Abstract
Despite global warming, total Antarctic sea ice coverage increased over 1979–2013. However, the majority of Coupled Model Intercomparison Project phase 5 models simulate a decline. Mechanisms causing this discrepancy have so far remained elusive. Here we show that weaker trends in the intensification of the Southern Hemisphere westerly wind jet simulated by the models may contribute to this disparity. During austral summer, a strengthened jet leads to increased upwelling of cooler subsurface water and strengthened equatorward transport, conducive to increased sea ice. As the majority of models underestimate summer jet trends, this cooling process is underestimated compared with observations and is insufficient to offset warming in the models. Through the sea ice-albedo feedback, models produce a high-latitude surface ocean warming and sea ice decline, contrasting the observed net cooling and sea ice increase. A realistic simulation of observed wind changes may be crucial for reproducing the recent observed sea ice increase.
Highlights
Despite global warming, total Antarctic sea ice coverage increased over 1979–2013
The Coupled Model Intercomparison Project phase 5 (CMIP5) models show Southern Ocean surface warming over most regions (Fig. 1d), there is no intermodel consensus in terms of warming at high latitudes
The temperature response associated with the Ekman upwelling of salty subsurface water dampens the direct Ekman–sea surface temperature (SST) response and this may be the reason why no significant Ekman–sea ice extent (SIE) relationship is found directly in the models over the 35-year period assessed. Both the observed and CMIP5 SIE trends are linked to the westerly wind jet intensification through the influence of SST
Summary
Total Antarctic sea ice coverage increased over 1979–2013. the majority of Coupled Model Intercomparison Project phase 5 models simulate a decline. Models have linked hemispheric-scale wind changes associated with the positive trend in the Southern Annular Mode[17] (SAM), attributed to increasing greenhouse gases and stratospheric ozone depletion[18,19,20], to Southern Ocean warming and a sea ice decline[21,22,23] This contrasts interannual variations, in which a positive SAM intensifies the westerly jet and shifts it polewards, resulting in cool sea surface temperature (SST) and increased sea ice extent (SIE) at most longitudes due to enhanced Ekman drift[24,25,26]. In (a) AS, Amundsen Sea; BS, Bellingshausen Sea; RS, Ross Sea; WS,Weddell Sea
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