Simulation the impact of shifts in Southern Ocean westerlies at LGM on ocean physics and atmospheric CO2

We explore the impact of a latitudinal shift in the we- sterly wind belt over the Southern Ocean (SO) on the Atlantic meridional overturning circulation (AMOC) and on the carbon cycle for Last Glacial Maximum background conditions using a state-of-the-art ocean general circulation model. For this “westerly wind hypothesis” (Toggweiler et al. 2006) we find that a southward shift in the westerly winds leads to an intensification of the AMOC (northward shift to a weakening). This agrees with other studies (Sijp & England 2009) starting from pre-industrial background, but the responsible processes are different. During deglaciation a gradual shift in westerly winds might thus be responsible for a part of the AMOC enhancement, which is indicated by various studies. The net effects of the changes in ocean circulation lead to a rise in atmospheric pCO2 of less than 10 μatm for both a northward and a southward shift in the winds. For northward shifted winds the zone of upwelling of carbon and nutrient rich waters in the Southern Ocean is expanded, leading to more CO2 out-gassing to the atmosphere but also to an enhanced biological pump in the subpolar region. For southward shifted winds the upwelling region contracts around Antarctica leading to less nutrient export northwards and thus a weakening of the biological pump. A shift in the southern hemisphere westerly wind belt is probably not the domi- nant process which tightly couples atmospheric CO2 rise and Antarctic temperature during deglaciation which is suggested by the ice core data