical record is usually said to show that during warm greenhouse climates of Earth’s past, moisture for plant growth was at least as plentiful as it is today in most land areas. Similarly, very cold glacial stages are usually interpreted as times of water scarcity, outside of a few small regions. Yet, for the warm greenhouse climates of Earth’s near future, comprehensive models largely project declines in time-mean soil moisture, and increases in physical drought and aridity measures.

This study attempts to understand these projections by using a simple, general, idealized land model coupled to a global atmosphere model (GFDL AM2.1) and a slab ocean. The model is run to equilibrium over a wide range of planetary temperatures, and the responses of land hydroclimates are examined. In particular, several different dimensionless water-scarcity metrics are tracked, including near-surface relative humidity, ratios between precipitation, actual and potential evapotranspiration, and the percent of maximum soil moisture.

These experiments are performed for a very wide range of land model parameters, atmospheric parameterizations, idealized continent geometries, and astronomical boundary conditions. If dimensionless water availability systematically declines with increasing planetary temperature regardless of these choices, then the model result might be viewed as robust. However, if certain modeling choices, metrics, and/or regions indicate the opposite behavior, in line with the geologic record, then the results could greatly aid in the interpretation of that record.

Back to top