Impact of Tropical Subseasonal SST Variability on Seasonal Mean Climate Simulations

Abstract

The purpose of this study is to examine the impact of subseasonal sea surface temperature (SST) variability on the simulation of the seasonal mean extratropical circulation using a state-of-the-art high-resolution atmospheric general circulation model (AGCM). The format is a case study for January through March 1989 (JFM89) and the primary emphasis is on regional scales over North America. The SST boundary conditions in the AGCM simulations were prescribed using observed weekly data. Experiments were made in which the week-to-week (subseasonal) SST variability was suppressed. In terms of the largest spatial scales, the subseasonal SST variability has only a modest impact; however, statistically significant modifications to the 500-mb height anomalies over North America were detected. Consistent with these changes in the height field, the seasonal mean North American rainfall anomalies were particularly sensitive to the subseasonal SST variations, especially over the Pacific Northwest. Two possible mechanisms for this sensitivity were investigated with additional AGCM experiments and model diagnostics. The first mechanism, referred to as a ?stochastic? effect, is defined by the hypothesis that the week-to-week SST variability only serves to enhance the amplitude of tropical precipitation variability, which, in turn, modifies the midlatitude response. With this stochastic effect, the details of the subseasonal SST evolution do not matter. In contrast, the second mechanism is a ?deterministic? effect in that the details of the evolution of the subseasonal SST matter. The experiments presented here indicate that the stochastic effect is small and that the details of the subseasonal SST produce significant differences. This conclusion is supported by experiments with very large ensembles using a somewhat lower-resolution AGCM and a nonlinear barotropic model. Finally, some implications of these results for real-time forecasting are discussed.