Climate Analysis Section

Climate Analysis Section (CAS) research has the goal of increasing our understanding of atmospheric and climate variability and climate change through parallel development and analysis of observational, assimilated, model-generated, and model-forcing datasets; and, by using these datasets for empirical studies, diagnostic analyses, model experimentation, and model evaluation, to document comprehensively the variability, its causes, and the processes involved.

In CAS, a central ongoing thrust of considerable importance to the research and university communities is the acquisition, evaluation, improvement, and restructuring of datasets, development of climatologies, and the documentation of results and methods of data access in catalogs available through the World Wide Web (WWW). The datasets are extensively used in diagnostic, theoretical, and modeling studies, including validation of the Community Climate System Model (CCSM). CAS collaborates closely with the Data Support Section of NCAR’s Scientific Computing Division (SCD) in all observational data-related activities. Tools to increase access to and display of data are being developed in conjunction with SCD's Data Portal activity.

CAS research is focused on the atmosphere and its interactions with the surface of the earth and oceans on a wide range of temporal and spatial scales. Included are studies of the diurnal cycle, intraseasonal variability, interannual variations such as the El Niño-Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), the Arctic Oscillation, the Tropospheric Biennial Oscillation (TBO), annular modes, extreme events, and interdecadal variations and longer period trends including paleoclimate. Also included are comprehensive diagnostic studies of the global and regional atmospheric moisture, heat, energy, and momentum budgets; observational analysis of upper ocean and sea ice variations; and heat and fresh water budgets in the ocean, and the water cycle on land. CAS scientists interact with other Climate and Global Dynamics sections especially through the analysis, validation, and diagnosis of divisional climate models.

CAS research spans many topics but includes several studies that are considered to be part of national and international programs including the Climate Change Science Program (CCSP), International Satellite Cloud Climatology Project (ISCCP), Global Energy and Water Cycle Experiment (GEWEX), Climate variability and predictability (CLIVAR), Past Global Changes (PAGES), Intergovernmental Panel on Climate Change (IPCC), Global Climate Observing System (GCOS), International Geosphere-Biosphere Programme (IGBP), and the World Climate Research Programme (WCRP) as a whole.

CAS staff members, and Dennis Shea, Lesley Smith, David Stepaniak, and Adam Phillips, in particular, continue to collaborate with SCD’s Data Support Section to acquire, evaluate, and reformat data. Ongoing efforts in these areas include analyses of conventional, satellite, and model data. In addition to the normal updates to existing datasets, some recent additions to the CAS data catalogs (http://www.cgd.ucar.edu/cas/dcats.html) include: ISCCP D2 level data and several global precipitation datasets. Stepaniak has developed and maintains a new Web page devoted to "Vertically Integrated Mass, Moisture, Heat, and Energy Budget Products Derived from the NCEP (National Centers for Environmental Prediction)/NCAR Reanalysis," providing the community with complete background and access to a total of 36 monthly mean fields spanning 1979 to 2001 – see http://www.cgd.ucar.edu/cas/catalog/newbudgets/. Shea continues to update the NCAR Instructional Aid entitled "An Introduction to Atmospheric and Oceanographic Datasets," which is available at http://www.cgd.ucar.edu/cas/tn404/. It serves as a ‘data-primer’ for students and those in other fields of research. It describes the general characteristics of atmospheric and oceanographic datasets and how to determine what data are available and where they are located.

Smith has responsibility for the on-line dataset catalog on the Web, which has been upgraded and added to in the past year. It documents all the observationally-based global datasets developed in CAS in the network Common Data Format (netCDF). Many datasets have been revamped and reconstructed into new formats, and Cassidy Rush (CAS), a student assistant, has helped a great deal and has provided a search engine. Continuing development of expert user guidance and commentary on quality and utility of datasets related to climate variability is led by Phillips, Clara Deser, and James Hurrell (all CAS). The primary purpose of this activity is to catalog and provide easy access to a basic set of historical datasets for interdisciplinary studies of climate variability. The need for this activity has been articulated at several U.S. CLIVAR planning workshops.

Sylvia Murphy (CAS) and Shea continue to participate in development of a general purpose data processing tool based on SCD’s NCAR Command Language. Processing and graphical capabilities continue to be added to facilitate climate research, in particular, model-to-model and model-to-observed comparisons. A Web site (http://www.cgd.ucar.edu/csm/support/) contains many up-to-date examples.

Several activities have been pursued including coordination of a diagnostics working group to facilitate the creation and dissemination of component (atmosphere, ocean, ice, and land) model diagnostics packages; the ability to access and process data in the Hierarchical Data Format-Earth Observing System format; and tutorial workshops on data processing and visualization.  Seven three-day workshops were held during 2003, four at NCAR and three at universities: University of California at Los Angeles (January), University of Alaska at Fairbanks (April), and Purdue University (November).

The data can be downloaded from the Web browser and all of the fields are available through anonymous ftp, via ftp://ftp.ucar.edu/cgd/cas/tn430/. The products are all global vertically-integrated grids as individual monthly mean time series from 1979 through 1993, or 2001 for NCEP, and monthly, seasonal, and yearly averages (climatologies).

The work on how well model coordinate results can be replicated in pressure coordinates and with data from a post-processor developed at NCAR has been published by Trenberth, Stepaniak, and Julie Caron (Climate Modeling Section, CMS), along with the role of vertical resolution. The standard 17-level reanalysis pressure level archive does not adequately resolve the atmosphere, and a new set of pressure levels that has 25 mb vertical resolution below 700 mb and 50 mb vertical resolution in the rest of the troposphere, giving 30 total levels, was proposed. This was communicated to NCEP and changes are underway at NCEP to address this issue. The diagnostics also revealed major problems in the NCEP reanalyses and ECMWF post-processing (see below).

Major problems with the NCEP reanalyses in the stratosphere found by Trenberth and Stepaniak were communicated to NCEP and are resulting in changes in model levels, a switch to hybrid coordinates (as recommended), and revamped upper boundary conditions. The problems are manifested most strongly as a spurious two-delta vertical wave in the divergence of the wind field above steep topography, especially where the wind increases with altitude in the stratosphere. It is present primarily above 50 mb at the topmost four levels in the NCEP model used for data assimilation and is directly related to the use of the sigma (terrain-following) coordinate system and the upper boundary condition in the assimilating model, and accordingly, it is a pathological problem.

Problems with ECMWF post-processing of fields onto pressure surfaces were found by Trenberth and Stepaniak, and they affect all ECMWF analyses (operational and reanalyses). These stem from the way the fields are truncated, which is a necessary step to avoid aliasing before putting the values out on a 2.5° or other grid. At ECMWF, the vector fields were truncated as two scalars (actually as a scalar multiplied by cos(phi)). A vector field requires an extra mode at each zonal wave number compared with a scalar field to properly depict it in spherical harmonics. Hence, the truncation of a vector as two scalars causes problems at high latitudes (because of the cos(phi) factor), often in wave 1. Moreover, it is highly desirable to compute the grid point values exactly rather than interpolating them from the Gaussian grid, as done by ECMWF, as the spatial interpolation causes slight loss of accuracy and loss of reversibility. ECMWF has now changed their post processing to address this issue.

Trenberth, Smith, and Stepaniak are participating in the ERA-40 reanalyses and have begun preliminary evaluations of the fields being produced. A new diagnostic package of vertically integrated quantities and fluxes related to the mass, moisture, heat, and energy budgets has been developed in conjunction with ECMWF, and these are being computed "on the fly" and will be made available as part of the official archive. This participation allows the reanalyses to be brought back to NCAR at minimal costs. Initial work is on vertically-integrated mass fields.

Hurrell, with Michael Alexander (Climate Diagnostics Center, CDC/NOAA), continued his role as co-chair of the CCSM Climate Variability Working Group. Together with James Hack (CMS), James Rosinski (CMS), Caron, and Shea, he constructed a new sea surface temperature (SST) and sea ice dataset as a lower boundary condition for the Community Atmosphere Model (CAM). A method was developed for "blending" SST products from NCEP and the Hadley Centre, and for reducing the number of spurious sea ice concentrations associated with the original SST products. The SST product is updated monthly and is available to the community. Both Geophysical Fluid Dynamics Laboratory (GFDL) and NASA use the new SST product in their modeling activities. With Phillips, Hurrell oversaw a number of production runs using CAM, and they are available for community use, including a 15-member ensemble forced with the observed time history of global SSTs since 1950. These data are available via the Web in netCDF format for easy community access.

Trenberth, along with Tom Karl (National Climatic Data Center) and Tom Spence (National Science Foundation), advocated implementation of a systems approach to climate observations in an article now published in Bulletin of the American Meteorological Society. This has led to extensive involvement by Trenberth in producing the "Second Report on the Adequacy of the Global Observing Systems for Climate in support of the United Nations Framework Convention on Climate Change," which was published this year. It has also led to close involvement in the Earth Observing Summit, held in Washington, D.C., on July 31, 2003, and development of implementation plans, as well as service on several NOAA committees.

Chester Newton’s (CAS) investigation of the associations between semiannual oscillations of the North Pacific cyclone track and upper-tropospheric circulations over the Eastern Hemisphere is now in press.

Advanced Study Program (ASP) postdoctoral fellow Richard Cullather is studying the climate of the Antarctic Peninsula, which has seen a warming trend of about 2.8°C in station temperatures since 1951. Assessments of temperature and sea level pressure fields in comparison to in situ station observations for the ECMWF reanalysis project, ERA-40 suggest that the fields are adequate for many locations over the period from 1972 until the present. These preliminary investigations and an assessment of the performance of the CCSM in the region will provide the necessary platform for a comprehensive study of the warming trend.

Hurrell, Trenberth, and Shea continue work to carefully evaluate variations in storm tracks associated with global patterns of circulation variability. They are making use of nearly 50 years of NCEP/NCAR reanalysis data band-passed filtered to retain synoptic time scales. The quality of the reanalyses for such a task is also being assessed.

Cullather, in coordination with Hurrell, evaluated the CCSM version 2 (CCSM2) using a feature-tracking algorithm applied to synoptic sea level pressure fields. The focus of the study was an assessment of the model storm tracks in comparison to observation-based analyzed fields, using a control simulation of the fully coupled model and an integration of the atmospheric model forced with observed 20th century sea ice and SST fields. Principal results highlighted the North Atlantic storm track, which was found to have a more zonal depiction in both coupled and uncoupled simulations than is observed, and the character of anticyclones over the central Arctic Basin, which are generally weaker and shorter lived in the model as compared with observations. These results were presented at the June 2003 CCSM Workshop.

Sungsu Park (ASP postdoctoral fellow) developed a new single column model to study mechanisms responsible for the properties of stratocumulus clouds in the upper part of the marine boundary layer. The key feature is an algorithm for evaluating decoupling parameters used to specify the statistical properties of thermodynamic and moisture variables at the base of the marine boundary layer (MBL) inversion. Model behavior of MBL and cloud properties under homogeneous static conditions and advective conditions over the northeastern subtropical and southeastern subtropical Pacific Oceans are in good agreement with observations along selected trajectories. This model represents a kind of maximum simplification of the MBL in the sense that omission of any major component would cause it to fail badly. The model is now being applied to a general circulation model (GCM). For this, satellite cloud products (MODIS) are being used for realistic tuning of internal model parameters over the Californian and Peruvian stratocumulus deck during September-October, 2000.

Trenberth has completed a comprehensive review article on energy on the planet Earth as an encyclopedia article.

This figure shows a schematic of the main processes involved in the Hadley circulation from the standpoint of the heat budget, especially in the subtropics, which are key drivers of the circulation. (Trenberth and Stepaniak, 2003.)

Cullather has been working with CAS scientists on topics related to the climate and tropospheric circulation in the polar regions. As part of his Ph.D. studies, Cullather diagnosed the annual cycle of sea level pressure over the Arctic Basin using observations and analyses. Above the Canada Basin/Laptev Sea side of the arctic, the annual cycle of surface pressure is dominated by the first harmonic, which has an amplitude of about 5 hPa and maximum pressure occurring in March. Along the periphery of northern Greenland and extending to the North Pole, a weak semiannual cycle was found in surface pressure with maxima in May and November. The presence of the semiannual variation was found to be highly variable. The progression of the annual cycle via the divergent atmospheric mass field indicated a transfer from Eurasia and into the Canadian Archipelago in spring and the reverse condition in autumn. Over the central Arctic Basin, springtime pressure increases result from an enhanced poleward mass transport from Eurasia. An increase of equatorward transport over the Canadian Archipelago in May and June results in central arctic pressure decreases into summer.

From a societal, weather, and climate perspective, precipitation intensity, duration, and frequency are as much of concern as total amounts, because these factors determine the disposition of rainfall once it hits the ground and how much runs off. Trenberth, Dai, David Parsons (Atmospheric Technology Division), and Roy Rasmussen (Research Applications Program) address the challenges in analysis of observations, modeling, and understanding precipitation changes, which are being taken up in the NCAR "Water Cycle Across Scales" initiative that will exploit the diurnal cycle as a test bed for a hierarchy of models to promote improvements in models. At the extremes of precipitation incidence are the events that give rise to floods and droughts, whose changes in occurrence and severity have enormous impact on the environment and society. Because the rate of precipitation, conditional on when it falls, greatly exceeds the rate of replenishment of moisture by surface evaporation, most precipitation comes from moisture already in the atmosphere at the time the storm begins, and transport of moisture by the storm-scale circulation into the storm is vital. Hence, the intensity of precipitation depends on available moisture, especially for heavy events. As climate warms, the amount of moisture in the atmosphere is expected to rise much faster than the total precipitation amount, which is governed by the surface heat budget through evaporation. This implies that the main changes to be experienced are in the character of precipitation: increases in intensity must be offset by decreases in duration or frequency of events. The timing, duration, and intensity of precipitation can be systematically explored via the diurnal cycle, whose correct simulation in models remains an unsolved challenge of vital importance in global climate change. Typical problems include the premature initiation of convection and events that are too light and too frequent.

Dai and Trenberth have used the new discharge data and the E-P fields derived from the ECMWF and NCEP/NCAR reanalyses to derive new estimates of meridional transport of fresh water by the oceans. Preliminary results show improved agreement with in situ data based on direct estimates. Compared with earlier indirect estimates, the new estimates show increased southward transport in the Atlantic Ocean and increased northward transport in the South Pacific.

Dai, together with Taotao Qian (CAS) and Trenberth, has started to estimate the global continental fresh water discharge since 1950 by combining historical records of streamflow and model-simulated surface runoff (forced by reanalysis and station data). The Community Land Model will be run offline and forced by bias-corrected reanalysis data to estimate the fresh water discharge with a goal to improve our understanding of discharge variability on interannual to decadal time scales.

Park explored variations of ship observed marine clouds and precipitation frequency. Marine cloud variations associated with ENSO could be roughly grouped into the following three categories: (1) storm track cloud variations in association with atmospheric teleconnection patterns, (2) MBL cloud variations in association with modulation of MBL static stability, and (3) deep convective cloud variations in association with modulated midtropospheric stability. He also found significant, simultaneous ENSO signals in ship-observed, visible-sky conditions over the northeastern Atlantic and western Mediterranean Sea (WM) region during late summer and autumn. Analysis of atmospheric flows indicates that the strong ENSO-WM correlation in the August through October season arises from two components: a previously unreported quasi-stationary Rossby wave propagating eastward from the western equatorial Pacific, and an anomalous component of the Asian wet monsoon circulation.

Dai, in collaboration with Adam Monahan (University of Victoria, Canada), examined the spatial and temporal structures of ENSO nonlinearity in several reconstructed SST datasets, surface air temperature (SAT) from ECMWF ERA-40 and NCEP/NCAR reanalyses, and SST data from millennial control runs from several state-of-the-art coupled GCMs. They found that in historical SST and SAT reconstructions, the anomaly spatial pattern that changes sign between El Niño and La Niña events (the "linear" signal) strongly resembled that of the first empirical orthogonal function (EOF), while that which does not change sign (the "nonlinear" signal) resembled the pattern of the second EOF. Furthermore, temporal subsampling of the long (130-year) SST reconstructions suggests that the magnitude of the nonlinear signal and its similarity to EOF 2 are functions of the strength of ENSO. This relationship was, however, absent in one of the three SST reconstructions, and it may have been suppressed by smoothing. Of several coupled GCMs considered, the spatial structure of the El Niño/La Niña asymmetry was partly captured only by the GFDL R30 model.

Meehl analyzed global coupled model simulations from the CCSM in collaboration with Johannes Loschnigg (University of Hawaii) and Julie Arblaster (CCR) to identify tropical-midlatitude interactions and dynamically coupled air-sea linkages important for the TBO. The CCSM experiments illustrated the role of coupled ocean dynamical mechanisms, including heat transport and Kelvin and Rossby ocean wave dynamics, in forming and maintaining anomalous SSTs in the Indian Ocean and the tropical Pacific that affect subsequent Indian monsoon strength (as previously documented). These experiments quantify the effects of the anomalous forcings, and they are also consistent with the corresponding patterns from previous singular value decomposition analyses of observations. The model upper ocean heat content anomalies, necessary to contribute to the persistence of SST anomalies, compared favorably to the observations in the TBO.

Along with Harry van Loon (Colorado Research Associates, CoRA) and Ralph Milliff (CoRA), Meehl addressed two aspects of the onset of warm events in the Southern Oscillation in the early 1990s. One involved the subtropical South Pacific High, which is weakened in early southern winter and contributes to reduced trade winds and upwelling along the equator. Another was the negative SST anomalies in the equatorial eastern Pacific cold-water tongue in the months that follow, and particularly during the mature phase of the warm event during southern summer, being displaced by positive anomalies. Using these indices, two distinct warm event onsets were identified in the early 1990s, one in 1991, and another in 1994.

Meehl collaborated with a group of international monsoon scientists led by Duane Waliser (State University of New York, Albany) to document behavior of monsoon intraseasonal variability in a group of atmospheric GCMs. Results showed that several of the models exhibit intraseasonal variability at or above that found in observations, including some form of northeastward propagation. However, the model patterns are typically less coherent, lack sufficient eastward propagation, and have smaller zonal and meridional spatial scales than the observed patterns.

Meehl collaborated with Arblaster and Hu to examine aspects of decadal variability in the PCM as seen in observations. The mechanism producing such decadal climate variations in tropical Pacific SSTs was studied to provide the context for modulation of interannual teleconnections, along with sensitivity experiments with the atmospheric model (Climate Community Model version 3, CCM3). Results showed that the subtropical cells in the Pacific Ocean, in concert with tropical-midlatitude interactions through the atmosphere, play an important role in decadal variability.

Deser, with Ramalingam Saravanan (Climate Dynamics and Predictability Section) and Gudrun Magnusdottir (University of California, Irvine), conducted a suite of experiments using the CCM3 to investigate the relative sensitivity of the model's atmospheric circulation to observed trends over the past 50 years in winter sea ice extent and SST over the North Atlantic/arctic sector. They found a stronger sensitivity to the sea ice anomalies than SST anomalies, and the response is a weak negative feedback on the NAO. The total response resembled closely the leading pattern of internal circulation variability in the model, although an additional "direct" near-local response was also apparent.

Hurrell and Robert Dickson (The Centre for Environment, Fisheries and Aquaculture Science) have completed and revised a chapter for the book Ecological Effects of Climatic Variations in the North Atlantic Ocean. Nils Chr. Stenseth (Center for Advanced Study, Norwegian Academy of Science and Letters) was the lead editor, and Hurrell was one of four co-editors of the book. The chapter provides an overview of the NAO, its forcing of the North Atlantic Ocean, and its impact on marine ecosystems. Hurrell also published a review article on the NAO for the Encyclopedia of Atmospheric Sciences.

Hurrell was the lead editor for a new American Geophysical Union Monograph, The North Atlantic Oscillation: Climatic Significance and Environmental Impact published in January 2003. The other editors were Martin Visbeck and Yochanan Kushnir (both of Lamont-Doherty Earth Observatory), and Geir Ottersen (Institute of Marine Research). Focusing exclusively on the NAO and its impacts, the monograph brings together for the first time atmospheric scientists, oceanographers, paleoclimatologists, and biologists to present a state-of-the-art assessment of the current understanding of this important climate phenomenon and its environmental and societal consequences. Indeed, the outstanding feature of the monograph is its multidisciplinary content presented in 12 papers thematically organized. Each paper provides a thorough overview of different facets of the NAO phenomenon and contains new research as well. A total of 42 specialists participated in writing the material for the book, and 36 expert referees made substantial contributions to the overall quality and content of the monograph. It is the first time that the NAO phenomenon is addressed in such a comprehensive manner, providing a current and authoritative survey of the ever-growing body of literature on the NAO. The monograph offers extensive information on different levels that can be useful to the students and scientists of climate and environmental studies, as well as to others who are interested in this topic. The book was advertised through short articles, authored by Hurrell and the other editors, in both CLIVAR Exchanges and Eos. Hurrell, Visbeck, Kushnir, and Ottersen also authored the lead chapter of the monograph.

Hurrell, along with Martin Hoerling (CDC), Phillips, and Taiyi Xu (CDC), completed two papers examining the role of tropical forcing in boreal winter North Atlantic climate change over the last half of the 20th century. In the first paper, diagnoses of ensembles of atmospheric GCM experiments demonstrate that the observed upward trend in the winter NAO index since 1950 is a virtually deterministic response to the temporal history of SSTs and that tropical SST forcing is of primary importance. The probability distribution function (PDF) of 50-year NAO index trends from the forced simulations are, moreover, appreciably different from the PDF of a control simulation with no interannual SST variability, although chaotic atmospheric variations are shown to yield appreciable 50-year trends. The results thus advance the view that the observed linear trend in the winter NAO index is a combination of a strong tropically forced signal and an appreciable "noise" component of the same phase. The changes in tropical rainfall of greatest relevance include increased rainfall over the equatorial Indian Ocean, a change that has likely occurred in nature and is physically consistent with the observed, significant warming trend of the underlying sea surface.

In the second paper, Hurrell and colleagues demonstrate that the simulated Northern Hemisphere atmospheric response to the linear trend component of tropic-wide SST change since 1950 projects strongly onto the positive polarity of the NAO and is a hemispheric pattern distinguished by decreased (increased) Arctic (mid-latitude) sea level pressure. Progressive warming of the Indian Ocean is the principal contributor to this wintertime extratropical response, as shown through additional atmospheric GCM ensembles forced with only the SST trend in that sector. The Indian Ocean influence is further established through the reproducibility of results across three different models forced with identical, idealized patterns of the observed warming. Examination of the transient atmospheric adjustment to a sudden "switch-on" of an Indian Ocean SST anomaly reveals that the North Atlantic response is not consistent with linear theory and most likely involves synoptic eddy feedbacks associated with changes in the North Atlantic storm track. The tropical SST control exerted over 20th century regional climate underlies the importance of determining the future course of tropical SST for regional climate change and its uncertainty.

Hurrell, Phillips, Christophe Cassou (CAS), Chris Folland, and Simon Brown (Hadley Centre, United Kingdom Meteorological Office) have completed their analysis of an ensemble of numerical experiments designed to assess the limitations of forcing atmospheric general circulation models (AGCM) with observed SSTs. The issue is examined using a consistent approach; namely, the simulated climate from a coupled ocean-atmosphere GCM is compared to that from an AGCM, where the latter is forced with the sea ice and SST fields internally generated by the former. The results confirm that coupling enhances lower tropospheric thermal variance and reduces net surface energy flux, but the effects are smaller than most previous studies indicate. Coupling can also slightly alter the amplitude (and persistence) of dominant modes of natural middle latitude variability, such as the NAO.

Hurrell and Folland have continued their examination of the annual cycle of climate and climate change over the Atlantic, investigating the mechanisms responsible for the variability through analyses of both observed and climate model data. They have published some initial results describing a significant change in the summer European climate, whereby a summer season of higher-than-average surface pressure over northern Europe is accompanied by reduced rainfall over the tropical North Atlantic and North Africa. Based on analysis of variance techniques that separate climate variability into forced (i.e., due to SST variations) and unforced (i.e., due to internal atmospheric dynamics) components, their results suggest the observed low-frequency extratropical changes in summer climate arise indirectly from processes that affect tropical Atlantic precipitation on long time scales, such as the inter-hemispheric gradient in tropical Atlantic SST. The results also suggest, however, a weaker direct atmospheric circulation response to SST forcing.

Hurrell and Mark Rodwell (ECMWF), a scientific visitor to CAS over the summer of 2003, completed work to examine the response in five different atmospheric GCMs (including CAM) to realistic, optimally chosen North Atlantic SST anomalies. Together with Marie Drevillon (CERFACS), Claude Frankignoul (Universite Pierre et Marie Curie), Holger Pohlmann (Max Planck Institute), Martin Stendel (Danish Meteorological Institute), and Rowan Sutton (Reading University), they illustrate a consistent response among the models that is similar to observational estimates. The results demonstrate the need to represent accurately low-frequency variations in the Atlantic SST (such as those anomalies associated with changes in the thermohaline circulation) if confidence in forecasts of regional climate change is to be obtained.

With Stenseth, Ottersen, Atle Mysterud (University of Oslo), Mauricio Lima (Catholic University of Chile), and Kung-Sik Chan (University of Iowa), Hurrell has completed two papers dealing with climate variability and ecology. The first, in Science, reviews how climate variations drive temporally and spatially averaged exchanges of heat, momentum, and water vapor that ultimately determine growth, recruitment, and migration patterns. The impact of large-scale climatic forcing on ecological systems is a relatively new concept for ecologists. In the second paper, they review two of the best-known patterns of climate variability, the NAO and ENSO, and show how these phenomena affect ecological patterns and processes in both marine and terrestrial systems.

Cassou, along with Hurrell, Deser, and Phillips, investigated the observed low frequency winter atmospheric variability of the North Atlantic-European region and its relationship with global surface oceanic conditions based on the climate and weather regimes paradigm. The main finding is a clear eastward shift of the reinforced Azores High for the positive phase of the NAO (as in the past 3 decades), when compared to the negative NAO phase. Model results from the Action de Recherche Petite Echelle/Grande Echelle (ARPEGE) AGCM (in collaboration with CERFACS, France) show that both tropical and extratropical SST anomalies alter the frequency distribution of the North Atlantic regimes, while the so-called Ridge regime is preferably excited during La Niña events, and the NAO regimes are associated with the North Atlantic SST tripole. The tropical branch of the SST tripole affects the NAO regimes occurrence and reproduces the observed link, while the extratropical branches tend to counteract the dominant influence of the tropical Atlantic basin.

Cassou, Hurrell, Deser, and Phillips also find, from observations and models, that the origin of the so-called summer North Atlantic Horseshoe (HS) SST mode, which is statistically linked to the next winter’s NAO, is from a remote footprint of tropical atmospheric changes. Anomalous convective patterns corresponding to a displacement and/or reinforcement of the Atlantic Intertropical Convergence Zone (ITCZ) generate forced atmospheric Rossby waves that propagate northeastward from the Caribbean basin and lead to changes in trade wind intensity over the northern subtropical Atlantic. The perturbed turbulent and radiative fluxes at the surface associated with the teleconnection pattern tend to imprint HS-shaped oceanic anomalies. The ARPEGE model is then used to test if the persistence of HS anomalies from summer to late fall can feedback to the atmosphere and have an impact on the next winter's North Atlantic variability. A weak but coherent early winter response projecting on the NAO is obtained and reproduces the observed HS/NAO relationship obtained from lagged statistics. The role of the transients is hypothesized to be of central importance to explain the nature and the sign of the model response.

Cassou, along with Alexander, Deser, Hurrell, and Phillips, implemented an intermediate complexity coupled model using the latest version of the CAM2.0, the NOAA/CDC Mixed Layer Ocean Model (MLM), and the thermodynamic version of the CCSM Sea Ice Model (CSIM4). Sensitivity experiments are being performed to study the so-called re-emergence mechanism in the North Atlantic and its impact upon the winter atmospheric circulation. The model is also being used to test the impact of the 2003 tropical Atlantic changes on the European summer climate. Clear anomalous warm conditions are extracted from a 50-year ensemble simulation where the observed northward displacement of the 2003 Atlantic ITCZ is prescribed in the model via diabatic heating anomalies. These warm conditions bear some resemblance with the observed signals, and teleconnection mechanisms are currently under investigation.

Cassou, along with Drevillon and Laurent Terray (CERFACS), have investigated the role of the so-called autumn Atlantic El Niño on the North Atlantic climate. Changes in rainfall and atmospheric meridional cells associated with the oceanic pattern are shown to affect the early winter NAO regimes occurrence as well as their persistence.

Dai, Meehl, Washington, and Wigley analyzed the climate change simulations under the DOE-sponsored Accelerated Climate Prediction Initiative Program using the PCM. An initialization to 1995 ocean conditions removes a large part of the unforced oceanic temperature and salinity drifts that occurred in the standard 20th century integration. The results suggest that the affect of small errors in the oceans (such as those associated with climate drifts) on coupled GCM-simulated climate changes may be negligible. The ensemble simulations of the 20th and 21st century climates using PCM and forced by historical and projected future greenhouse gas and sulfate aerosol forcings suggest similar global warming and precipitation changes as in the CSM version 1.3 single realization. However, the PCM ensemble-averaged changes of regional precipitation are better defined (higher signal-to-noise ratios) and may be able to provide more credible projections of future regional (e.g., the western U.S.) climate changes. In none of the members of the ensemble did the PCM reproduce the observed warming peak around 1940.

Meehl and a group of NCAR collaborators documented a new mechanism for how variations in solar input can influence surface climate. By analyzing output from 20th century simulations with the PCM, they showed that solar forcing most strongly influences clear-sky regions. This sets up patterns of forcing whereby regional circulation regimes in the tropics reinforce climatological precipitation maxima in response to increased solar forcing. This includes stronger monsoon regimes, as well as intensified precipitation associated with convergence zones over the tropical oceans.

Meehl also collaborated with Claudia Tebaldi (Environmental and Societal Impacts Group, ESIG), Linda Mearns (ESIG), Doug Nychka (Geophysical Statistics Project), and Arblaster to analyze changes in climate variability and extremes in ensembles of future climate projections. This work used statistical analyses of extremes, as well as threshold methods, to study changes in weather and climate extremes in the PCM. An analysis of frost days in the PCM showed that anomalous sea level pressure patterns in the future climate, indicative of regional atmospheric circulation changes, were prime contributors to the pattern of reductions in frost days in future climate, with soil moisture and clouds playing secondary roles.

In collaboration with Santer, Meehl, Washington, Ammann, and Arblaster, Wigley has also considered detection of an externally-forced signal in the observed record of tropopause height changes from ensemble-mean PCM results spanning 1890-1999 with greenhouse gas, direct sulfate aerosol, tropospheric and stratospheric ozone, solar and volcanic forcing as single forcing agents, and experiments with all forcings combined. Greenhouse-gas-induced warming of the troposphere and ozone-depletion-induced cooling of the stratosphere are the primary causes of long-term changes in tropopause height, whereas volcanic effects are the dominant short-term signals. The externally-forced signal is detected in the observations with high statistical confidence. The breakdown of the effects of different forcings shows that (a) this detection arises through the anthropogenic component of external forcing, and (b) both tropospheric warming and stratospheric cooling effects are necessary contributors, and strongly supports the reality of a significant warming of the troposphere over 1979-1999.