Comparing the impacts of tropical SST variability and polar stratospheric 3 ozone loss on the Southern Ocean westerly winds

24 Observed 850 hPa westerly wind trends over the Southern Ocean during 1979-2011 exhibit 25 strong regional and seasonal asymmetries. On an annual basis, trends in the Pacific Sector 26 (40°S-60°S; 70°W-160°W) are significant and three times larger than zonal-mean trends related 27 to the increase in the Southern Annular Mode (SAM). Seasonally, the SAM-related trend is 28 significant in austral summer, and many studies have linked this trend with stratospheric ozone 29 depletion. The Pacific Sector trends are significant in austral autumn. We propose that these 30 asymmetries can be explained by a combination of tropical teleconnections and polar ozone 31 depletion. A series of transient atmospheric model experiments, each forced with observed 32 tropical SSTs and a different combination of time-dependent radiative forcings, supports this 33 idea. In summer, the model simulates a positive SAM-like pattern, to which ozone depletion and 34 tropical SSTs both contribute, offsetting the insignificant direct contribution of non-ozone 35 radiative forcings (including greenhouse gasses). In autumn, the ensemble-mean response 36 consists of stronger westerlies over the Pacific Sector, explained by a Rossby wave originating 37 from the central equatorial Pacific. While these responses resemble observations, attribution is 38 complicated by intrinsic atmospheric variability. In the experiments forced only with tropical 39 SSTs, individual ensemble members can exhibit wind trend patterns that mimic the forced 40 response to ozone. When our analysis is applied to 1960-2000, the primary period of ozone loss, 41 ozone depletion entirely explains the model's SAM-like zonal wind trend. The time-varying 42 importance of these different drivers has implications for relating the historical experiments of 43 free-running, coupled models to observations.