A U.S. CLIVAR Project to Assess and Compare the Responses of Global Climate Models to Drought-Related SST Forcing Patterns: Overview and Results

The U.S. Climate Variability and Predictability (CLIVAR) working group on drought recently initiated a series of global climate model simulations forced with idealized SST anomaly patterns, designed to address a number of uncertainties regarding the impact of SST forcing and the role of land–atmosphere feedbacks on regional drought. The runs were carried out with ?ve different atmospheric general circulation models (AGCMs) and one coupled atmosphere–ocean model in which the model was continuously nudged to the imposed SST forcing. This paper providesan overviewof the experiments and some initial resultsfocusing on the responses to the leading patterns of annual mean SST variability consisting of a Paci?c El Nin ˜o–Southern Oscillation(ENSO)-likepattern,apatternthatresemblestheAtlanticmultidecadaloscillation(AMO),anda global trend pattern. One of the key ?ndings is that all of the AGCMs produce broadly similar (though different in detail) precipitationresponsestothePaci?cforcingpattern,withacoldPaci?cleadingtoreducedprecipitationanda warm Paci?c leading to enhanced precipitation over most of the United States. While the response to the Atlantic pattern is less robust, there is general agreement among the models that the largest precipitationresponseovertheUnitedStatestendstooccurwhenthetwooceanshaveanomaliesofoppositesigns.Further highlights of the response over the United States to the Paci?c forcing include precipitation signal-to-noise ratios that peak in spring, and surface temperature signal-to-noise ratios that are both lower and show less agreement among the models than those found for the precipitation response. The response to the positive SST trend forcing pattern is an overall surface warming over the world’s land areas, with substantial regional variations that are in part reproduced in runs forced with a globally uniform SST trend forcing. The precipitation response to the trend forcing is weak in all of the models. Itishopedthattheseearlyresults,aswellasthosereportedintheothercontributionstothisspecialissueon drought, will serve to stimulate further analysis of these simulations, as well as suggest new research on the physical mechanisms contributing to hydroclimatic variability and change throughout the world.