Antarctic ocean and sea ice response to ozone depletion : a two 1 timescale problem

6 The response of the Southern Ocean to a repeating seasonal cycle of ozone loss is studied 7 in two coupled climate models and found to comprise both fast and slow processes. The 8 fast response is similar to the inter-annual signature of the Southern Annular Mode (SAM) 9 on Sea Surface Temperature (SST), on to which the ozone-hole forcing projects in the sum10 mer. It comprises enhanced northward Ekman drift inducing negative summertime SST 11 anomalies around Antarctica, earlier sea ice freeze-up the following winter, and northward 12 expansion of the sea ice edge year-round. The enhanced northward Ekman drift, however, 13 results in upwelling of warm waters from below the mixed layer in the region of seasonal sea 14 ice. With sustained bursts of westerly winds induced by ozone-hole depletion, this warming 15 from below eventually dominates over the cooling from anomalous Ekman drift. The result16 ing slow-timescale response (years to decades) leads to warming of SSTs around Antarctica 17 and ultimately a reduction in sea-ice cover year-round. This two-timescale behavior rapid 18 cooling followed by slow but persistent warming is found in the two coupled models anal19 ysed, one with an idealized geometry, the other a complex global climate model with realistic 20 geometry. Processes that control the timescale of the transition from cooling to warming, 21 and their uncertainties are described. Finally we discuss the implications of our results for 22 rationalizing previous studies of the effect of the ozone-hole on SST and sea-ice extent. 23