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CGD 2008 Profiles in Science: Climate Analysis

The mission of the Climate Analysis Section is to increase the understanding of the atmosphere and climate system through empirical studies and diagnostic analyses of the atmosphere and its interactions with the Earth's surface and oceans on a wide range of scales with a particular goal of contributing to the building of a climate information system.

Emphasis of research is on the atmospheric and oceanic general circulations, meteorological phenomena such as tropical cyclones, global warming, the hydrological cycle, and climate variations over several time scales. Research has focused on interannual variations, such as the El Nino-Southern Oscillation and the North Atlantic Oscillation phenomena; decadal variations, such as the Pacific Decadal Oscillation; and longer-period trends, and their climate forcings. Attribution and mitigation of climate change are also topics of in-depth research. These studies are now framed in the context of the need for a climate information system for adaptation to climate change.

Rationale

The IPCC in its 2007 Fourth Assessment Report (AR4) found that global warming is "unequivocal" and is "very likely" caused by human activities. Moreover, the climate system inertia and existing human infrastructure guarantees that further climate change will occur. Accordingly, while mitigation of global warming is essential by reducing greenhouse gas emissions, adaptation to climate change is also essential. How we best do that is a major issue requiring interactions between physical scientists, who provide the best information possible, and social scientists, who are involved in how best society can adapt to the projected changes to minimize impacts, reduce vulnerability, and best use the information to improve decisions. We should adapt to climate change by planning for it and making better predictions of likely outcomes on several time horizons.

Climate Information System

Accordingly, we have argued that an imperative is to build a climate information system. The first step is a more comprehensive analysis and assessment of the current state of the climate system, as well as what has happened recently and an assessment of why. This means making full use of available observations of not just the state of the climate system, but also the forcings [the atmospheric composition (greenhouse gases and aerosols), the sun (across the electromagnetic spectrum (ultraviolet to infrared)), and the land surface (changes in vegetation and its health, changes in land use etc)]. It also means linking the observed changes to the forcings and fully understanding why recent climate anomalies have occurred (such as the role of the recent La Niña). This immediately informs the prospects for reliable predictions. However, we already have a lot of observations and analyses. Some are not processed as quickly as they might be (near real time) and it is only a first step to produce the information base. It is another step entirely to promulgate and dispense this information to decision makers and users through a full climate service that also seeks feedback from and informs users about prospects for new products.

CAS is making important contributions to all key components of a climate information system, including:

  • Observations: CAS evaluates observations and promotes their reprocessing and reanalysis into global fields. We advocate improved observations and analysis suitable for climate (satisfying the climate principles that are designed to ensure continuity of record). This especially includes those from space. CAS maintains a data catalog and works to enable access to data and products through the internet via the CAS web pages.
  • Analysis: CAS develops analytical and diagnostic techniques to process observations and model data, and facilitates their comparison and evaluation. In part, this is through the development, exploitation and promotion of NCL. Value-added derived products are also developed and archived in the data catalog and new datasets are made available.
  • Assimilation: CAS advocates analysis of observations into forms suitable for use in models and to initialize models. Within NCAR this is mainly done by MMM and the DART group in CISL. Assimilation enables reanalysis and model diagnostics that can be compared with observations to evaluate and improve models.
  • Attribution: CAS has carried out many studies on mechanisms and modes of variability that have contributed to observed climate anomalies. CAS helps develop capabilities that contribute to an operational attribution activity by pioneering studies and numerical experimentation that might be used in near real time to allow reliable statements to be made not only about what the state of the climate is, but also why it is the way it is. Studies involve the atmosphere and the fully coupled climate system.
  • Assessment: CAS uses the information from the analyses and other products to assess the state of the climate. Scientists participate in national (CCSP) and international (IPCC) assessments.
  • Prediction and predictability: Predictions are required on multiple time scales and it is evident from the inertia in the climate system and the forcings that there is some predictability associated with the initial state of the climate. CAS studies are carried out to assess the predictability associated with the initial state and thermal inertia, modes of variability, internal mechanisms and coupling among climate system components, and forcings. Evaluations of model strengths and weaknesses, possible improvements, comparisons among models and with observations, and evaluations to score their results in multi-model ensembles are underway. CAS scientists are also involved in regional climate model studies using embedded models (the Nested Regional Climate Model NRCM) in order to adequately represent scales of motion thought to be important.
  • Decision making: CAS contributes to how to reduce vulnerability and what the impacts will likely be associated with climate variability and change that in turn contribute to adaptation and risk assessment, such as to ecosystems, water resources, and communities. There is significant interaction between CAS scientists, the public and various stakeholder groups.

Topics of emphasis over the past year have included the following:

The global water cycle in the climate system and models

Components of the hydrological cycle studied include water vapor, precipitation (amount, frequency, intensity, type), evapotranspiration (evaporation plus transpiration from plants), soil moisture, runoff, streamflow and river discharge into the oceans, atmospheric moisture flows and divergence, and atmospheric moisture storage. The diurnal cycle, annual cycle, variability, extremes, and trends and their links to global warming are explored, along with effects of geoengineering. Evaluations focus on reanalyses and climate model simulations and projections. The runoff, river flow and river discharge into the oceans are being analyzed as time series for post 1948, and will be made available. The river discharge is being combined with estimates of precipitation minus evaporation over the oceans to explore fresh water budgets of the ocean, and thus implied flows of fresh water. Many variables in the hydrological cycle that are derived separately are being physically constrained to produce more reliable products.

CAS activities are thus a major contributor to NCAR's water cycle program.

Tropical cyclones and climate

How tropical cyclones have changed and their causes have been explored in several studies and suggest that observed century-timescale sea surface temperature (SST) changes cannot be explained solely by unforced variability of the climate system. Twentieth-century model simulations, with external forcing by combined anthropogenic and natural factors, are generally capable of replicating observed SST increases. Another major topic has been the energy and water cycles of hurricanes and their role in the climate system. Results demonstrate the overwhelming dominance of moisture convergence into the storms in precipitation, in spite of the critical role of the surface evaporative source, and have implications for the changing environment on hurricanes as climate changes. These model results have been related empirically to the maximum sustained wind in the model and the results used with the "best track" global observed data on tropical cyclones to deduce how surface fluxes and precipitation in hurricanes have changed since 1970. A detailed analysis of the Atlantic changes in tropical storms and the environmental variables has been completed. Hurricanes play a key and increasingly important role in climate as SSTs rise. The ability of NRCM to capture observed variations in tropical cyclones over the past decade has been analyzed, as this will be one tool to explore the up-scaling effect of hurricanes on climate.

Dataset development

Evaluation, exploitation and promotion of atmospheric reanalyses are important ongoing activities. New measurements of atmospheric structure using GPS Radio Occultation from the COSMIC mission are occurring. A new surface boundary forcing data set for uncoupled simulations with the Community Atmosphere Model has been developed and is updated monthly and freely available for community use.

Attribution and diagnostic analyses of ocean-atmosphere-sea ice variability using models and observations

Many studies document ocean-atmosphere coupling and phenomena in the ocean, including tropical instability waves, and the climatological semidiurnal and diurnal variations of surface winds in the tropical Pacific. New analyses have been made of the vertical structure of the atmosphere and how it evolves with El Niño.

Increased understanding of observed regional climate variability and change occurs through parallel development and analysis of observational and model-generated datasets and through systematic numerical experimentation to allow attribution of observed variability to processes and causes. Using suites of atmospheric general circulation model (AGCM) simulations forced by global, regional, and idealized SST variations, as well as Arctic sea ice and terrestrial snow cover variations, we are able to diagnose the factors contributing to a variety of circulation patterns and their changes over time, as well as their impacts on rainfall and air temperatures. Other new experiments were targeted specifically to elucidate the role of oceanic variability.

In particular, multi-decadal variations and trends in SST have primarily determined the spatial patterns, time history and seasonality of observed changes in African and North American climate over the past century, as well as being a key driver of wintertime North Atlantic climate change, including changes to the phase and amplitude of the North Atlantic Oscillation (NAO). Other examples include the role of the Bering Strait in the global climate response to freshening of the North Atlantic Ocean; the impact of year-to-year memory in the ocean mixed layer system upon the persistence of the NAO; the impact of future Arctic sea ice loss upon the atmosphere and climate; the role of the tropics in forcing the Pacific Decadal Oscillation, including the 1976-77 climate transition and it's impact upon marine ecosystems; and the direct and indirect roles of increasing greenhouse gases upon global atmospheric circulation trends.. Observational studies include the role of daily wind forcing of Arctic sea ice export through Fram Strait and the role of atmospheric circulation trends in contributing to the accelerating loss of Arctic sea ice. Dominant regional modes of atmospheric circulation variability have profound impacts on a variety of ecological processes and, consequently, patterns of species abundance and dynamics.

Present and Future Climate Change Research

The CCR Section has a Climate Change and Prediction (CCP) group which focuses on present and future climate change which is primarily funded by the DOE's Office of Biological and Environmental Research program which has a Climate Change subprogram with one of its goals being to "predict accurately any global and regional climate change induced by increasing atmospheric concentrations of aerosols and greenhouse gases." This is DOE's contribution to the U.S. Climate Change Science Program that integrates federal research on global change and climate change. CCP has concentrated on the decade to centuries time scales goal which is part of the priorities for the federal Climate Change Science Program (CCSP).

Energy budgets

Global energy and water cycle components contain considerable uncertainties but global constraints can be used to refine estimates over land, deduce ocean heat and water transports as residuals, and establish errors. CAS continues to update and extend previous analyses of the energy and moisture budgets in light of the new CERES top-of-atmosphere (TOA) radiation and several reanalyses, including the ECMWF ERA-40 and Japanese 25-year Reanalysis JRA-25, and we are interacting to produce new NASA reanalyses (MERRA).

Estimates of the atmospheric energy storage and transport, and estimates of the surface energy budget have been derived using many different datasets, including all the available reanalyses. These, combined with estimates of ocean heat content from several ocean datasets have contributed to an unprecedented analysis of the flow and storage of energy in the climate system and have contributed greatly to a quantification of existing uncertainties. These observationally based estimates are being used to evaluate reanalyses and global climate models from the AR4 CMIP3 archive, and reveal problems with the archive, problems with models, and insights into climate sensitivity.

The moisture and energy cycle equations have also been used to compute the contributions to diabatic heating and moistening of the atmosphere in three dimensions based on ERA-40 reanalyses. A partitioning has been performed into mean and transient transports of energy as well as horizontal and vertical fluxes. Comparisons with model generated fields were evaluated with help from observations of precipitation and radiation at top-of-atmosphere to reveal biases in the ECMWF assimilating model, and similarly for the Japanese model based on JRA-25 reanalyses. Variability over 1979 to 2002 of some energy transports and divergences highlight difficulties in dealing with changes in the observing system.

Climate forcings

Continued exploration of the role of solar forcing, and in developing scenarios for future climate and mitigation strategies have been pursued along with prospects and hazards of geoengineering solutions.

Climate predictions

All climate system predictions, regardless of timescale, may require initialization of coupled general circulation models with best estimates of the current observed state of the atmosphere, oceans, cryosphere, and land surface. Fundamental barriers to advancing prediction on time scales from days to years, as well as long-standing systematic errors in weather and climate models, are also partly attributable to our limited understanding and capability to simulate the complex, multi-scale interactions intrinsic to atmospheric and oceanic fluid motions. Therefore, considerable time over the past year has been devoted to building an effort to develop a community NRCM, which is a major new initiative toward prediction across scales. In addition, prototype initialized experiments using CCSM are being analyzed with a focus on the predictability of Atlantic climate on decadal time scales.

Progress has been made on all of the above topics in the past year, as detailed by the individual CAS scientist reports and support staff reports.

CAS Profiles in Science: Scientists

CAS Profiles in Science: Scientific Support Staff