Optimal Tropical Sea Surface Temperature Forcing of North American Drought

The optimal anomalous sea surface temperature (SST) pattern for forcing drought over North America is identified through climate model simulations in which the response of the Palmer Drought Severity Index (PDSI) is determined for each of 43 prescribed localized SST anomaly “patches” in a regular array over the tropical oceans. The robustness and relevance of the optimal pattern are established through the similarity of results obtained using two different climate models, and also by the good correspondence of the projection time series of historical SST anomaly fields on that pattern with the time series of the PDSI in separate model simulations with prescribed time-varying observed global SST fields for 1920-2005. It is noteworthy that this optimal drought-forcing pattern differs markedly in the Pacific from the dominant SST pattern associated with the El Niño-Southern Oscillation (ENSO), and also shows a large sensitivity of the drought to Indian and Atlantic Ocean SSTs.