Abstract Author: Richard Seager, Yochanan Kushnir and Mingfang Ting
Abstract Title: Mechanisms of tropical Pacific and tropical Atlantic forcing of North American hydroclimate
Abstract: The atmospheric dynamical mechanisms that link variations in tropical Pacific and Atlantic SSTs to drought over North America are analyzed. The results of a multi-model project of idealized simulations organized by the CLIVAR Drought Working Group are the starting point. It is shown that these robustly show that a cold tropical Pacific sets up drier conditions across southwestern North America. The multi-model experiments also show that a warm subtropical North Atlantic also forces dry conditions in the same area and during both summer and winter seasons. The Pacific link is analyzed in terms of changes in the mean flow and changes in transient eddy propagation in a manner that links tropical SST changes to changes in storm tracks, with a more northward displaced path of eddy propagation when La Nina conditions prevail. The summer season Atlantic influence seems to be mediated by a direct thermally forced subtropical stationary wave but the wintertime response seems to first involve an influence of Atlantic SSTs on tropical Pacific precipitation and a subsequent forcing of stationary waves that propagate from there poleward and eastward. These mechanisms are illustrated with controlled experiments with fully nonlinear global models that allow an analysis of the day-by-day evolution of the atmospheric circulation and moisture transport adjustments to instantaneously imposed tropical SST anomalies.