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The impacts of climate and weather on society and ecosystemsWhereas in the past, meteorology and climatology were separate fields, be it only because of disparate time (and length scales as well), it appears today that the two fields are strongly coupled, not only as the climate gives the boundaries for investigating the weather, but also because localized events can influence the larger climatological scales. The specific items on which ESSL scientists focused in this year are related to the role of aerosols in climate and weather, to the coupling of eco-systems, biochemistry and climate, to climate change, climate variability and extreme weather such as hurricanes, to interactions of the water cycle with climate and weather, to the impacts of climate and weather on society and ecosystems and finally to megacities and the effects of urbanization; the latter priority is a highlight for NCAR and concerns the international multi-agency field campaign that took place in Mexico city and combined interactive modeling as well. The laboratory highlights are related to the role of aerosols, to the regional carbon cycle, to a numerical simulation of turbulence, to landfall of hurricanes, to the global and regional water cycles and to polar climate. The impacts of climate and weather on society and ecosystems:
Polar climate [Highlight] - CGD
The impacts of climate and weather on society and ecosystems: Polar Climate
Arctic sea ice has undergone rapid retreat in recent decades and climate models project a continued decline into the foreseeable future, with the possibility of summer ice-free conditions being reached later this century. Considerable effort is underway to examine these observed and projected changes in the sea ice system and the consequences of a seasonally ice-free Arctic ocean for the climate system. Ongoing research indicates that the observed winter ice cover anomalies show less impact from the North Atlantic Oscillation in recent years and exhibit a more spatially uniform decrease since 2000. An analysis of projected changes in the future ice cover suggests that gradual, linear changes are unlikely. Instead, Community Climate System Model (CCSM) integrations exhibit abrupt reductions in the future summer sea ice cover, with the most extreme event going from 80% September ice coverage to 20% coverage in approximately 10 years (Figure). These transitions result from: 1) an increased efficiency of open water production as the Arctic ice thins, 2) rapid increases in ocean heat transport that trigger the events, and 3) the surface albedo feedback, which accelerates the ice retreat. Research is underway to further assess these factors within the CCSM and other climate model simulations, in particular those that are contributing to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4). The presence of seasonally ice-free conditions has potential impacts on the Arctic and global systems and CCSM experiments are being used to assess the impacts on global atmospheric circulation, the hydrological cycle and ocean conditions. This collaborative work involves numerous scientists within ESSL, at a number of different universities and at government laboratories. Associated with reductions in sea ice and Arctic warming are indications that the permafrost is warming and thawing. Large-scale thawing of permafrost is likely to induce a number of feedbacks to the hydrologic and carbon cycles of the Arctic system. Of particular concern, especially from a global perspective, is how permafrost thaw will affect the carbon balance in the Arctic. A high-latitude terrestrial climate change feedback project has been initiated to investigate this issue. This interdisciplinary project aims to improve our ability to simulate, understand, and predict high-latitude terrestrial climate feedbacks in CCSM with a particular goal to develop a version of CCSM that can address the critical carbon issues in the Arctic tundra. These issues include, but are not limited to, the accumulation and loss of carbon in organic or peatland type soil profiles, the partitioning of carbon emission between methane and carbon dioxide, hydrologic cycle change related to permafrost degradation, and the interaction between temperature, nitrogen cycling and the transition between herbaceous tundra and woody arctic shrubland. Initial efforts, in collaboration with university partner permafrost process specialists, have focused on improvements in the simulation of the thermal and hydrologic state of permafrost, namely inclusion of organic or peatland soil and a deeper soil column into the Community Land Model (CLM). This research is sponsored by NSF and DOE. Changing sea ice and permafrost conditions have important implications for the Arctic hydrological system change. Because of the proximity to deep water formation regions within the northern North Atlantic, this in turn can modify the global thermohaline circulation. Future projections of the Arctic freshwater budgets and their influence on deep water formation regions have been assessed in CCSM-3 IPCC-AR4 simulations. Additional experiments are underway to further examine the role of changing ice-ocean freshwater exchange on ocean circulation changes in future climate conditions. Additionally, the influence of the Bering Strait throughflow on the thermohaline circulation sensitivity to freshwater flux perturbations has been examined and found to play an important stabilizing role. Additional experiments to further examine and quantify these effects are underway.
The impacts of climate and weather on society and ecosystems: Water Cycle
As part of the Water Cycle Program, involving scientists across ESSL and well as other national lab and university colleagues, various aspects of the water cycle in observations and climate models has been examined. The focus is on precipitation, atmospheric water vapor, and land surface water fluxes, with the goal to improve our understanding and thus modeling and prediction of atmospheric moist convection, precipitation processes, and land surface hydrology on all time scales. The diurnal cycle of warm-season precipitation over the U.S. and other parts of the world has been employed as a means to systematically examine precipitation characteristics (onset, diurnal timing, frequency, intensity, duration, amount, type, etc.) in data and models, thus allowing a diagnosis of deficiencies in weather and climate models. The Water Cycle Program also interacts with other ESSL programs such as the Biogeoscience Program. Furthermore, data sets and model evaluation work produced under this project are helpful for improving the Community Climate System Model (CCSM) and other climate models. The project also leverages other NSF and NOAA-funded studies related to the water cycle. Recent work under this project includes 1) quantifying recent variability and trends in global surface humidity and total column-integrated water vapor content using surface and satellite data, 2) a new estimate of the mean global hydrological cycle that includes estimates of the main reservoirs and fluxes of water, as well as their annual cycle, with extensive use of reanalysis, satellite and surface data sets, 3) quantifying the various components (precipitation, evapotranspiration, soil moisture, streamflow, atmospheric water vapor flux, etc.) of the water cycle and their changes over the past 57 years and under global warming, such as potential drying over land (Figure), 4) documenting deficiencies in monitoring global cloudiness and recent trends in cloud cover, and 5) analyses of model-simulated precipitation and other hydrologic fields. These studies have resulted in a number (see Aiguo Dai and Kevin Trenberth) of refereed publications that are discussed extensively in the upcoming IPCC Fourth Assessment Report (AR4). For example, the work on the increased drying over land and drought during recent decades (Figure) is among the first to address a very important aspect of global warming; namely, changes in land surface moisture conditions that have a huge impact on agriculture, ecosystems, and water resources. Another important finding from this project is that water vapor content near the surface and in the atmosphere has been increasing during recent decades over global land and oceans together with increasing temperature, while trends in relative humidity are small. Hence water vapor provides an important feedback in the climate system through its greenhouse effect, roughly doubling the response to perturbations, as has generally been suggested in models. An analysis of the IPCC AR4 model precipitation shows that the latest generation of world’s climate models still rain too frequently at reduced intensity, a major deficiency for simulating future changes in extreme events such as heavy rainfall, flash floods and drought. Future plans call for further work with various models and joint work with modelers on various scales to see to what extent the models can be improved while still working with many different datasets (including those from recent satellite observations) to perform in depth diagnoses, including determining the vertical structure of water vapor and precipitation in the atmosphere. Funding Agencies: NSF, NOAA
Climate Change: Vertical temperature structure and extremes
NCAR was one of the first centers to study anthropogenic climate change with global coupled climate models starting in the late-1970s. Consequently, the earliest climate change experiments done at that time were pioneering at a national and international level. Few groups were doing climate change modeling as it was considered to be a sidelight to other more highly regarded modeling problems. NCAR climate change modeling (funded by DOE and NSF) was prominent in the DOE State-of-the-Art climate change assessments in the late 1980s, and in the first Intergovernmental Panel on Climate Change (IPCC) assessment in 1990 and the 1992 IPCC update, since only four groups in the world (including NCAR) had functioning global coupled climate models that were being used for climate change projections. Since then, climate change modeling has become a very prominent activity at NCAR, most recently through the Community Climate System Model (CCSM) effort. It is now a headline activity for ESSL. As climate change modeling evolves to include more complexity, we are moving toward an earth system model-type activity. This crosses division boundaries in ESSL and requires close cooperation with the other science divisions since such earth system models will include not only the basic atmosphere-ocean-land surface-sea ice global coupled model, but also components of chemistry, aerosols, dynamic vegetation and carbon cycle. In addition to multiple papers describing results from climate change modeling, ESSL scientists have been involved with research that has directly influenced and characterized national and international assessment activities. For example, one of the DOE and NSF funded models at NCAR, the Parallel Climate Model (PCM), has been run for the most extensive set of natural and anthropogenic forcing experiments anywhere in the world. These 20th century ensemble experiments include (singly and in various combinations), greenhouse gases (GHGs), sulfate aerosols, ozone, volcanoes and solar forcing. When performed for single forcing experiments, the unique effect of each forcing individually can be identified and used for detection/attribution applications. One such instance of these experiments being used for an assessment is the Climate Change Science Program (CCSP) assessment of vertical temperature changes. Zonal mean temperatures from the single forcing experiments were used to show that GHGs were the main cause of warming in the troposphere that is greater than at the surface, with ozone changes causing stratospheric cooling to extend further into the upper troposphere. The CCSP assessment product was motivated by previously reported discrepancies between the amount of warming near the surface and higher in the atmosphere that has occurred over the past 25 years. These discrepancies have been used to challenge the reliability of climate models and even the reality of human-induced climate change. The major conclusion of the report is that the discrepancy no longer exists, because errors in the upper atmosphere data have been identified and corrected. New data sets have been developed that do not show such discrepancies. Another focus of climate change research has been weather and climate extremes, and a number of papers have been written in the past several years studying the effects of anthropogenic climate change on frost days, heat waves and precipitation intensity. These analyses have dealt with looking at the NCAR models (CCSM3 and PCM) in addition to other models run for the IPCC Fourth Assessment Report (AR4). One such effort addressed changes in a number of extremes indices for both temperature and precipitation (Figure). Results from that study are being featured in the upcoming IPCC AR4. Future research priorities in climate change modeling include addressing the effects of including new forcings on the climate system response (e.g. carbon aerosols), regime-dependent changes of extremes (e.g. El Nino and extremes), the mechanisms involved with natural forcings on the climate change (e.g. solar), and interplay between externally forced climate change and internally generated climate variability (e.g. decadal).
Role of the oceans in climate
Covering 71% of the Earth, the oceans absorb the majority of the solar energy reaching the surface, and are the dominant source of evapotranspiration of water vapor to the atmosphere. The heat capacity of the upper three meters of the oceans exceeds that of the entire atmosphere, and the oceans contain approximately 50 times greater inventory of CO2 than the atmosphere. Ocean currents accomplish roughly equivalent energy transport from the tropics to higher latitudes as does the atmosphere. Through this capacity for storing and transporting energy, water, and radiatively active gases, the oceans act to moderate, modulate, and initiate climate variability and climate change. A comprehensive understanding of, and the ability to predict, the behavior of the climate system must therefore be based on an understanding of the physical, chemical, and biological processes operating in the oceans and their interaction with other components of the Earth system. Research to develop an understanding of ocean processes, and using this understanding in improving their representation in ocean models, OS ocean model development supports a broad spectrum of ESSL scientific objectives. These include: prediction of the Earth’s energy, water and biogeochemical cycles, and understanding natural and human influenced climate variability, including high impact variations such as sea level rise. In turn, the ESSL objective of understanding two-way scale interactions within the Earth system is central to improving our understanding of how ocean circulation features such as coastal upwelling zones, western boundary currents, and meso-scale eddies are affected by and affect the basin- to global-scale ocean circulation. Direct estimates of the climatological average global ocean heat budget are precluded by the paucity of observations of the ocean interior. ESSL scientists have combined several independent estimates of the air-sea heat flux based on ERA-40 and NCEP/NCAR reanalyses, and CERES and ERBE net top-of-atmosphere radiation fluxes with several analyses of historical ocean temperatures (NOAA/NODC, NOAA/GODAS, JMA) to arrive at estimates of the oceanic meridional heat transport and divergences, including their annual cycles. These estimates and their uncertainties will be used to evaluate the simulations of the ocean surface energy budget in the historical simulations submitted to the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC), including the Community Climate System Model (CCSM). Variations in ocean heat transport divergence can be forced by local changes in air-sea fluxes, as well as drive those changes. An example of the latter has been analyzed in simulations of CCSM. Anomalous ocean geostrophic heat flux convergence was found to be responsible for decadal time scale variability of surface heat fluxes in the Kuroshio extension region. The anomalous geostrophic transports are, in turn, a response to anomalous basin scale wind stress curl patterns 4 to 5 years earlier. The consequent feedback to the atmosphere maintains a coupled atmosphere-ocean mode of variability. This research is now being extended with more detailed investigations of the atmospheric response to imposed heat flux anomalies using CAM3. Using suites of atmospheric general circulation model (AGCM) simulations, ESSL scientists and collaborators have found that multi-decadal variations and trends in SSTs have determined the spatial patterns, time history and seasonality of observed changes in African rainfall since 1950. The multi-model ensemble mean from 80 separate 50-yr Global Ocean Global Atmosphere (GOGA) simulations from five different AGCMs realistically captured the observed seasonality and spatial structure of downward trends in African rainfall, not only over the Sahel during boreal summer, but across the continent following the seasonal migration of rainfall. That the observed drying trends fell within the distribution function of the simulated trends indicates that they were likely a consequence of 20th Century SST variations. Moreover, nearly half of the GOGA rainfall trends, as well as those observed, fell outside the distributions of trends from unforced coupled models, suggesting the responsible air-sea interactions were not arising from natural variations alone. It is estimated that 25% of the CO2 emitted into the atmosphere during the industrial age has been absorbed into the ocean. The ability of the ocean to continue to take up CO2 depends, in part, on the rate at which surface waters are subducted into the interior of the ocean. The degree to which this may change in response to global warming has been investigated with CCSM3 by including an idealized tracer called “age" that directly measures the mean time it takes a water parcel to travel from the sea surface to any point in the interior of the ocean. Results indicate that the ventilation of approximately 60% of the ocean volume will decrease, and the mean age will increase by approximately 10 years independent of the particular emission scenario considered. Waters associated with deep convection in the North Atlantic, which account for a significant fraction of the anthropogenic CO2 inventory, undergo some of the strongest changes, suggesting that ocean ventilation changes could provide a positive feedback on global warming. New simulations being undertaken will refine our estimates of ocean ventilation time scales first by computing a probability distribution for ages as a function of space, and second by carrying out the simulations with an ocean model in which the meso-scale eddies are resolved rather than parameterized. The role of eddies in maintaining the mean state of the climate has been investigated in several studies with high resolution ocean models. Tropical instability waves, a feature of the circulation in the Atlantic and Pacific Oceans were shown to pump heat from the atmosphere into the ocean, and are an important process in the heat budget of the mixed layer of the tropical oceans. High resolution global ocean simulations have revealed the existence of eddy-driven reversing zonal jets, dynamically analogous to those seen on the giant planets, as a feature of the time mean circulation of the ocean. Plans are underway to estimate how these structures influence tracer diffusivity. For long-term climate simulations ocean models in this class are prohibitively expensive, and meso-scale eddies must be parameterized. In coordination with the Climate Process Team (CPT) EMELIE organized under the U.S. CLIVAR program, improvements to the parameterization of ocean eddies in CCSM have been implemented and are currently being evaluated. Collaboration under another CPT is contributing to the improvement of the representation of the exchange of dense bottom waters OS narrative: Ocean model development across sills and through passages such as the Strait of Gibralter. While significant progress on parameterization of transient eddies is being made, there remain aspects of the time mean ocean circulation with comparably small spatial scales. A particularly important example for climate and ecosystem studies is the coastal upwelling regions found along most of the eastern margins of ocean basins. To address these problems, an effort to develop nested regional coastal ocean models, coupled to CCSM is underway.
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