Abstract

Interannual variability in the Southern Annular Mode (SAM) and sea ice covary such that an increase and southward shift in the surface westerlies (a positive phase of the SAM ) coincides with an expansion of the sea ice cover, as seen in observations and models alike. Yet, the sea ice extent decreases in response to sustained, multi-decadal positive SAM -like wind anomalies driven by 20th century ozone depletion in modeling studies. Why does sea ice appear to have opposite responses to SAM -like variability on interannual and multidecadal timescales? We demonstrate that the response of sea ice and the Southern Ocean to ozone depletion is a two timescale problem. The two timescales emerge because the fast processes of the surface mixed layer and the slow dynamics of the ocean interior drive Sea Surface Temperature anomalies of opposite sign in response to a SAM perturbation.

This behavior is seen in two models (one with an idealized geometry and another, more sophisticated, global climate model) although we find great uncertainties in the timescale of the transition from the fast to the slow response. Processes that controls this transition are clarified. Implications for the interpretation of observed decadal changes and future climate change in the Southern Hemisphere are discussed.