Effects of eddy variability on the response of the ACC to global warming (R'Eddy)
Final Report Abstract
The host of coarse- to high-resolution model simulations performed in this project have, most generally, emphasized the two key factors that have to be accounted for in simulations of the ACC response to changes in atmospheric conditions, either due to natural variability or anthropogenic climate trends: the mesoscale eddy field as a crucial component of the dynamics which fundamentally mediates changes in the mean flow and meridional overturning; and the concomitant changes in both intermediate and deep water masses formed within or south of the ACC. The simulations basically confirmed previous hypotheses of the “eddy saturation” in the ACC dynamics, implying that decadal trends in wind forcing become primarily manifested in the energy of the eddy fields, with little changes in the mean flow strengths. The resulting changes in the Southern Ocean eddy energies are, however, not spatially uniform, and in some regions effectively masked by intrinsic variations of a stochastic nature: in this regard the model results could help to identify those regions with an optimal signal-to-noise ratio needed to detect possible future changes, e.g., in sustained satellite observations. In contrast to the wind forcing-related changes in the eddy part, long-term changes in the mean flow of the ACC will more likely be governed by changes in the water masses, especially in the density of Antarctic Bottom Water (AABW): more specifically, if the currently observed warming trend of the abyssal waters around Antarctica (and thus, loss of dense AABW) should continue, the implied reduction in the meridional density gradient may result in a weakening of the ACC which could outweigh the weak effect of an intensification in the westerlies. The presence of these antagonistic effects clearly poses a ‘grand challenge’ for climate model studies aiming at simulations of future trends induced, e.g., by a continuation of the recent poleward shift in the southern hemisphere westerlies. Current work (e.g., by Patara in the Kiel Excellence Cluster “The Future Ocean”) building on the model developments and understanding gained in this DFG project, consequently aims to further elucidate the combined impact of these responses on the uptake of anthropogenic CO2, and to refine projections of future changes induced by the expected anthropogenic trends in the atmospheric conditions over the Southern Ocean.
Publications
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(2014): Abyssal ocean warming around Antarctica strengthens the Atlantic Overturning Circulation. Geophys. Res. Lett.
Patara L. and Böning C.W.
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An assessment of Southern Ocean water masses and sea ice during 1988-2007 in a suite of interannual CORE-II simulations. Ocean Modelling, Volume 94, October 2015, Pages 67-94
Downes S., R. Farneti, P. Uotila, S. M. Griffies, S. J. Marsland, D. Bailey, E. Behrens, M. Bentsen, C. Böning + 10 weitere
