Eddy compensation dampens Southern Ocean SST response to westerly winds

Edward Doddridge, John Marshall, Hajoon Song, Maxwell Kelley & Larissa Nazarenko

Abstract

Anthropogenic influences have led to a strengthening and poleward shift of the westerly winds blowing over the Southern Ocean (SO), especially during the austral summer months. We use observations, an idealized high-resolution eddying-sea-ice channel model, and a global coupled climate model to explore the response of the SO to a step-change in the westerly wind. Previous work has hypothesized a two timescale response for sea surface temperature. Initially, horizontal Ekman transport away from Antarctica cools the surface before sustained upwelling of warm subsurface water leads to warming on decadal timescales. We find that the fast timescale response is robust across our two models and in accord with our analysis of observations: it consists of Ekman driven cooling in the mixed layer, warming at the temperature inversion due to anomalous upwelling, and warming in the seasonal thermocline due to enhanced vertical mixing. The long-term response is inaccessible from observations. However, neither of our models shows a long term sub-surface warming. In our eddying channel this is a consequence of an eddy-driven circulation opposing the wind induced upwelling. This “eddy compensation” is also a feature of, although less pronounced in, our coupled climate model. Our results highlight the importance of accurately representing the mesoscale eddy contribution to the residual overturning circulation. We conclude that climate models which exhibit pronounced subsurface warming due to wind-induced upwelling are inconsistent with our understanding of SO dynamics and eddy compensation, and are unlikely to be able to capture the observed multi-decadal cooling SST trend around Antarctica.

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