A Pathway for Northern Hemisphere Extratropical Cooling to Elicit a Tropical Response

Abstract

Previous studies have found that Northern Hemisphere aerosol-like cooling induces a La Niña-like response in the tropical Indo-Pacific. Here, we explore how a coupled ocean-atmosphere feedback pathway communicates and sustains this response. We override ocean surface wind stress in a comprehensive climate model to decompose the total ocean-atmosphere response to forced extratropical cooling into the response of surface buoyancy forcing alone and surface momentum forcing alone. In the subtropics, the buoyancy-forced response dominates: the positive low cloud feedback amplifies sea surface temperature (SST) anomalies which wind-driven evaporative cooling communicates to the tropics. In the equatorial Indo-Pacific, buoyancy-forced ocean dynamics cool the surface while the Bjerknes feedback creates zonally asymmetric SST patterns. Although subtropical cloud feedbacks are model-dependent, our results suggest this feedback pathway is robust across a suite of models such that models with a stronger subtropical low cloud response exhibit a stronger La Niña response.