Extratropical energy forcing exerts a strong influence on tropical sea surface temperature (SST) via both oceanic and atmospheric teleconnection pathways. While recent studies have emphasized the role that coupled surface ocean-atmospheric feedbacks play in communicating subtropical temperature anomalies to the tropical Pacific, details of the subsurface adjustment remain unclear. In this work, we explore how the Indo-Pacific Ocean adjusts to Northern Hemisphere temperature forcing using a novel set of ocean-only simulations in which SST is relaxed to a specified anomaly in the Northeast Pacific. We clarify the timescale of the subsurface extratropicaltropical teleconnection (the so-called "ocean tunnel") by tracking the transient adjustment, and we find that cooling in the Northeast Pacific dynamically adjusts subsurface temperature and circulation through baroclinic wave activity. Within ten years of forcing, the equatorial Pacific features a shoaled thermocline and a La Niña-like SST cooling in the eastern equatorial Pacific. In the quasi-steady response, forced planetary waves create horizontal temperature asymmetries which then drive an equatorially asymmetric subtropical cell response via thermal wind balance. This cell transports heat to the cooled hemisphere. We find similar temperature patterns and cross-equatorial responses when forcing is applied in the Northwest and Southeast Pacific. Our results provide insight into how hemispherically asymmetric temperature forcing can drive cross-equatorial adjustments without any changes in wind. This adjustment has important implications for how the equatorial Pacific may respond to both natural and anthropogenic subtropical forcing, and it creates a more complete understanding of meridional energy transport within the coupled ocean-atmosphere system.

A document by Matt Luongo. Click on the document to view its contents.

Previous studies have found that Northern Hemisphere aerosol-like cooling induces a La Nina-like quasi-equilibrium response in the tropical Indo-Pacific. Here, we explore a coupled atmosphere-ocean feedback pathway by which this response is communicated. 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 are then communicated to the tropics via wind-driven evaporative cooling. In the deep tropics, the momentum-driven Bjerknes feedback creates zonally asymmetric SST patterns in the Indian and Pacific basins. 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 Nina response.

While the higher mean Equilibrium Climate Sensitivity (ECS) in CMIP6 has been attributed to more positive cloud feedbacks, it is unclear what causes the greater range of ECS values across CMIP6 models compared to CMIP5. Here we investigate the relationship between radiative forcing and cloud feedbacks across the two model generations to explain the very high ECS values in some CMIP6 models. The relationship is sensitive to the definition of the forcing, particularly in CMIP6, but fixed-SST simulations suggest the shortwave cloud feedback ($\lambda_{SW, cl}$) is anti-correlated with the forcing in CMIP5 and weakly positively correlated with the forcing in CMIP6. These relationships reflect the cloud adjustment to the forcing, which is anti-correlated with $\lambda_{SW, cl}$ in CMIP5 and positively correlated in CMIP6. Although we are unable to identify a systematic change across the model generations, we do show that modifications to the land components of climate models are not responsible for the change in the relationship between cloud adjustments and cloud feedbacks, and that cloud adjustments are generally driven by low and, especially mid-level clouds. Moreover, models derived from the MOHC and NCAR modeling centers seem to be responsible for much of the trend between CMIP5 and CMIP6. Our analysis is severely limited by the available simulations, highlighting the need for targeted simulations to probe the sources of intermodel differences in cloud adjustments.