Atmospheric Precipitation in Response to Equatorial and Tropical Sea Surface Temperature Anomalies

Abstract

Three sets of numerical experiments based on a GFDL GCM were developed to investigate the response of the large-scale tropical circulation and precipitation to the tropical and equatorial sea surface temperature (SST) anomalies. Specified SST anomaly (SSTA) with a small latitudinal scale of 13.5° was imposed in different regions of the Pacific Ocean in different sets of experiments and added to the climate-mean August SST to form a lower boundary forcing. Each set is composed of two experiments in which the SSTA possesses the same coverage and intensity but opposite sign. Anomalies of meteorological fields are calculated as the differences between the results of the warm and cold SSTA experiments. In all experiments, prominent anomalous low-level convergence and high-level divergence are observed over the warm SSTA regions. For experiments with warm SSTA placed at the equator, responses of the tropical streamfunction are similar to corresponding results found in other studies. When the warm SSTA is placed in the warmest SST region in the western North Pacific away from the equator, the excited anomalous streamfunction is different from that in the equatorial SSTA cases. A strong anomalous low-level cyclone and high-level anticyclone are generated, and strong anomalous westerly?southwesterly flow at lower levels and northeasterly?easterly flow at upper levels sweep through the southeastern part of the region. Anomalous rainfall is shown to be balanced mainly by anomalous convergence of stationary water flux; transient flux and anomalous evaporation from the warm water surface are secondary. Advection of water vapor by the large-scale flow and its anomaly were found to be significant in determining the rainfall pattern. Anomalous precipitation occurs in regions where the mean flow is down the SSTA gradient, or the anomalous flow is down the mean SST gradient. Mainly due to advection of water vapor by the divergent wind component and its anomaly, abnormal rainfall near the equator is biased toward the hemisphere where near-equatorial SST is warmer. Advection of anomalous water vapor by strong low-level equatorial easterlies in the eastern equatorial Pacific causes anomalous rainfall associated with the warm SSTA in the region to shift westward. Away from the equator, advection of water vapor by the anomalous rotational wind becomes important. It is this contributor that causes anomalous rainfall to shift away from the warmest SST region in the western North Pacific.