Oceanic Role in the Teleconnection from Southern Ocean to Tropical Pacific

Delayed warming of the Southern Ocean is one of the most robust features of CO2-driven climate response. This relative initial cooling in the Southern Ocean has recently been shown to have far-reaching impacts into the equatorial Pacific. The Southern Ocean-driven teleconnection mechanism has been examined in the atmospheric perspective, hence little is known about the role of ocean dynamics. In this study, we investigate the oceanic role in the teleconnection from the Southern Ocean to the tropical Pacific by applying a time-invariant zonally uniform surface heating between 40°S and 60°S, mimicking the effect of delayed warming of the Southern Ocean, using Community Earth System Model 1.2.2 (CESM1-CAM4). To better understand the oceanic role, we separate the contributions from buoyancy-driven and wind-driven ocean circulation by conducting two additional experiments in which the wind stress is prescribed to a repeating daily climatology, applied globally in one case and outside the tropical band between 10ºS and 10ºN in the other.
Consistent with previous studies, prescribed Southern Ocean warming leads to a weakening of the southern Hadley cell, inducing anomalous low pressure centered at 40°S and anomalous westerly wind from 20°S to 40°S. Anomalous westerly wind in the fully coupled experiment can cool the southwest Pacific by inducing northward Ekman transport, while both wind-stress prescribed cases do not experience anomalous Ekman transport and cooling effect. On the other hand, the southeast Pacific experiences more warming in the fully coupled experiment compared to other two cases, due to reduced coastal upwelling along the west coast of South America and weaker trade winds. In all experiments, reduced trade winds over the equator warm up the eastern Pacific, and smaller equatorial zonal temperature gradient induces weaker Walker circulation in fully coupled and tropical band coupled experiments, inducing further warming at the equatorial Pacific by reducing the Bjerknes feedback. However, even the global wind stress prescribed experiment experiences anomalous ocean heat release and warming, especially over the eastern equatorial Pacific, due to the shallow mean thermocline position and anomalously warmer thermocline. Ocean buoyancy change in the southern Pacific mid-latitudes can induce anomalous subtropical cell change in larger magnitude compared to the northern hemisphere, and this subtropical cell change interhemispheric asymmetry can induce heat convergence and warming of the equatorial thermocline.
These experiments reveal that a weakening of the equatorial upwelling motion is primarily driven by surface wind stress changes at low latitudes, while the equatorial sub-surface ocean heat convergence arises from the hemispherically asymmetric ocean subtropical cell changes driven by buoyancy changes. Not only the wind stress changes coupled to the southward ITCZ shift but also the buoyancy reduction in the southern Pacific mid-latitudes can contribute to the change in ocean circulation, contributing to the extratropics to tropics teleconnection effect. In conclusion, this research parsed out the importance of the buoyancy driven ocean circulation change for the extra-tropics to tropics teleconnection mechanism in the southern hemisphere.