Atmospheric Chemistry and Physics Past and future conditions for polar stratospheric cloud formation simulated by the Canadian Middle Atmosphere Model

Abstract. We analyze here the polar stratospheric temper-atures in an ensemble of three 150-year integrations of the Canadian Middle Atmosphere Model (CMAM), an interac-tive chemistry-climate model which simulates ozone deple-tion and recovery, as well as climate change. A key motiva-tion is to understand possible mechanisms for the observed trend in the extent of conditions favourable for polar strato-spheric cloud (PSC) formation in the Arctic winter lower stratosphere. We find that in the Antarctic winter lower stratosphere, the low temperature extremes required for PSC formation in-crease in the model as ozone is depleted, but remain steady through the twenty-first century as the warming from ozone recovery roughly balances the cooling from climate change. Thus, ozone depletion itself plays a major role in the Antarc-tic trends in low temperature extremes. The model trend in low temperature extremes in the Arc-tic through the latter half of the twentieth century is weaker and less statistically robust than the observed trend. It is not projected to continue into the future. Ozone depletion in the Arctic is weaker in the CMAM than in observations, which may account for the weak past trend in low temperature ex-tremes. In the future, radiative cooling in the Arctic winter due to climate change is more than compensated by an in-crease in dynamically driven downwelling over the pole.