INTERANNUAL TROPICAL RAINFALL VARIABILITY IN GENERAL-CIRCULATION MODEL SIMULATIONS ASSOCIATED WITH THE ATMOSPHERIC MODEL INTERCOMPARISON PROJECT

The interannual variability of rainfall over the Indian subcontinent, the African Sahel, and the Nordeste region of Brazil have been evaluat ed in 32 models for the period 1979-88 as part of the Atmospheric Mode l Intercomparison Project (AMIP). The interannual variations of Nordes te rainfall are the most readily captured, owing to the intimate link with Pacific and Atlantic sea surface temperatures. The precipitation variations over India and the Sahel are less well simulated. Additiona lly, an Indian monsoon wind shear index was calculated for each model. Evaluation of the interannual variability of a wind shear index over the summer monsoon region indicates that the models exhibit greater fi delity in capturing the large-scale dynamic fluctuations than the regi onal-scale rainfall variations. A rainfall/SST teleconnection quality control was used to objectively stratify model performance. Skill scor es improved for those models that qualitatively simulated the observed rainfall/El Nino-Southern Oscillation SST correlation pattern. This s ubset of models also had a rainfall climatology that was in better agr eement with observations, indicating a link between systematic model e rror and the ability to simulate interannual variations. A suite of si x European Centre for Medium-Range Weather Forecasts (ECMWF) AMIP runs (differing only in their initial conditions) have also been examined. As observed, all-India rainfall was enhanced in 1988 relative to 1987 in each of these realizations. All-India rainfall variability during other years showed little or no predictability, possibly due to intern al chaotic dynamics associated with intraseasonal monsoon fluctuations and/or unpredictable land surface process interactions. The interannu al variations of Nordeste rainfall were best represented. The State Un iversity of New York at Albany/National Center for Atmospheric Researc h Genesis model was run in five initial condition realizations. In thi s model, the Nordeste rainfall variability was also best reproduced. H owever, for all regions the skill was less than that of the ECMWF mode l. The relationships of the all-India and Sahel rainfall/SST teleconne ctions with horizontal resolution, convection scheme closure, and nume rics have been evaluated. Models with resolution greater than or equal to T42 performed more poorly than lower-resolution models. The higher resolution models were predominantly spectral. At low resolution, spe ctral versus gridpoint numerics performed with nearly equal verisimili tude. At low resolution, moisture convergence closure was slightly mor e preferable than other convective closure techniques. At high resolut ion, the models that used moisture convergence closure performed very poorly, suggesting that moisture convergence may be problematic for mo dels with horizontal resolution greater than or equal to T42.