On the acceleration of ENSO decay by northern tropical Atlantic SST anomalies

Ingo Richter, JAMSTEC Japan Agency for Marine-Earth Science and Technology, Application Laboratory, Research Institute for Value-Added-Information Generation, Yokohama, Japan, Hiroki Tokinaga, Kyushu University, Research Institute for Applied Mechanics, Fukuoka, Japan, Yu Kosaka, University of Tokyo, Research Center for Advanced Science and Technology, Tokyo, Japan and Takeshi Doi, JAMSTEC, Yokohama 236-0011, Japan
Abstract:
It has been known for some time that SST anomalies in the equatorial Pacific induce same-signed anomalies in the northern tropical Atlantic (NTA) through changes in the Atlantic trade winds. More recently, it has been suggested that the NTA, in turn, can influence the equatorial Pacific through changes in the surface zonal winds over the western part of the basin. The sign of these wind anomalies, however, would act to force equatorial Pacific SST anomalies that are opposite to those in the NTA. The present study takes a closer look at this apparent conundrum by analyzing GCM experiments with prescribed SSTs for the period 1982-2014. In the control experiment, observed SSTs are prescribed globally, while in two sensitivity experiments SST anomalies are removed from either the tropical Pacific or the tropical Atlantic. In the control experiment, NTA cold events during late winter and early spring are accompanied by a weakening of the Aleutian low and easterly wind anomalies in the western equatorial Pacific. This is consistent with the concomitant cool SST anomalies in the tropical Pacific. When tropical Pacific SST anomalies are removed, on the other hand, NTA warming coincides with a deepening of the Aleutation low and westerly anomalies on the equator. Moreover, when SST anomalies are removed from the tropical Atlantic, wind variability over the western equatorial Pacific is enhanced. These results suggest that SST anomalies in the NTA, though initially excited from the tropical Pacific, accelerate the decay of ENSO events and reduce variability in the tropical Pacific. This interbasin link may be further complicated by other influences on the NTA, such as local air-sea coupling and the North Atlantic Oscillation.