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Fast and slow responses of Southern Ocean sea surface temperature to SAM in coupled climate models
We investigate how sea surface temperatures (SSTs) around Antarctica respond to the Southern An- nular Mode (SAM) on multiple timescales. To that end we examine the relationship between SAM and SST within unperturbed preindustrial control simulations of coupled general circulation models (GCMs) included in the Climate Modeling Intercomparison Project phase 5 (CMIP5). We develop a technique to extract the re- sponse of the Southern Ocean SST (55◦S−70◦S) to a hypothetical step increase in the SAM index. We demonstrate that in many GCMs, the expected SST step re- sponse function is nonmonotonic in time. Following a shift to a positive SAM anomaly, an initial cooling regime can transition into surface warming around Antarctica. However, there are large differences across the CMIP5 ensemble. In some models the step response function never changes sign and cooling persists, while in other GCMs the SST anomaly crosses over from negative to positive values only three years after a step increase in the SAM. This intermodel diversity can be related to differences in the models’ climatological thermal ocean stratification in the region of seasonal sea ice around Antarctica. Exploiting this relationship, we use obser- vational data for the time-mean meridional and vertical temperature gradients to constrain the real Southern Ocean response to SAM on fast and slow timescales
Signatures of the Antarctic ozone hole in Southern Hemisphere surface climate change
Anthropogenic emissions of carbon dioxide and other greenhouse gases have driven and will continue to drive widespread climate change at the Earth’s surface. But surface climate change is not limited to the effects of increasing atmospheric greenhouse gas concentrations. Anthropogenic emissions of ozone-depleting gases also lead to marked changes in surface climate, through the radiative and dynamical effects of the Antarctic ozone hole. The influence of the Antarctic ozone hole on surface climate is most pronounced during the austral summer season and strongly resembles the most prominent pattern of large-scale Southern Hemisphere climate variability, the Southern Annular Mode. The influence of the ozone hole on the Southern Annular Mode has led to a range of significant summertime surface climate changes not only over Antarctica and the Southern Ocean, but also over New Zealand, Patagonia and southern regions of Australia. Surface climate change as far equatorward as the subtropical Southern Hemisphere may have also been affected by the ozone hole. Over the next few decades, recovery of the ozone hole and increases in greenhouse gases are expected to have significant but opposing effects on the Southern Annular Mode and its attendant climate impacts during summer