Eddy Compensation Dampens Southern Ocean Sea Surface Temperature Response to Westerly Wind Trends

Abstract

Anthropogenic influences have led to a strengthening and poleward shift of westerly winds over the Southern Ocean, especially during austral summer. We use observations, an idealized eddy-resolving ocean sea ice channel model, and a global coupled model to explore the Southern Ocean response to a step change in westerly winds. Previous work hypothesized a two time scale response for sea surface temperature. Initially, Ekman transport cools the surface before sustained upwelling causes warming on decadal time scales. The fast response is robust across our models and the observations: We find Ekman-driven cooling in the mixed layer, mixing-driven warming below the mixed layer, and a small upwelling-driven warming at the temperature inversion. The long-term response is inaccessible from observations. Neither of our models shows a persistent upwelling anomaly, or long-term, upwelling-driven subsurface warming. Mesoscale eddies act to oppose the anomalous wind-driven upwelling, through a process known as eddy compensation, thereby preventing long-term warming.