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

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. Plain Language Summary: Anthropogenic aerosols are an important radiative forcing on the climate system contributing to observed climate variability. In prior modeling studies, idealized aerosol‐like forcing applied to Northern Hemisphere high latitude regions has resulted in a tropical La Niña‐like response in the Eastern Equatorial Pacific. In this study, we investigate the pathway by which high latitude aerosol‐like cooling is communicated to the tropics via a sequence of ocean‐atmosphere positive feedback processes. We explore this pathway further by parsing out the total climate response into surface forcing which alters the buoyancy of seawater via heat or freshwater and surface wind stress forcing which alters the momentum of seawater. We find that subtropical patterns, arising from low clouds and wind‐induced evaporative cooling, are primarily buoyancy‐forced, while tropical asymmetries arise from momentum forcing. The results show that this pathway is robust across seven climate models such that stronger subtropical cloud responses elicit stronger sea surface temperature responses in the equatorial Pacific. The results highlight the important link between extratropical aerosol‐like forcing and La Niña‐like patterns via these coupled ocean‐atmosphere feedbacks. The equatorial Pacific can drive major climate variability, suggesting global implications for these results. Key Points: A pathway of three coupled ocean‐atmosphere feedbacks communicates extratropical anomalous cooling and sustains a tropical responseBroadly, buoyancy‐forced adjustments dominate in the subtropics while momentum‐forced adjustments create zonal asymmetries in the tropicsThis mechanistic pathway seems to be robust across several global climate models [ABSTRACT FROM AUTHOR]