Southern Ocean Cooling in a Warming World: Reassessing the Role of Westerly Winds

Abstract:

In contrast to the global warming trend and the loss of Arctic sea ice, the Southern Ocean has exhibited a gradual decrease in sea surface temperatures (SSTs) and a net expansion of the sea ice cover over recent decades. Moreover, historical simulations with CMIP5 global climate models do not reproduce the observed cooling around Antarctica and, instead, predict slow but steady warming and sea ice loss.

Here we identify enhanced Ekman transport as a possible mechanism allowing the Southern Ocean to cool. We further discuss the discrepancy between observations and CMIP5 historical simulations. The latter do not represent consistently the strengthening and the poleward shift of the Southern Hemisphere surface westerlies – due to internal variability or an inadequate representation of ozone forcing. We propose that under a realistic evolution of surface winds, CMIP5 models can produce cooling trends around Antarctica with magnitudes and spatial patterns similar to observations. To that end we consider the unforced preindustrial control runs of CMIP5 models and examine periods with multi-decadal trends in the speed and position of the Southern Hemisphere surface westerlies that are comparable to the 1979-2014 trends. Strengthening and southward displacement of surface winds produce an SST dipole around Antarctica: cooling south of 50S and warming in a zonal band along 30-50S, similar to observed patterns. The wind-induced cooling trends in the Southern Ocean are large enough to locally overwhelm the effect of greenhouse gas forcing.

We reconcile our findings with those of modeling studies which suggest that poleward intensification of the westerlies leads to warming rather than cooling trends around Antarctica. We show that the Southern Ocean response to a wind perturbation is non-monotonic in time. An initial cooling regime can transition into a warming regime. Some of the inter-model diversity in these fast and slow responses is related to differences in the models’ mean background stratification of the Southern Ocean.

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