Current Research

Current Research

Major research tasks have involved two broad ongoing activities:

analysis and interpretation of results from various model experiments (both coupled and uncoupled model versions), and

analysis and interpretation of observed data, often attempting to relate the observed results to characteristics of model simulations.

Research accomplishments during the past year include the following:

Ongoing studies of the tropospheric biennial oscillation (TBO) continued to identify tropical-midlatitude interactions and dynamically coupled air-sea linkages important for producing the TBO. Singular value decomposition (SVD) analysis is employed to quantify contributions from regional processes and large scale influences on a year by year basis using NCAR/NCEP reanalyses and the CMAP precipitation data. This analysis technique is applied to possible transition mechanisms associated with the Indian monsoon for 500 mb midlatitude height anomalies over Asia, Indian Ocean SSTs, and tropical Pacific SSTs. Additionally, it is has been applied to the Australian monsoon in terms of Southeast Asian precipitation, Indian Ocean SSTs, and tropical Pacific SSTs. A cumulative anomaly pattern correlation technique is used to quantify the annual contributions of these processes. Model sensitivity experiments using the NCAR CCM3 with climatological SSTs are performed to test the role of anomalous meridional temperature gradients over Asia, anomalous SSTs in the Indian Ocean, and anomalous SSTs in the tropical Pacific and subsequent Indian monsoon strength. These experiments quantify the effects of the anomalous temperature forcings, and are consistent with the corresponding patterns from the SVD analyses.

Collaborative work was continued with Aiguo Dai, Warren Washington, Tom Wigley and Julie Arblaster to run and analyze the influences of various anthropogenic forcings over the 20th and 21st centuries in the PCM. Experiments using a forcing dataset for 20th century climate, and ensembles of scenario integrations for future climate, have been used as input to the U.S. National Assessment and the IPCC Third Assessment Report.

Collaborative work with Peter Gent, Julie Arblaster, Bette Otto-Bliesner, Esther Brady and Tony Craig involved a comparison of the behavior of El Nino in various versions of the NCAR CSM and DOE PCM. The higher the amplitude El Nino variability, the lower the magnitude of the background ocean vertical diffusivity across the ten model versions analyzed. In spite of these differences in El Nino amplitude, all model versions simulate the approximate timescale of El Nino (3-4 years in the models and about 3-7 years in the observations), but all have similar systematic errors in the pattern of El Nino variability (extending too far west into the warm pool), and eastern Pacific seasonal cycle (SSTs too semiannual, double ITCZ). The PCM versions have a thermocline more intense than standard CSM versions but too shallow compared to observations.

Collaborative work with Julie Arblaster documented a decadal modulation of the interannual teleconnections between El Nino/La Nina events and Australian rainfall in the PCM as seen in observations. However, these linkages are intermittent in the model suggesting that either the model is not consistently capturing the observed teleconnections, or the limited period of the observations samples only a strong period of decadal teleconnectivity. During decadal periods of positive SSTs in the tropical Pacific in the model three effects are noted. First, El Nino variability is less, contributing to weaker interannual teleconnections to Australian rainfall. Second, the ascending branch of the Walker Circulation shifts eastward away from Australia and also contributes to reduced interannual teleconnections to Australian rainfall. Third, warmer western Pacific SSTs on the decadal timescale have a regional influence on Australian rainfall that could disrupt the interannual teleconnections to Australia from farther east in the Pacific. The mechanism producing such decadal climate variations in tropical Pacific SSTs is also being studied to provide the context for the modulation of interannual teleconnections.

Collaborative work with George Kiladis (NOAA), Roger Lukas (University of Hawaii), Klaus Weickmann (NOAA), Matthew Wheeler (BMRC, Australia), and Adrian Matthews (Univ. of Reading) built on previous work involving analysis of large-scale aspects of tropics-tropics and tropical-midlatitude interaction on a variety of time and space scales. We propose that climate base states set up by longer timescale phenomena (e.g. TBO, ENSO, and decadal timescale processes) could influence the manifestation of tropical-tropical and tropical-midlatitude interactions of shorter timescale phenomena (e.. MJO, 6-30 day timescale tropical convection), and vice versa in a continuing set of upscale and downscale interactions. We argue that neglecting any of the timescale phenomena ultimately dooms efforts to forecast the others, requiring a comprehensive research effort involving all time and space scales.

Collaborative work with Ben Santer and a team of scientists showed that trends in the lower tropospheric temperatures from the MSU satellite data were not inconsistent with a global coupled model that included known anthropogenic forcings for the period of the MSU data (including aerosols from Mt. Pinatubo). We are now addressing the issue of large El Nino events and their influence on trends in short data records (such as the MSU, where the 1997-98 El Nino event at the end of the record induced a warming trend that was absent prior to that event). We show that the models with El Nino amplitudes comparable to observations can produce trends nearly as large due to sampling of 20 year records similar to the observations.

Collaborative work with Linda Mearns and Julie Arblaster involves analysis of changes in variability and extremes in ensembles of future climate projections. This work is using statistical analyses of extremes as well as threshold methods to study changes in weather and climate extremes in the PCM.

Collaborative work with Warren Washington, Tom Wigley, Julie Arblster, and Aiguo Dai involves analyses of the solar forcing ensemble runs with the PCM for 20th century climate compared to the ensemble runs without solar forcing. Results show that the observed early century warming is only simulated when solar forcing is included. The main difference in climate system response in the solar runs compared to the runs without solar forcing is that the early century warming extends through the depth of the atmosphere, while the effects of greenhouse gases in the late century warming produces warming in the troposphere and cooling in the stratosphere.