Atmosphere-ocean interactions in the Pacific Southern Ocean

Frank Lamy, Alfred Wegener Institute (AWI), Helmholtz Center for Polar and Marine Research, Bremerhaven, Germany, Rainer Gersonde, AWI Bremerhaven, Bremerhaven, Germany, Conor Purcell, Alfred Wegener Institute Helmholtz-Center for Polar and Marine Research Bremerhaven, Bremerhaven, Germany; School of Earth and Ocean Sciences, Cardiff University, Cardiff, United Kingdom, Gisela Winckler, Lamont-Doherty Earth Observatory of Columbia University, Palisades, United States, Ralf Tiedemann, Alfred-Wegener-Institute, Bremerhaven, Germany and Gregor Knorr, Alfred Wegener Institute Helmholtz Center for Polar and Marine Research, Bremerhaven, Germany
Abstract:
Atmosphere-ocean interactions play an important role for understanding processes and feedbacks in the Southern Ocean (SO) that play a key role for explaining the variability in atmospheric CO2 concentrations. The most important atmospheric forcing at high and mid-latitudes of the Southern Hemisphere is the westerly wind belt which strongly impacts the strength and extension of the Antarctic Circumpolar Current (ACC), upwelling of deep-water masses, and also controls the back-flow of intermediate waters to the tropics.

We combine sea surface temperature, current strength, and mineral dust proxy data from the Pacific SO including Drake Passage with climate model results. Our data show that Drake Passage throughflow was reduced and the ACC generally weakened during the last glacial. The reduced Drake Passage throughflow was accompanied by a pronounced northward extension of the Antarctic cold-water sphere in the Southeast Pacific sector and stronger export of surface and intermediate water into the South Pacific gyre. These oceanographic changes are consistent with reduced westerly winds within the modern maximum wind strength zone over the subantarctic ACC and reduced wind forcing due to extended sea-ice further south. Despite of reduced winds in the core of the westerlies, we observe 3-fold higher dust deposition during glacial periods in the Pacific SO. This observation may be explained by a combination of factors including more expanded arid dust source areas in Australia and a northward extent or enhancement of the westerlies over Southeast Australia during glacials that would plausibly increase the dust uptake and export into the Pacific SO. Such scenario would imply stronger westerlies at the present northernmost margin of the wind belt coeval with weaker core westerlies and reduced ACC strength including Drake Passage throughflow during glacials. These results have strong implications for the global meridional overturning circulation and the interbasin exchange in the SO.

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