CGD's Climate Analysis Section

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. Emphasis 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; and longer-period trends and the climate forcings. Attribution and mitigation of climate change are also topics of in-depth research.

Large-scale moisture in the climate system and models

Components of the hydrological cycle studied include water vapor (Wang et al., 2006), precipitation (amount, frequency, intensity, type) (Dai et al. 2007, Tian et al. 2007)), evapotranspiration (evaporation plus transpiration from plants), soil moisture, runoff, streamflow and river discharge into the oceans (Qian et al. 2006, 2007), atmospheric moisture flows and divergence, and atmospheric moisture storage. The historical records and model simulations were analyzed to examine any changes associated with global warming in the water cycle, such as potential drying over land. A detailed study of the hydro-meteorology of the Mississippi river basin (Qian et al. 2007) utilizes both the energy and water budget constraints to put together a physically consistent picture of trends in the region. Trends from 1948 to 2004 in cloud and precipitation have reduced the solar energy available for evaporation ("dimming") and thus potential evaporation (or pan evaporation), but increased soil moisture has increased evapotranspiration as the actual evaporation has risen to become closer to the potential amount, even as runoff and streamflow have also increased. Diminished solar radiation is offset by reduced outgoing longwave radiation and the increased surface wetness has led to an increase in latent at the expense of a decrease in sensible surface heat flux.

A new estimate has been made of the global hydrological cycle for long-term annual means that includes estimates of the main reservoirs of water as well as the flows of water among them (Trenberth et al. 2007), see Fig. 1. In addition, the mean annual cycle of the atmospheric hydrological cycle based on 1979 to 2000 data includes monthly estimates of P, evapotranspiration E, atmospheric moisture convergence over land, and changes in atmospheric storage, for the major continental land masses, zonal means over land, hemispheric land means and global land means. The evapotranspiration was computed from the Community Land Model run with realistic atmospheric forcings, including precipitation constrained by observations for monthly means but with high frequency information taken from atmospheric reanalyses. Results for P-E from ERA-40 reanalyses show physically unrealistic results, especially in the tropics and subtropics. Effects of climate change on the hydrological cycle and precipitation are explored in Rasmussen et al. (2007) and Sun et al. (2007). An analysis of time series after 1948 reveals that the Mount Pinatubo volcanic eruption in 1991 was followed by record low land precipitation and river runoff (Trenberth and Dai 2007) that was so low it must have been associated with the volcanic aerosol forcing.

Analysis and evaluation of the various components of the global water cycle in climate models includes details on precipitation and moist convection (Dai et al. 2007, Sun et al. 2007), atmospheric moisture transport and land surface water fluxes and storage (soil moisture, evaporation, runoff and stream-flow, etc.).

Tropical cyclones and climate

Causes of changes in tropical cyclones have been explored in several studies. Santer et al. (2006) used 22 climate models to study the possible causes of SST changes in Atlantic and Pacific tropical storm cyclogenesis regions. The climate models examined suggest that observed century-timescale SST changes cannot be explained solely by unforced variability of the climate system. Model 20th-century simulations, with external forcing by combined anthropogenic and natural factors, are generally capable of replicating observed SST increases. Fasullo (2007) analyzed the observed best track hurricane record for sampling biases that might account for trends in the frequency of major storms, but the spurious influences of improvements in both the temporal sampling of storms and peak wind speed measurements are found to be weak.

Another major topic has been the energy and water cycles of hurricanes and their role in the climate system. One study has computed how much moisture that ends up as rain in hurricanes comes from local evaporation in the storm versus large-scale convergence (Trenberth et al. 2007a). This has been analyzed in a model framework using WRF at high resolution for realistic simulations run for observed storms, in particular Ivan in 2004 and Katrina in 2005. Models sensitivity runs have also been made with SSTs increased and decreased by 1°C. Results demonstrate the overwhelming dominance of moisture convergence into the storms, in spite of the critical role of the surface evaporative source (see Fig. 2), 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 (Trenberth and Fasullo 2007a), see Fig. 3. Hurricanes play a key role in climate and that role is increasing over time as SSTs rise.

Climate variability and observations

Evaluation, exploitation and promotion of atmospheric reanalyses is an important ongoing activity (Simmons et al. 2006). New measurements of atmospheric structure using GPS Radio Occultation from the COSMIC mission are documented in detail and climate applications are described (Anthes et al. 2007).

High-frequency ocean-atmosphere coupling are explored by Jochum et al. (2007) and Ueyama and Deser (2007). Tropical instability waves, which are due to internal ocean dynamics, have a significant impact upon surface zonal wind and precipitation variability in the tropical Pacific, and thus may be a source of "stochastic" noise in the climate system that may in turn augment the excitation of ENSO (Jochum et al. 2007). The climatological semidiurnal and diurnal variability of surface winds in the tropical Pacific, using 12 years of data from the Tropical Atmosphere Ocean Moored Buoy Array, feature a large-scale diurnal pattern of surface wind convergence in the tropical Pacific, which may have implications for the diurnal cycle of rainfall (Ueyama and Deser 2007).

Diagnostic analyses of extratropical ocean-atmosphere-sea ice variability using models and observations

Using suites of atmospheric general circulation model (AGCM) simulations forced by global, regional, and idealized sea surface temperature (SST) variations Hoerling et al. (2006) 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, e.g., see Fig. 4. No single 50-yr trend in unforced coupled ocean-atmosphere model simulations yielded a drying rate as large as observed (blue curve in Fig. 4) but the observed drying trend was likely a consequence of 20th Century SST variations, suggesting a role for more than natural variations alone.

A new monthly surface boundary forcing data set for uncoupled simulations with the CAM has been developed (Hurrell et al. 2007). It is a merged product based on the monthly mean Hadley Centre sea ice and SST data set version 1 (HadISST1) and version 2 of the NOAA weekly optimum interpolation (OI.v2) SST analysis from 1870 to the present: it is updated monthly, and it is freely available for community use. The merging procedure was designed to take full advantage of the higher resolution SST information inherent in the OI.v2 analysis, which arguably is the best global SST analysis currently available.

Understanding the transient evolution of the atmospheric circulation response in CCM3 to imposed North Atlantic/Arctic SST and sea ice anomalies was also pursued through a 240-member ensemble of experiments to show how the response evolves (Deser et al., 2007). In the first 2-3 weeks, the geopotential height response exhibits a baroclinic vertical structure localized to the vicinity of the forcing but evolves into an equivalent barotropic pattern that is hemispheric in scale and projects strongly onto the leading mode of internal variability in the control ensemble. The equilibrium response is maintained primarily by the transient eddy heat and vorticity flux convergences, while the initial response is maintained by anomalous diabatic heating associated with the imposed thermal forcing. An investigation of the impact of the SST ''re-emergence'' mechanism upon the winter-to-winter persistence of the NAO using an entraining ocean mixed layer model coupled to CAM2 (Cassou et al. 2007) shows that the reemergence of the SST "tripole" pattern in the North Atlantic enhances the winter-to-winter persistence of the NAO by 15-20%.

An analysis of North Pacific decadal variability in the 1000-yr control run of CCSM2 (Kwon and Deser 2007) has shown that anomalous geostrophic advection and associated heat divergence in the upper 200m of the ocean forces decadal (~20 yr and ~40 yr) SST anomalies in the Kuroshio Current Extension, with surface heat fluxes and Ekman currents responding to rather than driving the SST anomalies. The anomalous geostrophic currents are a result of basin-scale wind stress curl anomalies 3-5 years earlier. These results suggest that the simulated North Pacific decadal variability in CCSM2 owes its existence to two-way ocean-atmosphere coupling.

A combined observational and modeling analysis of the extent to which the 1976-77 climate transition over the North Pacific was forced by the tropics was carried out using the TOGA AMIP ensemble integrations with CAM3 for the period 1950-2001 (Deser and Phillips 2006). Approximately 75% of the "shift" is attributable to tropical SST forcing in the 10-member CAM3 AMIP ensemble. However, a similar ensemble size of CCM3 AMIP integrations shows no appreciable response in the Aleutian Low. The differences between the two model responses were traced to differences in their precipitation responses over the tropical Indian Ocean and, based on a combination of observational constraints and dynamical theory, the large and erroneous increase in rainfall over the tropical Indian Ocean after 1977 simulated by CCM3 is responsible for that model's poor simulation of the 1976-77 climate shift of the Aleutian Low. Alexander et al (2007) further explore ecosystem changes associated with the climate shift. Van Loon et al. (2007) explore observational links between solar activity and tropical Pacific coupled feedbacks to explain statistical results.

Changes in the Hadley cell and how this affects drought in the subtropics is a topic pursued by Frierson and Lu (2007) and Lu et al. (2007) that is extended to the United States by Seager et al (2007).

The role of the Bering Strait in climate is explored by Hu et al. (2007) using model experiments. Unusual temperature trends in California (Bonfils et al. 2007) are evaluated with help of model runs. Observed increases in annual-mean surface temperature are not always distinguishable from climate noise, depending on the dataset considered. However, the large positive trends in mean and daytime temperatures in late winter/early spring as well as in nighttime temperatures from January to September are inconsistent with natural internal climate variability, and thus require one or more external forcing agents to be explained.

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) (Fasullo and Trenberth 2007a,b; Trenberth and Fasullo 2007b). Effects on ocean sea level are explored by AchutaRao et al. (2007).

Serreze et al. (2007) have made a comprehensive energy analysis of the Arctic by synthesizing a variety of atmospheric and oceanic data to examine the large-scale energy budget of the Arctic. The seasonal cycle of atmospheric energy storage is strongly modulated by the net surface flux, which is also the primary driver of seasonal changes in heat storage within the Arctic Ocean. Further efforts (Fasullo and Trenberth 2007a,b) Trenberth and Fasullo (2007b) have led to the development of revised top-of-atmosphere radiation flux estimates, adjusted to agree with estimates of the global imbalances during their respective time intervals, and to provide continuity and consistency across known discontinuities in the satellite record. 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. In situ estimates of the annual variation of ocean heat content are found to be unrealistically large. The meridional structure of the annual cycle and mean energy budget of the climate system is given along with zonal mean global ocean transports and ocean basin contributions; see Fig. 5. Comparison of annual means with direct ocean observations gives reasonable agreement except in the North Atlantic, where transports from the ocean transects are slightly greater than the computed estimates.

Climate forcings

Foukal et al. (2006) analyzed physical mechanisms of solar luminosity variation, and its effect on climate. The variations measured directly from spacecraft since 1978 are too small to have contributed appreciably to the accelerated global warming over the past 30 years. Improved understanding suggests that the Sun may have had only a minor role in changes in climate over the past millennium and that brightening of the Sun is unlikely to have contributed much to global warming since the 17th century. Santer et al. (2006, 2007) have further been able to attribute changes in the ocean and water vapor to human activities using model studies. Santer and Wigley (2007) review more general progress in attribution studies.

Climate change mitigation pathways focus mostly on CO₂ but multi-gas emission scenarios can be used to meet climate targets and it is important to properly assess future emissions and how much remains airborne (Richels et al. 2007; Smith and Wigley 2006; Wigley et al. 2007; Wigley 2007). Sample pathways are derived for stabilization at 350 ppm to 750 ppm CO₂ concentrations and compared to WRE profiles. The probability of overshooting a 2°C climate target is derived by using different sets of radiative forcing peaking pathways. Geoengineering approaches may help address the climate problem but may be fraught with risk of drought or other problems (Trenberth and Dai 2007).

IPCC fourth assessment

A major achievement has been the IPCC AR4 report, and Chapter 3 of WG I, in particular (Trenberth et al. 2007c). Not only has Trenberth led this effort, but several CAS personnel, D. Shea, J. Hurrell, C. Deser, J. Fasullo and A. Dai, were contributing authors, and many of the figures were developed by Shea and Trenberth, while L. Butler contributed to the text development (and is listed in the acknowledgments). Chapter 3 has over100 pages (as well as the supplementary material), 47 figures with 126 panels, 8 Tables and 863 references. Trenberth also contributed to the Technical summary and Summary for Policy Makers, as well as reviewing chapters for both WG I and WG II.

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References

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Alexander, M., A. Capotondi, A. Miller, F. Chai, R. Brodeur and C. Deser, 2007: Decadal variability in the northeast Pacific in a physical-ecosystem model : The role of mixed layer depth and trophic interactions. J. Geophys. Res. (accepted) .

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Related Figures