The discrepancy between the observed lack of surface warming in the eastern equatorial Pacific and climate model projections of an El Niño-like warming pattern confronts the climate research community. While anthropogenic aerosols have been suggested as a cause, the prolonged cooling trend over the equatorial Pacific appears in conflict with Northern Hemisphere aerosol emission reduction since the 1980s. Here, using CESM, we show that the superposition of fast and slow responses to aerosol emission change-an increase followed by a decrease-can sustain the La Niña-like condition for a longer time than expected. The rapid adjustment of Hadley Cell to aerosol reduction triggers joint feedback between low clouds, wind, evaporation, and sea surface temperature in the Southeast Pacific, leading to a wedge-shaped cooling that extends to the central equatorial Pacific. Meanwhile, the northern subtropical cell gradually intensifies, resulting in equatorial subsurface cooling that lasts for decades.

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.

This study investigates the transient responses of atmospheric energy and momentum fluxes to a time-invariant extratropical thermal heating in an atmospheric model coupled to an aquaplanet mixed layer ocean with the goal of understanding the mechanisms and time scales governing the extratropical-to-tropical connection. Two distinct stages are observed in the teleconnection: 1) A decrease in the meridional temperature gradient in midlatitudes leads to a rapid weakening of the eddy momentum flux and a slight reduction of the Hadley cell strength in the forced hemisphere. 2) The subtropical trades in the forced hemisphere decrease and reduce evaporation. The resulting change to sea surface temperature leads to the development of a cross-equatorial Hadley cell, and the intertropical convergence zone shifts to the warmer hemisphere. The Hadley cell weakening in the first stage is related to decreased eddy momentum flux divergence, and the response time scale is independent of the mixed layer depth. In contrast, the time taken for the development of the cross-equatorial cell in the latter stage increases as the mixed layer depth increases. Once developed, the deep tropical cross-equatorial cell response is an order of magnitude stronger than the initial subtropical response and dominates the anomalous circulation. The analysis combines the momentum and energetic perspectives on this extratropical-to-tropical teleconnection and moreover shows that the subtropical circulation changes associated with the momentum budget occur with a time scale that is distinct from the deep tropical response determined by the thermal inertia of the tropical ocean.