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Summary of “Increase in Agulhas leakage due to poleward shift of Southern Hemisphere westerlies” by Biastoch et al. (2009; Nature)

Biastoch et al. (2009) uses a numerical simulation of the global oceanic circulation forced by realistic winds of the last 40 years to show that the poleward shift of the large-scale wind pattern over the Southern Ocean due to global warming has resulted into 1) a poleward shift of the boundary between the anticyclonic (Indian) and cyclonic (Southern Ocean) circulation and 2) a decrease in the transport of the Agulhas current followed by an increase of the leakage of this current into the South Atlantic Ocean. Interestingly, because of topographic barriers and strong nonlinearity within the Agulhas current, it is not possible to correlate the change in the large-scale wind and the change in the Agulhas transport as we would expect from Sverdrup theory. The increase of the leakage is about 1.2 Sv per decade. Such increase brings warmer and saltier water, inter alia, into the North Atlantic Ocean where it could disturb the formation of deep water. Such decadal increase in salinity is effectively being observed off Brazil within the North Brazil Current.