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GFDL Past Events & Seminars - 2016

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Date Speaker Affiliation Title of Presentation
Jan. 7Larissa BackUniversity of Wisconsin-MadisonDiagnosis of gross moist stability and related quantities in observational data.
Jan. 13Michael PrevidiLamont Doherty Earth Observatory at Columbia UniversityThe Antarctic Atmospheric Energy Budget: Observation and CMIP5 Simulation
In this talk, I will present an evaluation of 23 CMIP5 coupled atmosphere-ocean general circulation models in terms of their ability to simulate the observed climatological mean energy budget of the Antarctic atmosphere. While the models are shown to capture the gross features of the energy budget well (e.g., the observed two-way balance between the top-of-atmosphere (TOA) net radiation and horizontal convergence of atmospheric energy transport), the simulated TOA absorbed shortwave (SW) radiation is too large during austral summer. In the multimodel mean, this excessive absorption reaches approximately 10 W m-2, with even larger biases (up to 25-30 W m-2) in individual models. Previous studies have identified similar climate model biases in the TOA net SW radiation at Southern Hemisphere midlatitudes and have attributed these biases to errors in the simulated cloud cover. Over the Antarctic, though, model cloud errors are of secondary importance, and biases in the simulated TOA net SW flux are instead driven mainly by biases in the clear-sky SW reflection. The latter are likely related in part to the models' underestimation of the observed annual minimum in Antarctic sea ice extent, thus underscoring the importance of sea ice in the Antarctic energy budget. At the surface, substantial differences in the climatological energy fluxes between existing observational datasets preclude any meaningful assessment of model skill in simulating these fluxes. Finally, using simulations from one of the CMIP5 models (CESM-WACCM), I will also discuss the energy budget response to stratospheric ozone depletion during the late twentieth century.
Jan. 14Elizabeth BarnesColorado State UniversityThe tug of war on the jet-stream: perspectives on the jet response to tropospheric warming in a hierarchy of models
Climate models project enhanced tropospheric warming over the 21st Century in two regions: the tropical upper-tropospheric and the Arctic near-surface. The effects of warming in these two regions of the troposphere are largely expected to oppose one another, suggesting that the net response of the jet-stream will be the difference of two large numbers. Here, we explore the influence of enhanced warming in these two regions in both the present-day and future climate, paying particular attention to the response of the eddy-driven jet-streams. We will demonstrate that although there is evidence that Arctic warming may modulate the future jet response, the net response will likely be dominated by tropical upper-tropospheric warming. Furthermore, there is little evidence to suggest that the influence of Arctic warming on the jet-stream can already be detected in the observations. Finally, we will discuss the seasonality of the future jet-stream response in the context of the seasonality of this tug-of-war, highlighting the importance of seasonality in eddy-mean flow dynamics.
Jan. 21Ed GerberNYUWhat drives the Brewer-Dobson Circulation and its response to Global Warming?
The Brewer-Dobson Circulation describes the slow overturning circulation of the stratosphere, which transports mass up into the tropics and poleward until it returns to the troposphere in the extratropics. In concert with chemical processes, it sets the distribution of stratospheric ozone and water vapor, which have significant impacts on surface climate. The circulation can be understood as a response to mechanical wave driving through the "downward control" principal. Planetary-scale Rossby waves and small-scale gravity waves are the primary drivers, but as the latter cannot be properly resolved in most models, their effect must be parameterized. Climate models almost uniformly project an increase in the Brewer-Dobson Circulation in response to anthropogenic forcing, but differ significantly in explaining how this change is effected, disagreeing whether resolve waves or parameterized waves drive the change. Given this uncertainty (and the fact that observations, if anything, hint at a recent weakening of the circulation), there has been justifiable concern about the model projections. In this talk, I'll first show that understanding the response to global warming may actually be simpler than the question of what drives the climatological circulation. The so-called increase of the Brewer-Dobson Circulation might better be described as an upward shift, associated with the general lifting of the entire atmospheric circulation in response to greenhouse gas forcing. We relate the change in the circulation at any given pressure level to the change in the tropopause height, which can in turn be linked to changes in surface temperature and tropospheric stability driven by greenhouse gases. Second, an idealized atmospheric model allows us to explore the interaction between resolved Rossby waves and parameterized gravity waves, and thus explain how comprehensive models have come to such different conclusions as to what drives the response to global warming. These interactions lead us to suggest a new framework for interpreting downward control, and demand a more nuanced answer to the question: "What drives the Brewer-Dobson Circulation?
Jan. 27GFDL's Winter 2016 Poster Expo GFDL's Winter 2016 Poster Expo
GFDL's Winter 2016 Poster Expo
Jan. 28Paul EdwardsUniversity of MichiganKnowledge Infrastructures in the Age of Hypertransparency: the case of climate science
Abstract: Scientific knowledge infrastructures are shifting in the wake of Internet-related changes in the media landscape. Ideas and ideologies of transparency and the free circulation of information have led to increasing exposure not only of publications (open access), but also of materials (open data) and methods (open code). Today, a broad-based movement for "reproducible science"calls for new standards of openness throughout the sciences. Taking climate science as a case study, this talk places these changes in historical perspective. Constantly evolving, poorly standardized data practices created an ongoing need to re-examine and reanalyze climate data through much of the 20th century. Climate data analysis became a major focus of controversy in the 2000s, in such episodes as the "Climategate"affair of 2009-10. Recent clashes between NOAA and Senator Lamar Smith involving access to privileged internal communications are another case in point. Changing knowledge infrastructures presage possible futures that may either democratize and improve expert knowledge, or render it impotent in public policymaking — or both. Short bio: Paul N. Edwards is Professor of Information and History at the University of Michigan. He writes and teaches about the history, politics, and culture of information infrastructures. Edwards is the author of A Vast Machine: Computer Models, Climate Data, and the Politics of Global Warming (MIT Press, 2010) and The Closed World: Computers and the Politics of Discourse in Cold War America (MIT Press, 1996), and co-editor of Changing the Atmosphere: Expert Knowledge and Environmental Governance (MIT Press, 2001), as well as numerous articles.
Jan. 28Brown Bag Lunchtime SeminarBrown Bag Lunchtime Seminar
The physical basis for decadal climate predictions and the GFDL decadal climate prediction system Date: Thursday, January 28, 2016 at 12:00pm EST Speaker: Thomas L. Delworth, PhD, NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, NJ Abstract: There have been remarkable changes in the climate system over the last few decades, ranging from a rapid reduction in Arctic sea ice to changes in Atlantic hurricane actvity to prolonged drought in the western U.S. These changes are a combination of the response of the climate system to anthropogenic forcing and natural climate variability. There is tremendous societal interest in understanding the nature of such decadal scale climate changes, and the extent to which they can be predicted in advance. In this talk we present the physical basis for making predictions of how the climate system will change over the next decade due to both natural variability and anthropogenic radiative forcing changes. We summarize what parts of the climate system may be predictable on such timescales, and the physical processes that create such predictability. We then describe a prototype decadal prediction system that has been developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL), and present results from experimental decadal predictions that have been performed over the last several years. About the Speaker: Dr. Delworth is a Research Scientist at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) and member of the GFDL Science Board. He is also on the faculty of the Atmospheric and Oceanic Sciences Program at Princeton University.
Feb. 3Angel AdamesUniversity of WashingtonThe Madden-Julian Oscillation as a convectively coupled moisture wave
The Madden-Julian Oscillation (MJO) is the dominant mode of tropical intreaseasonal (20-90 day timescale) variability. In spite of decades of research, the mechanisms that lead to the eastward propagation of the MJO have remained elusive. Recently, a growing body of evidence suggests that tropospheric water vapor governs the maintenance and propagation of the MJO, which has led researchers to describe the MJO as a "moisture mode". In this study we expand upon the moisture mode theoretical framework by extending upon a linear wave theory, previously developed by Sobel and Maloney, in which column moisture is the only prognostic variable and the horizontal wind is diagnosed as the forced Kelvin and Rossby wave responses to an equatorial heat source/sink. A dispersion relation is derived that describes the MJO's signal in the wavenumber-frequency spectrum and defines the MJO as a dispersive equatorial moist wave with a westward group velocity. This dispersion is the result of the anomalous winds in the wave modulating the mean distribution of moisture such that the moisture anomaly propagates eastward while wave energy propagates westward. The moist wave grows through feedbacks involving moisture, clouds and radiation, and is damped by the advection of moisture associated with the Rossby wave. Additionally, a zonal wavenumber dependence is found in cloud-radiation feedbacks which causes growth to be strongest at planetary scales. Our results suggest that this wavenumber dependence arises from the non-local nature of cloud-radiation feedbacks, that is, anomalous convection spreads upper-level clouds and reduces radiative cooling over an extensive area surrounding the anomalous precipitation.
Feb. 10Jonathan GregoryUniversity of Reading, UKThe inconstancy of transient climate response
Thanks to the large number of contributing models and the experimental design, CMIP5 analyses have shown that various parameters in the forcing-feedback paradigm of global climate change are not truly constant. The transient climate response parameter (TCRP, global warming per unit increase in radiative forcing) increases under the scenario of CO2 increasing at 1% per year, because both the the ocean heat uptake efficiency and the climate feedback parameter decline. The former is due to the penetration of heat to greater depths in the ocean, essentially similar to the "cold start" effect. The latter could be due to a dependence on CO2 concentration, or to CO2 forcing increasing more rapidly than logarithmically with CO2 concentration. The climate feedback parameter declines (effective climate sensitivity increases) under constant 4xCO2, because of the evolving patterns of SST change, especially in the tropical Pacific; this is a small effect for the 1% CO2 scenario, but important for estimating equilibrium climate sensitivity. The climate feedback parameter in response to historical SST variations is larger (effective climate sensitivity smaller) than in response to idealised CO2 increase, and is not constant during the 20th century. This could be due to unforced variability or to varying sensitivity to historical forcing agents. The forcing-feedback paradigm is still a very useful interpretative picture, but more detailed investigation is needed of the physical processes responsible for non-linearities.
Feb. 11Jeff ArnoldU.S. Army Corps of EngineersBridging the Gap Between Climate Science and Water-resource Applications
The U.S. Army Corps of Engineers (USACE) has had primary responsibility for multi-purpose water resource operations on most major river systems in the states and territories of the U.S. for more than 200 years. In that time, USACE operations have largely proved robust against most of the range of natural climate variability they encounter. Now, though, for some variables in many watersheds climate change is known to be shifting the hydroclimatic baseline around which that natural variability occurs and changing the range of that variability as well. This makes historical stationarity an inappropriate basis for assessing continued water-resource operations under climate-changed futures [Milley et al., 2008]. Projections of possible future, specific, climate-change threats and associated impacts to regional-scale inland hydrology are still uncertain enough, however, to require explicit guidance on their interpretation and use. Revealing, reducing, and representing those uncertainties are essential steps for generating the plausible climate change narratives required to inform water-resource decision-making for climate- changed futures. And to be useful, such quantitative narratives, or storylines, of climate change threats and hydrologic impacts must sample from the full range of possible futures with their uncertainties across all parts of the modeling chain, from GCM results including simulations of natural climate variability, through regional climate downscaling by multiple techniques, and on to modeling affected hydrologic processes and water resources impacts. This talk will present part of the work underway now to reveal and reduce some important uncertainties for future generation of quantitative hydroclimatic storylines. Topics will include: 1- structural and parameter limitations of some methods widely used to quantify climate impacts to hydrologic processes [Gutmann et al., 2014; Newman et al,. 2015]; 2- new, spatially consistent, U.S. national-scale climate downscaling and hydrologic simulation capabilities directly relevant at the multiple scales of water- resource decision-making [Newman et al., 2015; Mizukami et al., 2015; Gutmann et al., 2016]; and 3-development and evaluation of advanced streamflow forecasting methods with tractable uncertainty estimates [Wood et al., 2014; Wood et al., 2015]. A key focus will be areas where climatologic or hydrologic science is under-developed for informing adaptation decisions - or is perhaps wrongly scaled or otherwise misapplied in practice - indicating the need for additional fundamental science and its interpretation for applications.
Feb. 16Marine Ecosystem Tipping Points meetingMarine Ecosystem Tipping Points meeting
Marine Ecosystem Tipping Points meeting
Feb. 17James SmithStrange Floods: The Upper Tail of Flood Peaks in the Conterminous US
We examine the nature of the upper tail of flood peaks in the US, principally through close analyses of two flood events. The 1927 flood in the Lower Mississippi River produced the largest flood peak discharge in the US Geological Survey (USGS) stream gaging record and was the most destructive flood in American history. The 14 June 1903 flood which devastated Heppner, Oregon was arguably the strangest flood in the conventional stream gaging record of the USGS. We reconstruct the storms and storm environments for the 1927 and 1903 flood events through downscaling simulations using the Weather Research and Forecasting model. We place the 1927 Mississippi River flood and 1903 Heppner flood in a hydroclimatological context through analyses of 20th Century Reanalysis fields and through intercomparisons with the largest flood events in the USGS stream gaging record. Analyses of the largest observed floods in the US point to fundamental difficulties in characterizing the upper tail properties of floods that are envisioned as key to development of societally acceptable design standards for flood hazards. We also point to advances in climate modeling that are needed to assess hazards from extreme floods in both current and future climates.
Feb. 18Colette HealdMITExploring Air Quality in a Changing World
Tropospheric gases and particles impact human and environmental health, visibility, and climate. While tremendous strides have been made to clean up the air in recent decades, poor air quality continues to plague many regions of the world. Uncertainty on the impacts of short-lived atmospheric constituents also dominates uncertainty in climate forcing. And critically our understanding of how global change will feedback on to atmospheric chemistry and surface air quality is limited. In this talk, I will highlight some of the connections between global change and air quality. In particular, I plan to discuss (1) how land use change plays an important, and often neglected, role in controlling air quality, (2) how meteorology drives air pollution and how accurately this is represented in global models, and (3) the uncertainties and importance of brown carbon.
Feb. 24DDT TrainingDDT Training
DDT Training
Feb. 25Dennis LettenmaierUCLAThe changing hydrology of the western U.S.
California is entering the fourth year of a severe drought, which in some respects (certainly in terms of mountain snowpack) is unprecedented in the instrumental record. The drought has mostly been characterized by dryness during the critical winter months, leading to anomalously low spring snowpacks and spring and summer runoff. However, winter 2013-14, and even more so 2014-15 were exceptionally warm - over much of the state, winter 2014-15 was the warmest of record. Furthermore, the exceptional warmth covered most of the Pacific Coast states of Washington and Oregon as well as California in winter 2014-15, and even though winter precipitation over the northernmost part of this region was near normal, over 80 percent of long-term snow courses in these three states had record low readings on April 1. These readings were lower (and in many cases, much lower) than 1977, the previous low of record for most of these stations. The Colorado River basin is also experiencing a continuing drought, which while somewhat less severe than California's has been more persistent and has now continued for more than a decade. An interesting aspect of the Colorado River drought is that streamflow declines, by some measures, have been more severe than predicted by hydrological models. I report on evaluations of long-term (~100-year) trends in winter temperature in California, as well as Washington and Oregon and the Colorado basin, and show that there are substantial differences (approaching factor of 2 over California) in the magnitude of the trend from various gridded data sets. Furthermore, while all data sets agree for California that the winter averages of daily temperature minima have increased over the last 100 years, even the sign of changes in daily temperature maxima are inconsistent across data sets. I report on work that has used the Variable Infiltration Capacity (VIC) model to reconstruct long-term snow water storage over California with and without the estimated trend from the various data sets, and from these results infer the contribution of long-term warming to the winter 2013-14 and 2014-15 snow water storage anomalies.
Mar. 2Gabriel ChiodoColumbia UniversityUnderstanding the effect of solar irradiance changes on climate
An accurate assessment of the impact of solar forcing on climate is a key step towards a proper quantification of natural and anthropogenic climate change. One of the best documented modes of variability of solar irradiance is the 11-year sunspot cycle (SC), over which total solar irradiance varies by 1 W/m**2. There is some observational and modeling evidence of SC signals in the climate system (see Gray et al., review paper 2010). One apparent SC signal in observations is a warming of 1 K in the tropical lower stratosphere (Frame and Gray, 2010). This signal appears to be key in the downward transfer of the SC signals from the stratosphere to the troposphere (Haigh et al., 2005). However, the weakness of the forcing, paired with strong intrinsic variability of the climate system and lack of sufficiently long observations hamper the separation of the SC signal from other sources of variability in the tropical stratosphere. By using transient simulations with the Whole Atmosphere Community Climate Model (WACCM), I will quantify the relative role of volcanic eruptions, ENSO, and the QBO in the quasi-decadal signal in the tropical stratosphere with regard to temperature and ozone commonly attributed to the 11 yr solar cycle. By doing so, I will provide answers to the long-standing debate on the nature of the observed SC signal in the tropical lower stratosphere, with implications for the interpretation of tropospheric SC signals in reanalysis data. Another outstanding issue is the evolution of future solar activity, with a descent into a solar minimum state as a plausible scenario. With aid of experiments from WACCM, I will determine whether a reduction in solar activity could modulate the regional-scale climate change patterns in the Northern Hemisphere, and provide insights into the sensitivity of climate change projections to the uncertainty in spectral irradiance forcing. Results show that a future reduction in visible irradiance (350-700 nm) drives changes in the stationary wave pattern of the North Pacific and sea-ice cover. A decrease in UV irradiance (200-350 nm) leads to smaller surface signals, although its regional effects are not negligible. These results point to a distinct but additive role of UV and visible irradiance in the Earth's climate, and stress the need to account for spectrally-resolved solar forcing as a source of uncertainty in future projections. One final issue that I will explore in my talk concerns the role of the stratospheric ozone chemistry. Owing to its large computational cost, the bulk of modeling studies to date have been performed without interactive stratospheric photochemistry: the impact of this simplification on the modeled climate system response to solar forcing remains largely unknown. I will quantify this impact, by comparing the response of two model configurations to an idealized irradiance increase, with and without interactive ozone chemistry. I will show that neglecting stratospheric photochemistry leads to a sizable overestimate of the surface response to changes in solar irradiance. I will discuss the implications for the simulations of the climate in the Last Millennium, as well as the future directions in sun-climate research.
Mar. 3Adam ScaifeUK MET Office/Exeter, UK Tropical Rainfall, Rossby Waves and Regional Winter Climate Predictions
Skilful climate predictions of the winter North Atlantic Oscillation and Arctic Oscillation out to a few months ahead have recently been demonstrated, but the source of this predictability remains largely unknown. Here we investigate the role of the tropics in this predictability. We show high levels of skill in tropical rainfall predictions, particularly over the Pacific but also the Indian and Atlantic Ocean basins. Rainfall fluctuations in these regions drive clear signatures in tropical and extratropical atmospheric circulation that are approximately symmetric about the equator in boreal winter. We show how these patterns can be explained as steady poleward propagating linear Rossby waves emanating from just a few key source regions. These wave source "hotspots"become more or less active as tropical rainfall varies from winter to winter but they do not change position as they are anchored to regions of strong vorticity gradient associated with the climatological jets. Finally, we show that predicted tropical rainfall explains a significant fraction of the predicted year to year variation of the winter North Atlantic Oscillation.
Mar. 7Anja WestemayerMunich ReStatistical modelling of thunderstorms in the present and future climate
A statistical model was developed for the occurrence of electrified convection across Central Europe, based on ERA-Interim reanalysis data and lightning detection data from the European Cooperation for Lightning Detection. The model was developed by fitting an additive logistic regression to multiple selected physical parameters, whose individual relation to lightning occurrence was studied a priori. In doing so, it was found that lightning occurrence is strongly dependent on mid-tropospheric humidity. A re-application of the model to ERA-Interim reanalysis data shows that it can reproduce the annual cycle of thunderstorms in northern and southern half of the Central European domain rather accurately. Applying the model to the entire ERA-Interim dataset, which starts in 1979, reveals long-term changes in thunderstorm occurrence and variability. Subsequently, the model was applied to EuroCORDEX regional climate simulations to create predictions of changes in thunderstorm probability until 2100 according to the rcp 4.5 and rcp 8.5 climate scenarios. These changes and the processes responsible causing them are studied by an evaluation of the changes of individual parameters included in the model.
Mar. 9Aditi SheshadriColumbia UniversityStratospheric variability and tropospheric responses
Variability of the polar stratospheric vortex impacts weather and climate patterns at the Earth's surface on timescales from weeks to decades. In this talk, I will discuss polar vortex variability on seasonal and decadal timescales, as well as the dynamics of tropospheric responses to forcing from the stratosphere (and, indeed, to external forcing in general, such as rising concentrations of greenhouse gases). I will introduce an idealized, dynamically comprehensive atmospheric GCM that captures the observed hemispheric differences in stratospheric seasonal variability, and use this model setup to study the dynamics of tropospheric responses to stratospheric ozone depletion. I will discuss the implications of interactive chemistry in capturing these responses correctly, and demonstrate that it is not always accurate to describe the atmosphere's responses to external forcing by a single "annular mode". This will lead into a discussion of the application of Principal Oscillation Pattern analysis to extratropical winds, results from which suggest the existence of coupled, propagating annular modes, describing systematic latitudinal migrations of the mid-latitude jet.
Mar. 10Tom RyersonESRL/CSDField measurements and analyses to address climate and air quality issues
Usefully constraining regional- and global-scale chemical simulations is complicated by the difficulty of assessing large-scale model performance using in-situ observations that are typically made over limited spatial and temporal scales. Field research and analysis in the NOAA Chemical Sciences Division is designed to connect these very disparate scales by providing process-level diagnostics to not only benchmark model performance, but to also suggest specific improvements to model inventories and chemical process parameterizations. I will give examples of field data and analyses intended to improve the simulation of climate- and air quality-relevant issues in large-scale models, and that illustrate the potential for beneficial GFDL-CSD collaborations in the near future.
Mar. 16Max PoppGFDLA study of cloud radiative effects with strong radiative forcing
Water-rich planets such as Earth are expected to become eventually uninhabitable, because liquid water turns unstable at the surface as temperatures increase with solar luminosity. Therefore, an active subfield of research in planetary science studies climate impacts of large radiative forcings and their influence on habitability. One of the largest uncertainties in the evolution of Earth and similar planets under the brightening sun is the behavior of clouds. Here I present results of the first study focused on cloud-mechanisms in a hot climate. Simulations with both a one-dimensional column model and a modified version of the atmospheric general circulation model ECHAM6 suggest that clouds have a large impact on the habitability of Earth-like planets. Positive cloud feedbacks destabilize the climate as it warms beyond 300 K. The resulting climate transition leads to a new steady state with global-mean surface temperatures above 330 K. In this hot climate the sign of the cloud feedbacks changes and cloud feedbacks become strongly negative. The changes in cloud feedback are mostly a consequence of the weakening large-scale circulation and of changes in the shortwave relative to the longwave cloud-radiative effect with increasing surface temperatures. These changes eventually provide a stabilizing effect against a Runaway warming of the climate. However, in the hot climate the upper atmosphere is sufficiently moist to allow for a substantial loss of water to space. In a matter of hundreds of millions to a few billions of years this would lead to the loss of the habitability of a planet in this state. Furthermore, since the climate transition is caused by cloud-radiative effects, CO2-induced forcing can as easily lead to a climate transition to the hot climate as solar forcing and thus poses an equal threat to the habitability of an Earth-like planet.
Mar. 17Mike EkNCEP/EMCLocal Land-Atmosphere Interactions
Mar. 21Computer User Support TeamComputer User Support Team
Computer User Support Team
Mar. 23Robert Tuleya, Morris Bender, Thomas KnutsonImpact of upper Tropospheric temperature anomalies and vertical wind shear on tropical cyclone evolution using an idealized version of the operational GFDL Hurricane Model
The GFDL hurricane modelling system, initiated in the 1970s, has progressed from a research tool to an operational system over four decades. This system is still in use today in research and operations, and its evolution is briefly described. This study used an idealized version of the 2014 GFDL model to test its sensitivity across a wide range of three environmental factors that are often identified as key factors in tropical cyclone (TC) evolution: SST, atmospheric stability (upper air thermal anomalies), and vertical wind shear (westerly through easterly). A wide range of minimum central pressure intensities resulted (905 to 980hPa). The results confirm that a scenario (e.g., global warming) in which the upper troposphere warms relative to the surface will have less TC intensification than one with a uniform warming with height. For example, a warming of 2oC aloft, and increase of 2oC SST will lead to an decrease of ~10hPa in SLP; with no relative warming aloft, the decrease would be ~20hPa. TC rainfall is also investigated for the SST-stability parameter space Rainfall also increases for combinations of SST increase and increasing stability similar to global warming scenarios, and consistent with climate change TC downscaling studies with the GFDL model. The GFDL 2014 forecast system's sensitivity to vertical shear was also investigated. The idealized model simulations showed weak disturbances dissipating under strong easterly and westerly shear of 10 m s-1. A small tendency for greater intensity under easterly sheared versus westerly sheared environments was found at lower values of SST. The impact of vertical shear on intensity was different when a strong vortex was used in the simulations. In this case none of the initial disturbances weakened, and most intensified to some extent.
Mar. 30Carolina DufourPrinceton AOSOn the role of ocean mesoscale and fine-scale topography in the meridional circulation of the Southern Ocean
The Southern Ocean plays a crucial role in the climate system by taking up and redistributing heat and carbon at the global scale. Because the Southern Ocean is one of the most under-sampled regions of the global ocean, questions still remain regarding the meridional circulation and its response to climate change. Two important players in the Southern Ocean circulation are the transport effected by transient mesoscale eddies and the constraint exerted by topography on the mean flow. Transient eddies, which are ubiquitous features of the order of 10 to 100 km, oppose the wind-driven circulation and transport physical and biogeochemical tracers. Topographical steering of the mean flow gives rise to standing meanders, that augment transient eddy activity, and plays an important role in the overflow of dense waters over continental shelves. However, the effects of mesoscale transient eddies and fine-scale topographic obstacles on the circulation are challenging to observe and model. In this seminar, we will investigate the role of transient mesoscale eddies and fine-scale topography in the Southern Ocean meridional circulation using a suite of GFDL climate models at eddying resolutions. We will first examine the role played by transient mesoscale eddies in transporting heat and biogeochemical tracers across the intense circumpolar fronts of the Southern Ocean. We will then explore the contribution from transient mesoscale eddies and fine-scale topographic features to the vertical stratification of the Weddell Sea which controls the formation of the large Weddell Sea polynya. In both cases, transient mesoscale eddies are found to play an important role in the meridional circulation and the transport of climate relevant tracers, while fine-scale topographic features are found to provide strong geographical constraints on, and control the intensity of, the mean flow. Our results demonstrate the importance of accounting for the non-zonality of the Southern Ocean circulation, and doing so provide a new picture of the Southern Ocean circulation that departs from the traditional two-dimensional zonally-averaged view. In addition, our results highlight the challenges faced by the current generation of climate models that does not resolve mesoscale ocean processes and fine-scale topographic features.
Mar. 31Sandrine BonyUniversity Pierre et Marie CurieDoes convective aggregation matter for climate?
Tropical convection often organizes into convective cloud clusters, which are responsible for most of rainfall and cloudiness over the Tropics. This organization of convective systems is usually thought to be rooted in external in fluences such as large-scale surface temperature gradients, wind shear or rotation. However, simulations using cloud-resolving models run with high resolution under conditions of radiative-convective equilibrium have shown that convection can organize spontaneously even in the absence of external in fluences, a behavior called `self-aggregation'. These simulations further suggest that the aggregation of convection depends on mean surface temperature, and that it dramatically alters the large-scale state of the atmosphere, including its humidity and cloudiness. The question thus arises as to how much convective aggregation matters for climate variability and sensitivity, and whether climate models are able to represent this fundamental interaction between convection, circulation and eventually climate. I will show that the impact of convective aggregation on the large-scale state of the atmosphere is also evident in observational data, and that the phenomenon of self-aggregation also occurs in General Circulation Models. Physical mechanisms underlying the triggering of self-aggregation in such models will be explained. I will further show that convection tends to aggregate more as the climate warms, and that it is associated with a decrease of the anvil cloud amount and a narrowing of large-scale convergence zones. The physical mechanisms responsible for this behavior will be discussed, as well as their potential implications for large-scale circulations, cloud feedbacks and climate sensitivity.
Apr. 6Ben SulmanPrinceton AOSMicrobes, minerals, and roots: Next-generation soil carbon in the GFDL land model
Apr. 7Thierry PenduffCNRS, Laboratoire de Glaciologie et Geophysique de l'EnvironnementMulti-scale chaotic variability in the ocean: insights from a global ocean/sea-ice simulation ensemble
Idealized models have demonstrated the chaotic behavior of current systems at high Reynolds number, up to multi-decadal timescales. Unlike laminar ocean models used in most current climate projections, eddying OGCMs that will be used in future climate projections spontaneously generate a substantial intrinsic variability from eddy scales to multi-decadal/basin scales, with a chaotic character, and a marked signature on SSH and SST where air-sea fluxes are maximum in Nature. Whether and how this ocean­-driven low-frequency chaotic variability may ultimately impact biogeochemistry, the atmosphere and climate is an important but unsettled question. Before addressing this question in fully-coupled mode, it is important to explicitly simulate, characterize and quantify over a long period the stochastic character and specific scales of the low-frequency oceanic variability at high Reynolds number under full reanalyzed forcing, with a focus on climate­-relevant indexes. In the framework of the OCCIPUT ANR/PRACE project, we have performed and are analyzing a 50­-member ensemble of 1/4° global ocean/sea­-ice NEMO-based 1/4° hindcasts driven by the same reanalyzed 1958-2014 atmospheric forcing. After a common spinup, the spread of the ensemble is seeded by applying stochastic perturbations within each member for one year; eddy­-eddy interactions then take control of the subsequent growth of the ensemble spread and of its cascade toward long space and time scales. Along with reduced-size North Atlantic sensitivity experiments, this global ensemble simulation provides a probabilistic description of the global ocean/sea-ice state and evolution over the last 5 decades, and a measure of the actual constraint exerted by the atmosphere on the weekly-to-decadal ocean variability. We will present our strategy, describe the strong imprints of this atmospherically-forced stochastic variability on temperature, AMOC, SSH and water mass properties with a focus on interannual and longer time scales.
Apr. 14Robert HalbergGFDLNext Generation Earth System Prediction: Strategies for Subseasonal to Seasonal
This talk will present an overview of a recently released National Research Council report laying out research strategies for advancing subseasonal to seasonal forecasting over the next decade. Many aspects of this report will be of interest to the GFDL community. Hallberg is a member of the committee that prepared this report.
Apr. 20Desiree TommasiValue of Ocean Prediction for Fisheries Management
Fish populations are strongly influenced by climate variability. The inability of fisheries managers to anticipate such environment-driven fluctuations in fish productivity can lead to overfishing and stock collapses. Recent advances in dynamical global climate prediction systems allow for skillful sea surface temperature (SST) anomaly predictions at a seasonal scale over many shelf ecosystems. These coastal systems produce 80% of the annual global fish catch, raising prospects for improved utility of these tools for living marine resource problems. Here, the utility of SST predictions at this "fishery relevant"scale to improve fisheries management decisions is assessed using Pacific sardine as a case study. SST is a robust indicator of environmental factors influencing Pacific sardine productivity and past SST variability is presently incorporated in the harvest guideline for this stock. The value of SST anomaly predictions to management was quantified under four harvest guidelines differing in their level of integration of SST data and predictions. The harvest guideline that incorporated stock biomass forecasts informed by skillful SST predictions led to increases in stock biomass and yield, and reductions in the probability of yield and biomass falling below socioeconomic or ecologically acceptable levels. However, to mitigate the risk of collapse in the event of an erroneous forecast, it was important to combine such forecast-informed harvest controls with additional harvest restrictions at low biomass. A preliminary analysis of potential decadal SST predictability at this coastal ecosystem scale, and its relevance to fisheries management decisions, will also be discussed.
Apr. 21Rita ColwellUniversity of MarylandClimate, Oceans, and Human Health
Apr. 28Elsie SunderlandHarvard UniversityImpacts of global emissions and climate variability on marine mercury concentrations
Apr. 29Steven KleinLawrence Livermore National LaboratoryEmergent constraints for the climate prediction of clouds and precipitation
Emergent constraints are physically-explainable empirical relationships between characteristics of the current climate and long-term climate prediction that emerge in collections of climate model simulations. When combined with an observational estimate of the current climate characteristics, they seemingly offer the prospect of constraining long-term climate prediction. In this talk, I will review three constraints discovered by researchers in the LLNL-UCLA Cloud Feedbacks project involving tropical and extra-tropical low-level clouds and global mean precipitation. Because emergent constraints identify a source of model error that projects onto climate predictions, they deserve extra attention from those developing climate models and climate observations. While a systematic bias cannot be ruled out, the emergent constraints I review suggest larger cloud feedbacks and smaller precipitation sensitivity. Emergent constraints are physically-explainable empirical relationships between characteristics of the current climate and long-term climate prediction that emerge in collections of climate model simulations. When combined with an observational estimate of the current climate characteristics, they seemingly offer the prospect of constraining long-term climate prediction. In this talk, I will review three constraints discovered by researchers in the LLNL-UCLA Cloud Feedbacks project involving tropical and extra-tropical low-level clouds and global mean precipitation. Because emergent constraints identify a source of model error that projects onto climate predictions, they deserve extra attention from those developing climate models and climate observations. While a systematic bias cannot be ruled out, the emergent constraints I review suggest larger cloud feedbacks and smaller precipitation sensitivity.
May. 4Salvatore PascaleCaltechNorth American monsoon Gulf of California moisture surges: tropical and extratropical controls and the impact of horizontal resolution in a suite of coupled global climate models.
Far from being a steady circulation, the North American Monsoon (NAM) features subseasonal variability on different timescales. One important mode of variability is associated with transient disturbances traveling along the Gulf of California (GoC). These disturbances, named GoC moisture surges, appear most pronouncedly as periods of anomalous northward winds and moisture transport along the GoC followed by enhanced convective activity over the southwestern United States. Given the complex topographic features of the GoC region, modeling studies of the GoC surges have so far been performed with Regional Climate Models. Most GCMs (e.g., CMIP3, CMIP5) still feature an atmospheric grid spacing that is too coarse (larger than 100 km) to adequately resolve the GoC. In this talk I will discuss the impact of horizontal resolution on the climatology of GoC moisture surges in a suite of global circulation models (CM2.1, FLOR, CM2.5, CM2.6, HiFLOR) developed at the Geophysical Fluid Dynamics Laboratory (GFDL) featuring horizontal resolution up to 25 km in the atmosphere and up to 0.1 degree in the ocean. Increasing horizontal atmospheric resolution results in a drastic improvement in the model's capability of accurately simulating surge events. The climatological near-surface flow, moisture and precipitation anomalies associated with GoC surges are overall satisfactorily simulated in all higher-resolution models. The number of surge events agrees well with reanalyses but models tend to underestimate July-August surge-related precipitation and overestimate surge-related September rainfall in the southwestern United States. Large-scale controls supporting the development of GoC surges such as tropical easterly waves, tropical cyclones and trans-Pacific Rossby wave trains, are also well captured, although models tend to underestimate the tropical easterly wave/tropical cyclone magnitude or number. Near-surface GoC surge features and their large-scale forcings do not appear to be substantially affected by a finer representation of the GoC at higher ocean resolution.
May. 5Xuebin ZhangClimate Research Division, Environment Canada, Toronto, CanadaChanges in Extreme Precipitation: From Climate Research to Engineering Applications
Floods are among the leading natural disasters worldwide. Many engineering infrastructures, from sewers, street curbs and gutters to large water reservoir dams, are built to prevent and to control floods. The design of such infrastructures depends critically on the knowledge and information about extreme precipitation during the lifespan of the structures. In a typical engineering application, a design value of extreme precipitation is statistically inferred based on historical records and under the assumptions that the historical data adequately represent the past climate and that the climate did not change in the past and will not change during the lifespan of the structures. Human influence on the climate is clear: this manifests not only in the warming of planet Earth, but also in the changes in water cycle. An increase in extreme precipitation would result in reduced level of protection from floods by a structure than that it is designed for and consequently an increased risk of flood damage. Infrastructure adaptation requires the quantification of future changes in extreme precipitation at regional and local scales, posing a significant challenge to the climate research community. This presentation reviews some of the challenges in identifying past changes in extreme precipitation, in understanding the causes of those identified changes, as well as in projecting future changes.
May. 9Geeta Persad Final Public Oral ExamGeeta Persad Final Public Oral Exam
Geeta Persad Final Public Oral Exam
May. 11Kate MarvelNASA GISS at Columbia UniversityImplications for climate sensitivity from the response to individual forcings
Climate sensitivity to doubled CO2 is a widely-used metric of the large-scale response to external forcing. Climate models predict a wide range for two commonly used definitions: the transient climate response (TCR: the warming after 70 years of CO2 concentrations that rise at 1% per year), and the equilibrium climate sensitivity (ECS: the equilibrium temperature following an abrupt doubling of CO2 concentrations). Many observational datasets have been used to constrain these values, including temperature trends over the recent past, inferences from paleo-climate, and process-based constraints from the modern satellite era. However, different classes of observational constraints produce incongruent ranges. In this talk, I'll show that climate sensitivity can only be accurately derived from the historical period after accounting for the efficacy of each forcing. When we use single forcing experiments to estimate these efficacies and calculate climate sensitivity from the observed twentieth-century warming, our estimate of both TCR and ECS is revised upward compared to previous studies, improving the consistency with other independent constraints.
May. 12Danny FelthamUniversity of Reading, UKParameterising sub-grid scale sea ice physics for climate models: anisotropic rheology and melt ponds
Climate model representations of sea ice are incomplete, being limited by insufficient knowledge of sea ice physics and the level of complexity it is practical to represent. A consequence is that increasing the resolution of sea ice models does not, in itself, resolve new sea ice process: sub-grid scale parameterisations are required. The construction of suitable parameterisations presents challenges: salient aspects of the physics must be captured; fundamental principles such as conservation of mass and material frame indifference must be respected; and the model must be computationally tractable. The development of parameterisations involves judgment calls: it should not be expected that a parameterisation is the best possible description of the physics. In this talk, I outline briefly the case for improving sea ice physics in climate models, and, in particular, motivate and describe two recently developed sea ice parameterisations: for anisotropic rheology, and melt ponds. I will indicate why the parameterisations were built as they were, touching upon the issues mentioned above, and show some example simulations indicating the benefits that more realistic parameterisations bring to emergent sea ice properties in climate simulations and seasonal prediction.
May. 18Fernando Gonzalez TaboadaGFDLAssessing the predictability of marine net primary production using an empirical statistical model
Oceanic net primary production (NPP) accounts for roughly half of biological carbon fixation at the global scale, determining upper bounds for fisheries and export production. Assessing our ability to predict changes in NPP has thus major implications for the analysis of climate change impacts and for the management of living marine resources. Here, we used an empirical statistical approach to analyze NPP estimates from 18 years of remote sensing data and from simulations of a fully coupled, ocean-atmosphere Earth System Model (ESM). We used a reduced dimension, linear dynamical spatio-temporal model to project the evolution of NPP anomalies from the time decay and interactions among a reduced set of major NPP modes. This approach allowed us to assess potential limits to the predictability of NPP at the seasonal scale, and to assess the agreement in regional patterns of predictability based on remote sensing and ESM NPP estimates at large scales. Our results revealed that the high predictability of NPP in subtropical latitudes is a consequence of the decay of major modes at short time scales, with a prevalence of slow moving modes related to El Nino-Southern Oscillation and the Atlantic Multidecadal Oscillation. This decay dominated short term predictions in both remote sensing and ESM NPP, although the modes prevailed a longer time in ESM simulations. The dominance of short time persistence was gradually substituted by interactions between modes at longer lead times related to the propagation of major climate modes. The approach provides a novel set of diagnostics to assess the performance of ESMs, and it allowed us to identify potential regions where the prediction of NPP might lead to an improved management of living marine resources.
May. 19Simona BordoniCalifornia Institute of TechnologySeasonally migrating convergence zones on Earth and other planetary atmospheres
Idealized modeling studies have provided the basis for significant progress on our conceptual understanding of the fundamental dynamics of tropical circulations and associated convergence zones. Here, we study seasonally migrating convergence zones on an aquaplanet, i.e., over a uniform slab ocean of fixed depth, in a wide range of configurations and climates. In particular, we explore dynamical and thermodynamical mechanisms mediating the response of tropical precipitation to changes in surface heat capacity, planetary rotation rate and atmospheric long wave optical depth. Implications for observed monsoons on Earth and seasonal weather patterns on terrestrial planets are also discussed.
May. 24Ray YamataCourant Institute, NYUMoisture and wave-mean flow interactions in the general circulation of Earth's atmosphere
May. 24Dry Deposition MeetingDry Deposition Meeting
Dry Deposition Meeting
May. 24Dry Deposition MeetingDry Deposition Meeting
Dry Deposition Meeting
May. 25Hiroyuki MurakamiGFDLDominant Role of Subtropical Pacific Warming on the Extreme 2015 Eastern Pacific Hurricane Season
The 2015 hurricane season in the Eastern and Central Pacific Oceans (EPO and CPO), particularly around Hawaii, had been extremely active - including historical record of tropical cyclone (TC) number and the first instance of three simultaneous Category 4 hurricanes in the EPO and CPO. A strong El Niño had developed during the 2015 boreal summer season, and it has been anticipated the cause of extreme number of TCs. However, using a suite of targeted high-resolution model experiments, the extreme 2015 EPO and CPO hurricane season was not primarily induced by the 2015 El Niño's tropical Pacific warming, but by warming in the subtropical Pacific Ocean, so called a "warm blob". This warming is not typical of El Niño, but is associated with a different mode of climate variability, the "Pacific Meridional Mode (PMM)", superimposed on long-term anthropogenic warming. The state-of-the-art model projects an increase in the frequency of extremely active TC year like 2015 in the future in EPO, CPO, and the Hawaiian domain due to anthropogenic warming in the future. However, the probability of occurrence of the extreme year depends on a phase of natural variability. The long-term warming of the subtropical Pacific Ocean is key to projections for increased risk of such active tropical-cyclone years in the EPO, CPO, and Hawaiian Islands over the next few decades.
May. 26John HigginsPrinceton UniversityExtending the ice core record of atmospheric composition and the global carbon cycle beyond 1 million years
Ice cores serve as a critical archive of past environmental conditions, providing constraints on global atmospheric composition and the climate of polar regions. Reconstructions of atmospheric CO2 and CH4 from air trapped in ice cores dating as far back as 800 ka indicate a link between greenhouse gases and global climate in the form of 100 kyr glacial cycles. These climate cycles are recorded in proxy records from deep-sea sediments reflecting variations in ocean temperature and continental ice volume. Deep-sea records indicate that the 100 kyr glacial cycle developed only ~900,000 years ago (the Mid-Pleistocene transition or MPT). Prior to this, and going back to 2.8 Ma, glacial cycles lasted on average 40 kyr. The origins of both the 100 kyr and 40 kyr glacial cycles, their links to orbital forcing, and changes in atmospheric greenhouse gases, are hotly debated. In this talk I will present the first direct ‘snapshots' of atmospheric composition during the MPT from an ice core drilled at Site BIT-58 in the Allan Hills blue ice area and discuss implications for the MPT. I will also present a new record of atmospheric O2/N2 over the last 800,000 years from ice cores and show how this record provides novel insights into the processes that control atmospheric CO2 and O2 on geologic timescales.
May. 27Michael DunphyIFREMER, Plouzane, FranceInvestigating the Impacts of Mesoscale Circulation on Low-Mode Internal Tides
May. 31Peng GongTsinghua University Environmental monitoring and Earth system modeling at Tsinghua
Jun. 1Alon SternGFDLIce shelf melting and breaking: implications for Antarctic Climate
The freshwater flux from the Antarctic continent into the global ocean occurs through basal melting and the calving of icebergs off the edge of Antarctic ice shelves. The meltwater from basal melting affects local hydrography and plays a role in driving local ocean currents around Antarctica. In contrast, icebergs can drift long distances from their calving origins before melting entirely, depositing their meltwater remotely and affecting sea ice formation and climate away from the Antarctic coastline. This talk focuses on the mechanisms (and ocean currents) that move heat and freshwater towards and away from the Antarctic ice shelves. These heat delivery mechanisms are discussed using a combination of laboratory experiments, analysis of field observations, and both complex and simple numerical models. With these mechanisms in mind, a fully-coupled general circulation model with an iceberg component is used to investigate how the partitioning of the Antarctic freshwater flux into basal melting and iceberg calving affects the greater climate system. Finally, we discuss some new ideas and innovations for iceberg and ice shelf modeling, which are a step towards creating a unified ice modeling framework.
Jun. 2Alessandra GianniniColumbia UniversityExploring the dynamical uncertainty in projections of Sahel rainfall change
I will review current understanding of the influence of the oceans on Sahel rainfall, linking the variability of the recent past epitomized in late 20th century drought to projections of change, and sketch a reason for uncertainty in projections based on the dynamics of monsoonal response to warming.
Jun. 3Dr. Robert WillsETH ZurichStationary-eddy influence on changes in the hydrological cycle
The "wet gets wetter, dry gets drier" paradigm explains the expected moistening of the extratropics and drying of the subtropics as the atmospheric moisture content increases with global warming. Here, we show, using precipitation minus evaporation (P - E) data from climate models, that it cannot be extended to apply regionally to deviations from the zonal mean. Wet and dry zones shift substantially in response to shifts in the stationary-eddy circulations that cause them. Additionally, atmospheric circulation changes lead to a smaller increase in the zonal variance of P - E than would be expected from atmospheric moistening alone. The P - E variance change can be split into dynamic and thermodynamic components through an analysis of the atmospheric moisture budget. This reveals that a weakening of stationary-eddy circulations and changes in the zonal variation of transient-eddy moisture fluxes moderate the strengthening of the zonally anomalous hydrological cycle with global warming. In order to understand the mechanisms for these changes in stationary-eddy circulations, we examine the climate change response of stationary eddies in idealized GCM experiments with simple zonal asymmetries: a midlatitude Gaussian mountain and a zonally anomalous ocean heating pattern in the tropics. Through an analysis of the atmospheric energy budget in these experiments, we explain the different mechanisms responsible for their responses to global warming. Through this work we lay out a framework for understanding changes in the zonally anomalous hydrological cycle in terms of changes in zonal-mean climate.
Jun. 6Ron Stouffer SymposiumRon Stouffer Symposium
For the past 38 years, Ron Stouffer's comprehensive research contributions at GFDL have expanded scientific understanding of the atmosphere, oceans, and climate through high performance supercomputing with mathematical models of the Earth system. Ron's accomplishments include the development, along with Suki Manabe, of the first coupled atmosphere-ocean models for global climate warming projections, new understanding of natural modes of climate variability, palecolimate, atmosphere and ocean responses to natural and human-influenced factors, and ongoing leadership of both model development within GFDL and community synthesis efforts through the Coupled Model Intercomparison Project and the Intergovernmental Panel on Climate Change. To commemorate Ron Stouffer's illustrious career, we are in the early planning stage to hold a one day Symposium at GFDL. We are planning three forward-looking science sessions focusing on topics on which Ron's career has had demonstrable impact. The morning session will foucs on scientific talks relating to climate change and community assessments therof, followed by a panel discussion on the increasing overlap between climate modeling and impact assessments and to what extent can they be merged. The afternoon session will include a range of climate-related talks on regional patterns, variability, sea level rise, the carbon system and other topics on which Ron has demonstrated scientific leadership. For more information click here
Jun. 7Daniel RothenbergMITUnderstanding Fundamental Aerosol-Cloud Interactions and their Contributions to the Aerosol Indirect Effect
By mediating the formation of cloud droplets, aerosol play a critical role in setting both the state of the climate system and its sensitivity to change. However, observational and modeling efforts to date have done a poor job of constraining the sensitivity of cloud radiative effects to aerosol perturbations. To help resolve some of this uncertainty, we introduce a new metamodeling framework for parameterizing the lynchpin of aerosol-cloud interactions: droplet activation. For simplified, single-mode aerosol populations, our parameterization better predicts activation dynamics compared to two leading, widely-deployed schemes. Furthermore, we show how the framework is easily extensible to handle the sophisticated aerosol descriptions predicted in modern global models, and incorporate such a scheme in a global aerosol-climate model, the CESM-MARC. By comparing with several others schemes we show that small differences in activation estimates grossly impact the model's sensitivity to aerosol perturbations, especially in maritime regions and heavily polluted parts of the globe. Changes in baseline cloud microphysical properties resulting from the different activation schmes lead to a difference in indirect effect estimates of ~1 W/m2. Aerosol also influence glaciated clouds. Using empirical data collected by a new instrument (the SPIN) during the Fifth Ice Nucleation Workshop, we derive an updated parameterization for predicting potential ice nuclei based on the empirical scheme of DeMott et al (2010), and incorporate this scheme in our model. By coupling aerosol to the ice phase microphysics in MARC-CESM, we halve its sensitivity to aerosol perturbations. The new scheme amplifies the cloud radiative effective in the longwave by greatly increasing the average ice cloud water path and ice crystal burden. We show that careful consideration of ice nucleation frameworks across thermodynamic and cloud regimes is critical when including components that couple to the available aerosol fields.
Jun. 8Baird LangenbrunnerUCLAIdentifying leading spatial patterns of model uncertainty in multi-model ensembles and perturbed physics ensembles
The hydrological cycle continues to be a major aspect of uncertainty in global climate models, especially at the regional level in historical climatologies and end-of-century changes. Proper simulation of precipitation requires that models correctly capture the interaction of large-scale dynamics with local, sub-grid scale processes, which themselves rely on accurate parameterizations. Model uncertainty will therefore arise from structural model differences (present in a multi-model ensemble) as well as differences in a given model's physics or parameterizations. Objective spatial analysis techniques are applied to the CMIP5 ensemble to visualize patterns of intermodel uncertainty in end-of-century precipitation change and in the historical climatology. This analysis pinpoints a source of intermodel spread in projected precipitation changes along the North American west coast, especially for the Southern California region. Similar techniques are applied to a perturbed physics ensemble to visualize the parameter sensitivity of precipitation and other fields. Models of parameter dependence (termed model emulators or metamodels) are constructed and offer a computationally cheap alternative to brute-force sampling of a climate model's parameter space. These metamodels provide a tool to climate model developers exploring questions of parameter optimization and sensitivity. Concepts from multiobjective optimization are used to visualize inevitable tradeoffs that arise when attempting to improve performance of a climate model in multiple fields simultaneously.
Jun. 9Welcome Aboard Intern BriefWelcome Aboard Intern Brief
Welcome Aboard Intern Brief
Jun. 10Arnaud Czaja Imperial College, London, UKThe impact of extra-tropical oceans on climate: warm and cold paths
The variability displayed by satellite estimates of surface currents and that of temperature and salinity fields in the Argo data over the recent decade is fascinating. The extra-tropical oceans in particular draw attention with respect to anthropogenic heat uptake and lateral exchange of heat between mid and high latitudes. But are the climate models able to represent the impact of these oceanic changes on weather patterns and on the low frequency variability of storm-tracks? Or are there none? In this talk, I will suggest that an important mechanism of interaction between the extra-tropical oceans and the atmosphere is missing in coarse coupled climate models. The mechanism relies on the impact of warm water advection by western boundary currents on the warm sector of atmospheric cyclones. The mechanism will be highlighted by high resolution (12km) simulations with the Met Office UM model and reanalyses data. It will also be compared to more traditional mechanisms of oceanic forcing associated with shallow, low level, diabatic heating in the cold sector of atmospheric cyclones.
Jun. 16Roberto BuizzaECMWF, Reading UKThe ECMWF coupled data assimilation system for climate reanalysis Authors: Roberto Buizza and Patrick Laloyaux Affiliation: European Centre for Medium-Range Weather Forecasts (ECMWF)
The European Centre for Medium-Range Weather Forecasts (ECMWF) has developed a coupled variational assimilation system that ingests simultaneously ocean and atmospheric observations. In this variational system, which has been named CERA, full coupling of the ocean and atmosphere components is achieved at the outer-loop level. Employing the coupled model constraint in the variational method implies that assimilation of an ocean observation has immediate impact on the atmospheric state estimate, and, conversely, assimilation of an atmospheric observation affects the ocean state. Within the European Union FP7 project ERA-CLIM2, this coupled approach has been applied to produce the first coupled global reanalysis of the 20th-century (CERA-20C), which will provide a century-long record of low-frequency climate variability and change using a consistent set of observations. The evolution of the global ocean and atmosphere for the period 1901-2010 is represented by a ten-member ensemble of 3-hourly estimates for ocean, surface and upper-air parameters. The ensemble technique has been used to take into account inevitable uncertainties in the observational record and the forecast model, and provides an indication of the data confidence. In this talk, the ECMWF activities in reanalysis will be briefly reviewed, the CERA system will be described, and some preliminary results from the CERA-20C reanalysis, which is expected to be completed by the end of May, will be presented.
Jun. 17Jordan SchnellUniversity of California, IrvineSurface ozone, extreme air quality episodes, and heat waves: New applications for chemistry-climate models
Global change is driving chemistry, climate, and atmospheric composition to new regimes over the coming century, threatening attainment of air quality standards globally. The global chemistry models used to evaluate the effect of future change on air quality are often plagued with large systematic surface ozone biases, making it difficult to directly compare them with observations and to accurately quantify future changes. In addition, previous studies often rely on only point-based data and a fixed threshold exceedance to investigate air quality extremes and consequently do not capture the large-scale, spatially coherent structures of the worst pollution episodes. Here, we develop novel statistical approaches to commensurately compare observations and models with a specific focus on extreme pollution episodes. First, a generalizable interpolation algorithm was developed to convert the heterogeneously spaced hourly ozone site data from surface networks over North America and Europe into maps of grid cell averaged ozone, which allowed a direct comparison with a global model grid cell. Air quality extreme (AQX) events are identified locally as statistical extremes of the ozone climatology. AQX episodes are found to occur predominantly in clustered, coherent, multiday episodes with spatial extents of more than 1000 km. Additionally, a global chemistry model was found to demonstrate skill in hindcasting these extreme episodes, thus identifying a new diagnostic to test global chemistry-climate models. Second, the global chemistry-climate models that participated in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) were evaluated in their ability to simulate observed, present-day surface ozone climatology over North America and Europe. The models were tested on temporal scales from diurnal to multi-year variability and on statistics from median geographic patterns to the timing and size of AQX episodes. Third, the effect of future climate change on surface ozone was evaluated using present-day and future hourly surface ozone simulated by the ACCMIP models. Finally, the developed methods were extended in an effort to characterize the co-occurrence of surface ozone, particulate matter, and temperature extremes, providing further diagnostics for global chemistry climate model evaluation and enabling an investigation of the multi-stressor impacts of poor air quality and heat waves.
Jun. 21Junyi Chai Final Public OralJunyi Chai Final Public Oral
Junyi Chai Final Public Oral
Jun. 22Tra DinhPrinceton UniversityProcesses linking atmospheric radiative forcing and the hydrological cycle response
The physical constraints of the global hydrological cycle,including the energetic balance of the atmosphere and the governing equations for sensible heat flux and surface evaporation, are discussed in the context of increasing atmospheric CO2. Subject to the CO2 forcing, the atmosphere changes until at equilibrium the sum of the changes in latent heat flux and sensible heat flux from the surface into the atmosphere balances the change in the radiative flux divergence from the atmosphere. The causality of this balance remains incompletely understood, with some authors emphasizing surface condition change, and others atmospheric radiative budget change. Here, we seek to disentangle the processes that link the atmospheric radiative forcing to changes in the near-surface air conditions. For the so-called "direct" response where the surface temperature has not yet adjusted, both sensible heat flux and latent heat flux decrease with increasing CO2. In GFDL aqua-planet simulations, these changes in sensible and latent heat fluxes are mainly associated with increasing temperature and specific humidity in the near-surface air layer, while changes in the surface wind and drag are secondary terms. We show that such changes of the near-surface air temperature and specific humidity are forced by the increase in the radiative heating rate (decrease of radiative cooling) due to increasing CO2. Via this process, we demonstrate the causality (forcing-response) connection between atmospheric radiation and the near-surface air conditions, correspondingly the connection between atmospheric radiation and sensible and latent heat fluxes and thus the global hydrological cycle.
Jun. 23Jessica LundquistUniversity of WashingtonWater Towers of the World: Innovative Methods to Quantify Mountain Precipitation
Mountain rivers provide water for 40% of the world's population, but our ability to model and manage that water supply is limited by our lack of knowledge of how much precipitation falls over mountain terrain. Atmospheric models provide our best path forward to quantify this precipitation, but these models are limited by a lack of ground truth data to aid model development and parameterizations. Focusing on the western United States and particularly, on the Sierra Nevada of California, we present uncertainties in multiple gridded precipitation datasets and how these uncertainties can be narrowed by carefully considering snow and streamflow measurements.
Jun. 29V. BalajiOrigins of model diversity
In this talk, I will cover sources of diversity in climate models. We will look at the process of model development at different institutions and show how many factors: including history and culture, scientific priorities, community of users, developers and "customers", and computational constraints, play a role in defining a climate model. We will explore how this may evolve in the future.
Jul. 6Yizhen LiWoods Hole OceanographicProcesses regulating formation of low-salinity high-phytoplankton biomass lenses near the edge of the Ross Ice Shelf
Both remotely sensed and in situ observations in austral summer of early 2012 in the Ross Sea suggest the presence of cold, low-salinity, and high-biomass eddies along the edge of the Ross Ice Shelf (RIS). Satellite measurements include sea surface temperature and ocean color, and shipboard data sets include hydrographic profiles, towed instrumentation, and underway acoustic Doppler current profilers. Idealized model simulations are utilized to examine the processes responsible for ice shelf eddy formation. 3-D model simulations produce similar cold and fresh eddies, although the simulated vertical lenses are quantitatively thinner than observed. Model sensitivity tests show that both basal melting underneath the ice shelf and irregularity of the ice shelf edge facilitate generation of cold and fresh eddies. 2-D model simulations further suggest that both basal melting and downwelling-favorable winds play crucial roles in forming a thick layer of low-salinity water observed along the edge of the RIS. These properties may have been entrained into the observed eddies, whereas that entrainment process was not captured in the specific eddy formation events studied in our 3-D model—which may explain the discrepancy between the simulated and observed eddies, at least in part. Additional sensitivity experiments imply that uncertainties associated with background stratification and wind stress may also explain why the model underestimates the thickness of the low-salinity lens in the eddy interiors. Our study highlights the importance of incorporating accurate wind forcing, basal melting, and ice shelf irregularity for simulating eddy formation near the RIS edge. The processes responsible for generating the high phytoplankton biomass inside these eddies remain to be elucidated.
Jul. 11Amanda O'RourkeUniversity of Michigan Impacts of Nonlinear Advection and Wind Stress on the Ocean Surface Kinetic Energy Budget at Low Frequencies in GFDL's CM2-O Model Hierarchy
Low frequency variability within the ocean surface can be excited by both external forcing, such as atmospheric exchanges of heat and momentum, as well as the nonlinear transfer of energy between ocean eddies. Recent studies have shown that nonlinear eddy interactions at short timescales can excite an energy transfer from high to low frequencies similar to the transfer of energy between spatial scales in two dimensional turbulence. As this energy exchange is sensitive to the existence of oceanic eddies, the process of energy exchange across frequencies may be sensitive to ocean resolution. We GFDL's CM2-O hierarchy of fully coupled ocean-atmosphere models to address the transfer of surface kinetic energy and temperature variance between synoptic and decadal timescales utilizing a cross-spectrum diagnostic. One question related to this research is whether low frequency modes are primarily driven from internal mechanisms, such as nonlinearity, or external forces from the atmosphere. Diagnostics of energy flux and transfer within the frequency domain will be compared between three models at 1, 1/4th, and 1/10th degree ocean resolution to address the importance of eddy resolution in the driving of energy to low frequencies.
Jul. 11Brian ArbicUniversity of MichiganThe internal gravity wave spectrum in two high-resolution global ocean models
Internal gravity waves (IGWs) are a topic of continuing interest to oceanographers, in part because breaking IGWs drive a substantial amount of mixing in the subsurface ocean. The sea surface height (SSH) signal of IGWs is large enough to be of interest in satellite altimetry, especially wide-swath satellite altimetry, which will map SSH at unprecedented horizontal resolution. Motivated by these applications and by US Navy operational concerns, we have inserted tidal forcing into global 1/12th and 1/25th degree simulations of the HYbrid Coordinate Ocean Model (HYCOM). Recently we have shown that the HYCOM simulations are beginning to develop an IGW continuum spectrum, which improves as model resolution is increased. At the same time, some very recent global simulations of the MITgcm have been performed with up to 1/48th degree resolution and tidal forcing. Therefore, the community now has two models in which to explore the global internal tide and IGW continuum fields. This talk will briefly cover some technical aspects of global internal tide modeling, and will then move on to scientific applications of such simulations, including comparisons of the internal tides and IGW spectra in HYCOM, MITgcm, in-situ observations, and altimetry.
Jul. 13Courtney Schumacher (Texas A and M University)Texas A and M UniversityThe complexity and relevance of observed mesoscale convective organization
This talk will describe, from an observational frame of reference, the definitions of mesoscale organization, the factors that organize and maintain mesoscale convective systems, and the large-scale impacts of organized convective systems in the tropics. Recent results using the TRMM precipitation radar will be shown to highlight important aspects of mesoscale convective organization and its interaction with the larger scale environment, with special emphasis on the tropical precipitation-column moisture relationship.
Jul. 14Allison Wing (Lamont-Doherty Earth Observatory)Lamont-Doherty Earth ObservatoryOrganization of tropical convection: Self-aggregation and spontaneous tropical cyclogenesis
Tropical clouds and relative humidity play a key role in both the planetary energy balance and the sensitivity of global climate to radiative forcing. Both clouds and relative humidity are also strongly modulated by the organization of tropical convection, which results in a large fraction of tropical cloudiness and rainfall. Here, we investigate the organization of tropical convection in the context of self-aggregation, a spontaneous transition in idealized numerical simulations from randomly distributed to organized convection despite homogeneous boundary conditions. Specifically, the System for Atmospheric Modeling is used to perform 3-d simulations of radiative-convective equilibrium in a non-rotating framework, with interactive radiation and surface fluxes and fixed sea surface temperatures. We characterize the fundamental physical mechanisms that lead to self-aggregation as well as its growth rate and spatial scale. Cloud-radiative feedbacks and surface flux feedbacks are found to be important in the initial instability. Self-aggregation has primarily been studied in a non-rotating framework, but it has been hypothesized to be important to tropical cyclogenesis. In numerical simulations of tropical cyclones, a broad vortex or moist bubble is often used to initialize the circulation. Here, we instead allow a circulation to develop spontaneously from a homogeneous environment in 3-d cloud-resolving simulations of radiative-convective equilibrium (RCE) in a rotating framework, and compare the resulting tropical cyclogenesis to non-rotating self-aggregation. We find that in the initial development of a broad circulation, the feedback processes leading to cyclogenesis are similar to the initial phase of non-rotating aggregation. Sensitivity tests in which the degree of interactive radiation is modified are also performed to determine the extent to which the radiative feedbacks that are essential to non-rotating self-aggregation are important for tropical cyclogenesis.
Jul. 15Brian Mapes (University of Miami)University of MiamiToward an understanding of form-function relationships for mesoscale convection
Toward an understanding of form-function relationships for mesoscale convection Abstract: Seen from the large-scale viewpoint that drives atmospheric science, convection performs various functions in the atmosphere. How do these functions depend on its form? In particular, what is the importance of theoretically and computationally inconvenient mesoscale structure in space-time? To address these questions, we need ways to measure function that are both well defined and relevant (which requires approaches that are not too intrusive), yet at the same time we need techniques for conditioning or controlling meaningful aspects of form. I will review various convection research approaches and results from this viewpoint. A currency of tolerances (how good is good enough?) is also needed to orient and prioritize efforts. The postulate that mesoscale structure is inessential, despite its observed ubiquity, will be entertained.
Jul. 18Xiu-Qun YangImpact of Urbanization and Aerosols on the East Asian Monsoon
Jul. 20George PhilanderPrinceton UniveristyThe Recurrent Ice Ages; On the Importance of Asking the Right Question
Glaciers that wax and wane, originally the protagonists in the dramatic amplification of climate fluctuations over the past 3 million years, are now viewed as members of a large cast of interacting phenomena in different parts of the globe. Questions that focus on glaciers in isolation are therefore misleading, but questions that concern the broad spectrum of climate variability are intractable. The trio of dominant cyclic signals in the records -- each member is distinctive and is related to a different facet of orbital sunlight variations -- poses manageable questions and provides stringent tests for models that predict future climate changes. Those tests can reduce uncertainties in estimates of climate sensitivity.
Jul. 21Maryam AbdioskoueiMethane in the GFDL Atmospheric Chemistry Model
Methane in the GFDL Atmospheric Chemistry Model
Jul. 27Xueshun ShenCenter for Numerical Weather Prediction, China Meteorological AdministrationGlobal/Regional Non-hydrostatic Numerical Weather Prediction Model Using Semi-implicit and Semi-Lagrangian Method: progress and challenges
Since 2001, the China Meteorological Administration (CMA) began to develop the new generation NWP system. This system consists of a unified global/regional numerical model and a variational data assimilation system, which is called as the GRAPES (Global and Regional Assimilation & Prediction System). The governing equations of GRAPES are the fully compressible non-hydrostatic ones with shallow atmosphere approximation, which makes the model suitable for use at very high resolutions (such as grid size ). The GRAPES model was developed to be used for all production models in NWP, i.e., so called unified NWP model. The prognostic variables include 3-dimensional wind components, potential air temperature, non-dimensional pressure (The Exner function) derived from mass conservation and mixing ratio of water species. To solve the equations numerically, the spherical coordinate in the horizontal and terrain-following height coordinate in the vertical are utilized. To optimize balance in the model and to eliminate computational modes, C-grid staggering in the horizontal and Charney-Phillips staggering in the vertical were chosen. It should be noted that Charney-Phillips staggering presents a problem when used in the boundary layer mixing as care is needed to define the mixing coefficients appropriately as momentum and temperature are at different levels. For the time stepping scheme, the off-centered two-time-level SISL discretization is applied to each of the prognostic equations. The only distinction between scalar and vector quantities is that the departure and arrival point components and their directions are discretized together to include the change of vector direction from departure to arrival point with the help of a mid-point and a rotation matrix. The use of semi-implicit time scheme results in a 3-dimensional elliptic equation for solving the non-dimensional pressure. This is solved iteratively using the generalized conjugate residual technique (GCR) with appropriate preconditioning. For the spatial discretization, the equations of GRAPES model are evaluated on an Arakawa-C grid in the horizontal and a Charney-Phillips grid in the vertical as mentioned above. Standard second-order accurate finite differencing is applied to all terms. To apply conservation to tracers and non-negative quantities (monotonicity) under semi-Lagrangian advection, a conservative semi-Lagrangian scheme based on the piece-wise rational function is used (Shen, Wang and Xiao, 2011). However, the algorithm cannot be used to conserve the dry air mass. The solution procedure of GRAPES model is rather straightforward. For the dynamics part of GRAPES, 3-dimensional elliptic equation of non-dimensional pressure is solved at first. Then, the other prognostic variables are evaluated through substituting the solved non-dimensional pressure into the individual discretized equations. After finishing the integration of dynamics, the physics are computed by using the simple parallel-splitting method except for the microphysics. Before calling the microphysics, all the prognostic variables are updated to include the time changes of variables due to dynamics, radiation, land surface process, PBL and vertical diffusion as well as cumulus convection. The computation of microphysics is the final step of GRAPES model integration within one time-stepping loop. 2. Main applications of GRAPES model The successful development of GRAPES model and data assimilation system symbolizes the big change of CMA NWP system from technological dependence on the imported NWP into the self-development. Since 2006, a regional forecast system-GRAPES_Meso has been implemented for operation in National Meteorological Center (NMC) for meso-scale forecast over China, in Shanghai Typhoon Institute for typhoon track forecast, in Guangzhou Tropical Oceanographic and Meteorological Institute for monsoon and heavy rainfall forecast in succession. At present, the highest resolution of GRAPES_Meso is 3km. Since 2007, a pre-operational version of global forecast system-GRAPES_GFS (GRAPES global model: 0.5°x0.5°L36+ GRAPES global 3DVAR:1.125°x1.125°) has been established for global medium-range (0~10days) weather forecast. From June of 2016, GRAPES_GFS was updated with the horizontal resolution 0.25°x0.25°and 60 levels in the vertical, and became operation officially. And, a rapid update cycle system-GRAPES_RUC, based on GRAPES_Meso system, has been implemented for the very short range severe weather forecast (0~1day). 3. Future GRAPES model development Until present, applications of SISL scheme to the fully compressible equations have been proved the effectiveness. But, for the future high-resolution applications on massively parallel computers ( or more processors) as well as for the foreseeable seamless prediction from weather to climate, there exit great challenges for the future GRAPES model . As with many grid-point models using implicit time-stepping, the non-locality of 3-dimensional elliptic equation solver becomes a critical issue for the efficiency and scalability. Any non-local process will limit the speed of the model as will data transposes. The anisotropy of the latitude-longitude grid results in a large number of solver iterations, particularly at high resolutions. More uniform grids on the sphere and locally intensive algorithm are necessary. Traditional semi-Lagrangian schemes lose any conservation properties due to the use of point-wise interpolation. Although it is argued that mass conservation may not be so critical for weather forecast, but mass conservation should be revisited for the future earth system prediction, for example, the seamless weather-climate modeling system, chemical weather forecast, etc. In the current GRAPES model, conservation of moist species is guaranteed by using conservative SL scalar advection. Dry air mass and any other conservative properties will be the key issue for further development of GRAPES, either by developing the conservative Lagrangian algorithm or through revising the governing equations as well as the temporal-spatial discretization method. From 2012, we started to develop the new dynamical core based on the multi-moment constrained finite volume method. The basic consideration is to design a new core which has high accuracy, high scalability, better conservation properties and grid flexibility. Up to now, fundamental researches have been finished, including various tests in global shallow water and 2-D slice non-hydrostatic frameworks. Presentation on this new dynamics will be given during the visiting period.
Aug. 2Spencer Hill Final Public OralSpencer Hill Final Public Oral
Spencer Hill Final Public Oral
Aug. 3E. Predybaylo/S. OsipovVolcanic Impact on ENSO / Aerosol Impact on the Red Sea
Informal Seminar
Aug. 5Summer Intern PresentationsSummer Intern Presentations
Summer Intern Presentations Bridge: 877-918-1365/4351085
Aug. 12Jeff Strong Final Oral PresentationJeff Strong Final Oral Presentation
Jeff Strong Final Oral Presentation
Aug. 24Nadir JeevanjeeGFDL Post DocWhy 3% per Kelvin? A Quantitative Theory for Mean Precipitation Change with Warming
Global warming simulations show that mean precipitation increases at ~ 3% per Kelvin, but we do not know what sets this value, or how to predict it. Mean precipitation is constrained by radiative cooling, however, and radiative cooling is dominated by water vapor, which behaves very simply with warming due to Clausius-Clapeyron. We leverage these connections to derive a simple analytical equation for how radiative cooling, and hence precipitation, change with warming. This equation can predict the ~3% per Kelvin increase directly from base-state diagnostics, and furthermore yields some intuition for this value. We validate our approach with tropical RCE simulations, as well as AMIP and AMIP4K GCM output.
Sep. 14James PorterChief, Reservoir Operations, NYC Environmental Protection Bureau of Water SupplyThe Operations Support Tool (OST): How ensemble streamflow forecasts support risk-based decision-making for the New York City water supply
The New York City water supply system provides over one billion gallons of high-quality water each day to more than nine million people. The water is delivered from a system of 22 reservoirs and lakes spread over 2,000 square miles north and west of the city. Releases are made from these reservoirs to support downstream interests including flood attenuation, cold water fisheries, salinity repulsion, drought mitigation, and water supply in the Delaware and Hudson basins. Operating such a large, complex, and multi-objective system is challenging, especially in an era of increasing regulations, heightened stakeholder expectations, and climate change. To help meet this challenge, DEP built the Operations Support Tool (OST). OST ingests ensemble streamflow forecasts from the National Weather Service and real-time environmental data to drive models of reservoir operations and water quality. The models provide ensemble output from which probabilistic information can be derived, allowing managers to make risk-informed decisions about system operations. This talk will describe the water supply system, give an overview of OST, and present use cases illustrating how ensemble streamflow forecasts enhance operational decision-making by providing additional information not available from traditional deterministic forecasts.
Sep. 21Louise Nuijens of MIT Sensitivity of shallow cumulus and deeper shallow cumulus (congestus) in subsiding branches of large-scale circulations
Shallow clouds have received much attention in recent years for their role in the uncertainty of climate sensitivity, related to their influence on Earth's albedo. In particular, climate models diverge in their modeled behavior of shallow cumuli in regions with weak subsidence - areas which cover much of the subtropical and tropical ocean. It is also increasingly recognized that shallow clouds and the convection that underlies them might influence the climate system in other important ways, such as through circulations tied to regions of deep convection. In this talk I will use a long record of atmospheric profiling at the edge of the subtropics and tropics to show that cloudiness from shallow cumuli is rather insensitive to changes in the large-scale flow. I will discuss how shallow cloud behaves differently in and among climate models, and hence why climate-model cloud feedbacks might be overestimated. Furthermore, I will show that deeper modes of shallow cumuli or congestus, which readily precipitate, have greater variability.
Sep. 23Jaya Khanna FPOTBD
Sep. 28Isaac HeldGFDLIdealized atmospheric models and tropical cyclone climatology
Starting with the realistically configured HiRAM model, a number of idealized models have been used to probe fundamental controls on tropical cyclone climatology. These include aquaplanets with realistic meridional temperature gradients, aquaplanets with uniform SSTs, and f-plane radiative-convective equilibria with fixed SSTs and with slab ocean boundary conditions. Five papers have been published on this work, led by graduate students and post-docs (Zhou, Ballinger, Merlis). I will try to place this series of papers in a broader context, emphasizing open questions.
Sep. 30Todd Mooring FPOTodd Mooring FPO
Todd Mooring FPO
Oct. 4Andrew StuartUCLAEddy mixing and transport at the Antarctic margins
Ocean processes occurring along the Antarctic continental slope admit shoreward transport of heat toward the continent's marine-terminating glaciers and export newly-formed dense waters from the continental shelf. Recent modeling studies indicate that both eddies and tides may modulate these processes in all sectors of the Antarctic margins. However, due the computational cost of resolving the small (~20km) scales of Antarctic shelf/slope eddies, previous analyses have been limited to regional models and idealized process studies. In this study we investigate eddy/tidal transport and dynamics around the entire Antarctic shelf break using output from recent global ECCO2 simulations run at 1/24th and 1/48th degree horizontal resolutions. We use energy and vorticity budgets to characterize the eddy/tidal-mean flow interaction in the Antarctic Slope Current (ASC). We show that tidal forcing produces a distinct dynamical regime arises in which the core of the ASC flows at almost exactly the same speed as the overlying sea ice, resulting vanishing surface momentum and energy forcing that is accommodated by lateral momentum and energy fluxes. Eddies generated by baroclinic instabilities in this steep slope front effect shoreward heat transport across the continental slope, particularly where the continental shelf break is interrupted by troughs in the ocean bed. We also discuss how this dynamical regime varies between different sectors of Antarctica.
Oct. 5David Raymond New Mexico Institute of Mining and Technology, Socorro, New MexicoBalanced dynamics and convection in the tropical atmosphere
Balanced dynamics and convection in the tropical atmosphere
Oct. 5Xiao LiuUniversity of Southern CaliforniaImpact of Fine-scale Physical Processes on Marine Ecosystem Dynamics and Carbon Cycling in the North Pacific Subtropical Gyre
Subtropical gyres contribute significantly to global ocean productivity. As the climate warms, the strength of these gyres as a biological carbon pump is predicted to diminish due to large-scale stratification and depleted surface nutrients. The impact of fine-scale processes (1-10 km) on the subtropical gyre ecosystems, however, has been poorly understood due to observational and computational constrains. We developed a new statistical tool to quantify fine-scale surface patchiness from high-resolution (1 km) satellite sea surface temperature. Chlorophyll concentrations in the North Pacific Subtropical Gyre were shown to be enhanced by fine-scale frontal dynamics with an average increase of 38% (maximum of 83%) during late winter. The magnitude of this enhancement is comparable to the observed decline in chlorophyll due to a warming of ~1.1°C, suggesting that future trends in the changes of fine-scale physics may either compensate or exacerbate the predicted weakening of biological carbon pump. In order to assess the responsiveness of ecosystem and carbon dynamics to fine-scale physics, we also introduced a new modeling approach, the Spatially Heterogeneous Dynamic Plankton (SHiP) model, which allows for subgridscale heterogeneity in physical and biogeochemical environments through probabilistic representation of fine-scale, episodic disturbances. The model was applied to the Hawaiian Ocean Time-series site in the NPSG. Significant differences were noticed between nutrient concentrations, phytoplankton size composition, and carbon export dynamics when the model was run in a temporally and spatially heterogeneous mode relative to the traditional homogeneous approach. We also suggest that the temporal scale (e.g. duration) of the disturbance events is extremely relevant to the responses of ecosystem and carbon dynamics. These findings indicate that the incorporation of fine-scale bio-physical interactions in global-scale climate models is important for accurately modeling and predicting changes of marine ecosystems under future climate projections.
Oct. 6Annalisa BraccoGeorgia TechDelta-MAPS: From spatio-temporal data to a weighted and lagged network between functional domains. A climate application
I will discuss a recently developed methodology based on complex network analysis and its application to CMIP5 simulations to assess their performances, quantify uncertainties, and uncover changes in global linkages between past and future projections. Network properties of modeled sea surface temperature over 1956-2005 are constrained towards observations or reanalyses, and their differences quantified using two metrics. Projected changes from 2051 to 2300 under the scenario with the highest representative and extended concentration pathways (RCP8.5 and ECP8.5) are then determined. The network of models capable of reproducing well major climate modes in the recent past, change little during this century. In contrast, among those models the uncertainties in the projections after 2100 are substantial, and are primarily associated with divergences in the representation of the modes of variability, particularly of ENSO, and their connectivity, and therefore with their intrinsic predictability, more so than with differences in the mean state evolution. Next I discuss a further evolution of the above network methodology, called delta-Maps, that first identifies the distinct spatial components of the underlying system, referred to as domains, and second infers the connections between them. In the delta-Maps case we compute a domain as the maximum-sized set of spatially contiguous cells that include the detected core and satisfy a homogeneity constraint, expressed in terms of the average pairwise cross-correlation across all cells in the domain. Major advantages of this method are that domains may be spatially overlapping, and the correlated activity between domains can be lagged in time. We illustrate the application of delta-Maps on different SST reanalysis data sets and on one climate model.
Oct. 11Sylvia SullivanGeorgia Techmulti-scale modeling of in-cloud ice crystal formation
Atmospheric models at all scales struggle to accurately predict both localized precipitation intensity and supercooled liquid fraction. The in-cloud ice crystal number concentration can control both, either via cold phase-initiated precipitation or the Bergeron process. In order to improve predictions of cloud lifetime or phase partitioning, this work takes a two-fold approach to more accurately represent ice crystal formation, both by primary nucleation and secondary production. Existing ice nucleation parameterizations are extended for large-scale sensitivity and attribution analyses in the Community Atmosphere Model and Goddard Earth-Observing Model. Using first-order sensitivities, discrepancies between field and laboratory data-based nucleation spectra are identified, in both nucleation regime and efficiency. Attribution metrics, defined from these sensitivities, show that different inputs drive temporal variability in output nucleated ice crystal number for different models. Far less effort has been devoted to the parameterization of secondary ice production, such as rime-splintering or breakup upon collision, and a smaller-scale approach is taken here. The potential enhancement in ice crystal number due to this secondary production is shown, both from parcel model simulations and a case study with the COSMO regional weather model.
Oct. 12V. BalajiGFDLCPMIP: Measurements of Real Computational Performance of Earth System Models
Traditional metrics of computational efficiency such as performance counters and scaling curves do not tell us enough about real sustained performance from climate models on different machines. They also do not provide a satisfactory basis for comparative information across models. We introduce a set of metrics that can be used for the study of computational performance of climate (and Earth System) models. These measures do not require specialized software or specific hardware counters, and should be accessible to anyone. They are independent of platform, and underlying parallel programming models. We show how these metrics can be used to measure actually attained performance of Earth system models on different machines, and identify the most fruitful areas of research and development for performance engineering. We present results for these measures for a diverse suite of models from several modeling centres, and propose to use these measures as a basis for a CPMIP, a computational performance MIP.
Oct. 13Ming XueUniversity of OklahomaData Assimilation, Ensemble Prediction and Predictability of Convective-Scale Weather over the Continental United States
Convective-scale, severe, hazardous weather, including strong local winds, heavy precipitation, tornadoes and hail, has high socioeconomic impacts. Prediction of such weather requires nonhydrostatic models running at convection-resolving grid spacings while uncertainties of such forecasts require the use of ensemble. The models also need to be initialized with convective-scale observations through advanced data assimilation. Since 2007, the Center for Analysis and Prediction of Storms (CAPS) at the University of Oklahoma has been producing realtime convection-allowing (3-4 km) storm-scale ensemble forecasts (SSEF) and convection-resolving (1-2 km) deterministic forecasts for continental United States (CONUS) domains, as contributions to the NOAA Hazardous Weather Testbed (HWT) Spring Experiments and more recently for the Hydrometerology Testbed (HMT) Flash Flood and Intense Rainfall Experiment (FFaIR). These realtime forecasts aim to address the scientific issues including: the values and cost-benefit of convection-allowing-resolution ensemble versus coarser-resolution short-range ensembles and even-higher-resolution convection-resolving deterministic forecast; optimal design of a storm-scale ensemble system including the initial condition perturbation methods, physics perturbations, and the use of multiple models; proper handling and use of lateral and lower boundary perturbations; the value and impact of assimilating high-resolution data including those from WSR-88D radars; the most effective ensemble post-processing and forecast products for the convective storm scales; and the value and impact of such unique products for forecasting guidance and warning. The data sets have also been used to study predictability of precipitation at different scales, performance of new microphysics and PBL parameterization schemes, and understand physical processes of a number of high-impact events. In this talk, methodologies used by the CAPS SSEFs and key findings obtained based on the SSEF forecasts will be presented. The Community Leveraged Unified Ensemble (CLUE) forecast project organized by HWT in 2016 will be briefly introduced. There is also plan to enhance and run the GFDL FV3 model with a ~3 km nest over CONUS starting in spring 2017 to evalaute its performance for convective-scale forecasting.
Oct. 17George KiladisEquatorial Waves and the Performance of the NCEP and ECMWF Operational forecast models during NOAA's 2016 El Nino Rapid Response Campaign
The El Nino Rapid Response (ENRR) field campaign targeted equatorial Pacific atmospheric convective activity during January-March 2016 through enhanced observations. The 2015-16 El Nino had much in common with the events having similar amplitude sea surface temperature (SST) anomalies during 1982-83 and 1997-98, but also differed in several key aspects. All of these episodes featured enhanced convectively coupled Kelvin wave activity crossing the entire Pacific basin, while during December 2015 a large amplitude Madden-Julian Oscillation (MJO) was observed, with a convective signal that propagated unusually far to the east (~150W). A second MJO-like event occurred during the latter part of February, 2016, but despite similar SST and convective heating fields, the basic state flow was much different than during December, with strong upper level westerlies favoring the intrusion of extratropical Rossby wave energy into the equatorial eastern Pacific region. This second MJO event was accompanied by an unexpected lack of storm activity and associated precipitation along the west coast of North America. Based on the preliminary results of AMIP simulations using observed SSTs, these differences are hypothesized to be due to internal variability within the storm track itself that may have been overriding the large scale forcing by the tropical diabatic heating field. We assess the quantitative precipitation forecasts (QPF) skill of the NCEP GFS and ECMWF IFS forecasts during ENRR through a comparison with GPM satellite precipitation estimates for the same period. Results reveal that, in general, initial conditions were reasonably well estimated in both forecast systems, as indicated by relatively good QPF scores for the 6-12 hour forecasts. In the tropics, a previously well-documented overestimation of light precipitation develops almost immediately, being more prevalent in the GFS than the IFS. In general, extratropical Northern Hemisphere QPF in the GFS was superior to the IFS at all lags and for all sectors, but this was reversed in the tropics, with the IFS scoring somewhat better. Tests of the Grell-Freitas "scale aware" convective parameterization are shown to improve the performance of the GFS in the tropics at all forecast lead times.
Oct. 19Daniel WardGFDLEarth Wind & Wildfire: Predicting fire emissions in the GFDL ESM
Smoke from fires is a major global source of carbonaceous aerosols and greenhouse gases, and leads to a diverse set of impacts on climate and air quality. Here we find substantial changes in tropospheric ozone, aerosol optical depth, sea surface temperature and atmospheric circulation from global fire emissions in fully-coupled ESM2Mb simulations (500yr) with preindustrial forcing: one simulation with no fire emissions, one with constant annual emissions, and a third simulation where fire emissions are interannually varying. Efforts to understand how these impacts may have evolved in the past, or might evolve in the future, due to anthropogenic activities and climate changes, requires credible, "process-based"modeling of fires. Long preindustrial control simulations and historical period ensemble simulations from ESM2Mb and CESM1 show that the previous generation of fire models underpredicts interannual variability in fire emissions and also disagree on the sign of the trend in 20th century fire emissions. A new process-based fire model has been introduced into the GFDL ESM that dramatically improves our prediction of area burned by fires globally. Further improvements to the fire model are also underway including the introduction of prognostic fire intensity and multi-day fire duration, with an aim toward correcting the low bias in area burned and interannual variability for boreal fires. Within this model, carbon consumed by fires is converted to aerosol and trace gas species and can now be injected into the atmosphere model using a plume height parameterization. We use this model to estimate the contribution of agricultural fires, both on croplands and pasture, to global aerosol optical depth given present day land use.
Oct. 20Sergey KravtsovUniversity of Wisconsin-MilwaukeeOn semi-empirical decomposition of multidecadal climate variability into forced and internally generated components
This study combines CMIP5 historical simulations and observations of surface temperature to investigate relative contributions of forced and internal climate variability to long-term climate trends. A suite of estimated forced signals based on surrogate multi-model ensembles mimicking the statistical characteristics of individual models is used to show that, in contrast to earlier claims, scaled versions of the multi-model ensemble mean cannot adequately characterize the full spectrum of CMIP5 forced responses, due to misrepresenting the model uncertainty. The same suite of multiple forced signals is also used to derive unbiased estimates of the model simulated internal variability in historical simulations and, after appropriate scaling to match the observed climate sensitivity, to estimate the internally generated component of climate variability in the observed temperatures. On average, climate models simulate the non-uniform warming of Northern Hemisphere mean surface temperature well, but are overly sensitive to forcing in the North Atlantic and North Pacific, where the simulations have to be scaled back to match observed trends. In contrast, the simulated internal variability is much weaker than observed. There is no evidence of coupling between the model simulated forced signals and internal variability, suggesting that their underlying dominant physical mechanisms are different. Analysis of regional contributions to the recent global warming hiatus points to the presence of a hemispheric mode of internal climate variability, rather than to internal processes local to the Pacific Ocean. Large discrepancies between present estimates of the simulated and observed internal climate variability suggest that our ability to attribute and predict climate change using current generation of climate models is limited.
Oct. 26Dan FeldmanLawrence/Berkley LabMeasuring and Modeling the Radiative Forcing from Carbon Dioxide and Methane
Carbon dioxide (CO2) and methane (CH4) are greenhouse gases due to their interactions with both mid- and near-infrared radiative energy in the Earth's atmosphere. While atmospheric CO2 mixing ratios have been rising at an increasing rate since the late 1950s, CH4 exhibited a plateau in atmospheric mixing ratio increases between 1995 and 2006. Since 2006, CH4 mixing ratios have been increasing in a spatially heterogeneous manner, but there is an unresolved debate regarding the cause(s) of this hiatus and its end. We expect that rising quantities of these greenhouse gases will perturb the surface energy balance through radiative forcing, but this needs to be observed and quantified. This presentation will show, through detailed observations at two ARM sites, the first time-series of observations of mid-infrared radiative forcing from CO2 and CH4. The CO2 time-series shows the increasing greenhouse effect due to rising atmospheric concentrations and agrees with theoretical predictions. This time-series of CH4 is more complicated, but does show the signal of the plateau and end thereof, but also is heavily influenced by the amount of atmospheric water vapor. This presentation will conclude by presenting preliminary results, prospects, and challenges for observing the near-infrared forcing from CO2 and CH4 and the scientific opportunities that such observations may enable.
Oct. 27Michael WehnerLawrence Berkeley National Laboratory and University of California, BerkeleyThe need for high resolution in probabilistic extreme event attribution.
The attribution of the human influence to the likelihood and severity of extreme weather events has become a cottage industry as of late. With an annual BAMS State of the Climate Supplement, together with the more standard journals, the body of such literature has increased dramatically since the pioneering work of Stott, Stone and Allen's analysis of the deadly 2003 European heat wave. As the public's interest in such analyses has increased, the scientific community has been asked to opine about many high impact events during the news cycle. In this talk, we examine whether the climate models typically used are "fit for purpose"for such statements, both for rapid assessment and for more careful detailed analyses published long after the events in question. Novel experimental designs to assess the anthropogenic effect of very rare and unusual weather events are presented along with a frank discussion of their usefulness and limitations.
Oct. 31Guy DaganThe Weizmann Institute of Science, Rehovot IsraelNon-monotonic response of warm convective clouds to changes in aerosol loading
The mutual effect of the environmental conditions and aerosol loading on the time evolution of warm convective clouds and rain is studied. For a given thermodynamic conditions, an increase in aerosol concentrations leads to a competition between processes that amplify clouds development versus those that suppress it. Specifically, the invigorating processes are driven by increase in the condensation efficiency, longer condensational growth and better mobility of the condensate allowing the liquid water to move upward higher in the atmosphere. On the other hand, the suppressing processes are enhanced by stronger mixing with the surrounding sub-saturated air and larger water loading. This competition dictates an optimal aerosol concentration for which key macrophysical properties of the cloud (like total mass and rain yield) reach their maximal values. This optimal aerosol concentration (Nopt) depends on the thermodynamic conditions such that deeper clouds would have larger Nopt. The coupling between the clouds to the cloud field's thermodynamic properties adds another layer of complexity. Under polluted conditions rain suppression acts to increase the atmospheric instability whereas cloud and rain forming in clean conditions tend to stabilize the lower atmosphere. In this work we explore aerosol effects in both the cloud and the field scales using numerical simulations. We show how changes in cloud's properties are manifested in the field's properties and vice versa.
Oct. 31Da YangUniverstiy of California, BerkleyTropical waves, the MJO, and double ITCZs
What is the origin of the double Intertropical Convergence Zones (ITCZs)? What leads to the eastward propagation of the Madden-Julian Oscillation (MJO)? These are long-term mysteries in tropical meteorology and climate dynamics. In this talk, I will show that both double ITCZs and the MJO can self-emerge over an ocean surface with uniform temperature. This is counterintuitive based on prevailing paradigms that double ITCZs arise from nonuniform external forcing, and that the eastward propagation of the MJO relies on a particular background moisture distribution. To explain the finding, I propose a new theoretical framework that both double ITCZs and the MJO arise due to multi-scale interaction between short-lived atmospheric waves and individual rainstorms. I will present supporting evidence from idealized simulations and observations. This multi-scale view of tropical convection suggests that accurate representation of individual rainstorms is not only important to local weather forecast but also crucial to global climate prediction.
Oct. 31Chia-Ying LeeColumbia University Towards a new tropical cyclone risk assessment system - stochastic modeling of tropical cyclone activity
We are developing a system to estimate the risk of a rare, high-impact tropical cyclone landfall event by generating ensembles of synthetic tropical cyclones whose properties depend on the climates in which they occur. In this talk, I will present results from three components of the system: a genesis model, a beta-advection track model and an auto-regressive (AR) TC intensity model. The genesis model seeds weak vortices throughout the domain, which are then passed to the intensity and track models to determine the rest of their life cycles. The genesis model's seeding rate is computed based on the local environment, vorticity, shear, relative sea surface temperature and other factors. The AR TC intensity model includes an empirical deterministic component dependent upon the large-scale environment and a stochastic forcing representing nonlinear influences on TC intensification. Models are developed and verified against reanalysis and historical best-track TC data. The best simulated intensity climatology was found using stochastic forcing and the use of nonlinear potential intensity terms in the deterministic forcing. The modeled genesis and track densities are similar to those of the historical records although the tracks are slightly smoother. I will also discuss the estimated risk from the integrated model using all three components.
Nov. 1Bjorn StevensMax-Planck Institute - Hamburg GermanyOpaque aspects of convection-circulation coupling in the tropical atmosphere.
Opaque aspects of convection-circulation coupling in the tropical atmosphere.
Nov. 9Mitch BushukGFDLSeasonal prediction and predictability of Arctic sea ice in the GFDL forecast system
The rapid decline of Arctic sea-ice extent (SIE) has created an increased need for skillful seasonal predictions of sea ice. Currently there is a significant gap between the potential predictability of SIE and the forecast skill of operational prediction systems. In this work, we explore avenues for closing this gap in the context of the GFDL prediction system. First, using a 700-year control integration and a suite of initialized forecast ensemble experiments, we identify sea-ice thickness and the ice-albedo feedback as key sources of predictability for summer SIE. We find that thickness anomalies are persistent on interannual timescales and, moreover, that these anomalies are enhanced over the summer months by a positive feedback between the sea-ice state and surface albedo. Second, we move towards stakeholder-relevant spatial scales, investigating regional SIE prediction skill in a suite of retrospective seasonal forecasts spanning 1981-2015. The regional SIE prediction skill scores are highly region and target month dependent, but generically exceed the skill of a damped persistence forecast. Notably high skill is found for winter and spring SIE in the Barents and Labrador Seas, which is partially attributable to the model's initialization and persistence of subsurface ocean temperature anomalies.
Nov. 15Early Career Town Hall MeetingEarly Career Town Hall Meeting
Early Career Town Hall Meeting - Led by Charles Stock
Nov. 16Brandon ReichlGFDLAn Energetic Model of the Upper Ocean Turbulent Boundary Layer Including Surface Wave Effects
It is necessary for ocean climate simulations to model the impact of turbulent vertical mixing within the ocean surface boundary layer. Reasons for this include accurate modeling of air-sea fluxes of heat and gases and accurate simulation of the exchange of properties (heat and chemical compounds) between the surface ocean and the ocean interior. Present methods for modeling the upper ocean turbulent mixing for climate applications are unsatisfactory for a variety of reasons (e.g. computational inefficiency, lack of energetic consistency, tenuous physical assumptions). Therefore, a new energetically constrained, computationally appealing vertical mixing parameterization (the Energetic Planetary Boundary Layer, or EPBL) has been under development at GFDL for the past several years. EPBL has recently undergone rigorous testing and been further developed through careful coefficient optimization and improved parameterization of physical processes. My talk will discuss the features of the EPBL model and methods to improve and test its performance, including guidance from more complete and higher resolution (but computationally demanding) simulations. Because the surface of the ocean is dominated by interfacial waves, any physically realistic model must also include their impact on upper ocean processes. Therefore, I will also discuss how we will implement surface gravity wave impacts within EPBL to better capture physical processes that drive mixing in certain regimes. Preliminary results suggest that the improved model reduces biases in sea surface temperature and mixed layer depth in GFDL's CM4 climate simulations. Remaining biases correlate well with regions of strong surface wave activity, suggesting their improved representation will result in further model improvement.
Nov. 28David TrossmanJohns Hopkins UniversityBottom-flow topography interaction and roles of large-scale ocean circulation in climate
This talk is divided into two parts. The first part is about the roles of the ocean circulation in setting the pace of 1) ocean deoxygenation and 2) transient climate change. The dominant factors involved in the decrease of oceanic oxygen concentrations under anthropogenic climate change are investigated using an ensemble of fully coupled climate change simulations under a 1% to doubling of carbon dioxide scenario. To better quantify the ocean circulation's role in ocean deoxygenation, these simulations are compared to identical ones in which the ocean circulation is held constant and an oxygen budget analysis is performed. The role of the ocean circulation in transient climate change is addressed by performing additional simulations in which the cloud radiation fields, or both the ocean circulation and cloud radiation fields, are held constant. Hypothesized mechanisms for ocean circulation-cloud interaction are outlined. The second part of this talk is about the interaction between oceanic bottom flows and the underlying topography. A theory (Garner, 2005) that accounts for the drag associated with the generation of internal lee waves arising from geostrophic flow impinging upon rough topography and topographic blocking is assessed using observations in two Southern Ocean regions where internal tides are small. The theory is inserted into an eddying global ocean model at two different horizontal resolutions by affecting its momentum equations in the bottom 500 meters. Model results with and without the theory included as a parameterization are compared to satellite altimetry and current meter observations. An energy budget analysis is performed to address a hypothesized mechanism for the dynamical changes in the simulations. Reasons for the relatively small impacts on sea surface statistics in response to large increases in abyssal dissipation are further explored in an ensemble of quasi-geostrophic turbulence and more realistic ocean model simulations. Future directions are outlined for each of the topics discussed here.
Nov. 29Samantha StevensonNCARThe El Nino/Southern Oscillation and Climate Change: Using Large Ensembles to Advance Dynamical Understanding
Understanding the sensitivity of the El Nino/Southern Oscillation (ENSO) to climate change is critical for improvements to projections of future risks associated with climate variability. However, efforts to assess the degree to which shifts in ENSO characteristics can be attributed to external forcing are subject to uncertainties due to both differing model physics and internal ENSO variability. New community ensembles created at the National Center for Atmospheric Research and the NOAA Geophysical Fluid Dynamics Laboratory are ideally suited to addressing this problem, providing many realizations of the climate of the 850-2100 period with a combination of both natural and anthropogenic climate forcing factors. Here the impacts of external forcing on ENSO dynamics are assessed using four sets of simulations: the CESM Last Millennium Ensemble (CESM-LME), which covers the 850-2005 period and provides long-term context for forced responses; the Large Ensemble (CESM-LE), which includes 20th century and 21st century (RCP8.5) projections; the Medium Ensemble (CESM-ME), which is composed of 21st century RCP4.5 projections; and a large ensemble run with the GFDL ESM2M, which includes 20th century and RCP8.5 projections. Over the 20th century, the CESM-LME shows that greenhouse gas and anthropogenic aerosol influences tend to counteract one another, and that a leading cause of alterations in El Nino evolution may be shifts in the seasonal cycle of sea surface temperature due to land use changes. In the 21st century the effects of greenhouse gases naturally dominate, and with a sufficiently large model ensemble the differences between projections with differing magnitudes of greenhouse forcing become detectable. However, comparison of CESM and ESM2M 21st century projections shows that the mechanisms for climate change responses differ drastically. This seems to be strongly linked with differences in mean-state climate shifts between the CESM and ESM2M, where upwelling-driven changes in sea surface temperature tend to be favored under climate change in CESM and suppressed in ESM2M. To definitively attribute inter-model and inter-ensemble differences in ENSO behavior, dedicated process studies will be required, and potential future directions will be discussed.
Nov. 29Indrani PalColumbia UniversitySpatio-temporal variability and changes in streamflow drought indicators in the major U.S. river basins
Extreme low streamflow characteristics are important indicators of water scarcity within the broader context of droughts. We explore spatial variability patterns of annual low flow magnitudes and frequency of dry days (i.e. number of days a river goes dry), historical evolution of their trends for the region-specific data, and report their periodicity patterns, using data for the U.S. "Data-driven" extreme value clustering technique is used for the spatial variability study. Complete Ensemble Empirical Mode Decomposition (CEEMD), which is suitable for nonlinear and nonstationary signals, is used for the trend detection, Multitaper Spectral Analysis method (MTM) is used for periodicity detection. Long-term data indicate that more river basins are having positive significant linear trends in low flows than negative significant trends; however, the fraction of total number of stations with negative trends has been increasing in the last four decades, making a gradual reduction in the wetting tendency in low flows in the U.S. We also report a reversal in linear trends observed for some of the northern regions, since 1980s, which yielded a further analysis indicating that most of the river basins, in fact, display nonlinear trends in annual low flow magnitudes - indicating region-level impacts of multiple factors on low flow characteristics. Most notably, recent increasing trends are noticed over the northeast and east, while decreasing trends are in evidence in the southern plains and the Pacific northwest. In addition, regionally consistent, El-Nino Southern Oscillation (ENSO)-type periodic signals are particularly evident for the low flows in the Great Lakes region, the central part of the U.S., and also the Southern Plains where low flow magnitudes indicated a periodicity ranging from 3 to 6 years. Cumulative frequency of dry days yields large-scale climate connections, which I also discuss in the end. Overall, these studies indicate connections between climate and water systems in the U.S. during the driest time of the year.
Nov. 30Benjamin SulmanPrinceton/GFDLNext generation nitrogen cycle processes in GFDL's land model
Nitrogen limitation to plant growth has the potential to limit terrestrial carbon cycle responses to increasing atmospheric CO2 concentrations, and as a result including the terrestrial nitrogen cycle is important for developing accurate next-generation ESMs. Previous ESMs have either omitted the nitrogen cycle, assuming that nitrogen is not limiting, or used a law-of-the-minimum approach that limits plant growth based on nitrogen released from independent soil decomposition processes. I present a new coupled model of plant-soil carbon and nitrogen cycling that simulates interactions between plants, symbiotic fungi, and soils, implemented within the GFDL land model. Within this framework, plants can spend carbon to mobilize soil nitrogen when they become nitrogen limited, while at the same time accelerating the decomposition of soil carbon and increasing soil CO2 emissions. The model represents known differences in nitrogen acquisition strategies between plant functional types, and successfully reproduces observed ecosystem responses from elevated CO2 experiments. In global simulations, the model successfully predicts global distributions of plant functional groups and carbon and nitrogen stocks as well as gaseous and aqueous nitrogen fluxes. Preliminary model results suggest that plant nitrogen acquisition strategies under increasing atmospheric CO2 have important consequences for the future evolution of terrestrial carbon pools.
Dec. 1Isla SimpsonNational Center for Atmospheric ResearchFuture predictions of the Northern Hemisphere mid-latitude circulation: mechanisms and model diversity
A critical aspect of human-induced climate change is how it will affect regional hydroclimate around the world. To leading order, the increased ability of the atmosphere to hold moisture as it warms, intensifies moisture transports, making sub-tropical dry regions drier and mid-latitude wet regions wetter. But regional changes in hydroclimate will also depend on how the atmospheric circulation responds to warming. Here, the predictions of the future of the Northern Hemisphere circulation during wintertime by the current generation of global climate models will be discussed, with a particular focus on circulation changes that impact on regional hydroclimate. In the Pacific-North American Sector, during Northern Hemisphere winter, changes in the stationary waves are a leading order effect. The focus will be on two aspects of circulation change in this region. The first is the predicted change in the meridional wind field over North America, which is closely linked to North American hydroclimate change. The second is the predicted poleward shifting of the westerlies in the mid-latitude Pacific and its relation to tropical circulation change. There is considerable diversity among the models in both these aspects of future circulation change, with important implications for regional hydroclimate predictions.
Dec. 2Hanna Zanowski Final Public Oral ExamThe Influence of Antarctic Open-Ocean Polynyas on the Abyssal Ocean
Hanna Zanowski Final Public Oral Exam
Dec. 7Stuart EvansGFDLSimulating dust variability and its effects on regional climate
Observations of atmospheric dust from various regions of the world have shown high variability at seasonal, interannual, and decadal time scales. Models, on the other hand, typically produce very little variability. Here we introduce a soil water and vegetation aware dust emission parameterization that increases long term dust variability, and dramatically improves the relationship between Australian dust and El Nino. We find that the radiative effects of this dust inhibits local precipitation and enhances El Nino related droughts in Australia. Finally, we discuss additional implications of this dust variability, particularly for the oceanic carbon cycle.
Dec. 9James WilliamsMcGill UniversityDynamic Preconditioning of the Minimum September Sea-ice Extent
There has been an increased interest in seasonal forecasting of the Arctic sea ice extent in recent years, in particular the minimum sea ice extent. Here, a dynamical mechanism, based on winter preconditioning, is found to explain a significant fraction of the variance in the anomaly of the September sea ice extent from the long-term linear trend. To this end, a Lagrangian trajectory model is used to backtrack the September sea ice edge to any time during the previous winter and quantify the amount of sea ice advection away from the Eurasian and Alaskan coastlines as well as the Fram Strait sea ice export. The late-winter anomalous sea ice drift away from the coastline is highly correlated with the following September sea ice extent minimum (r = -0.66). It is found that the winter mean Fram Strait sea ice export anomaly is also correlated with the minimum sea ice extent the following summer (r = -0.74). To develop a hindcast model of the September sea ice extent—which does not depend on a priori knowledge of the minimum sea ice extent—a synthetic ice edge initialized at the beginning of the melt season (1 June) is backtracked. It is found that using a multivariate regression model of the September sea ice extent anomaly based on ice export from the peripheral Arctic seas and Fram Strait ice export as predictors reduces the error by 38%. A hindcast model based on the mean December-April Arctic Oscillation index alone reduces the error by 24%.