Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 8 4 2008 10.5194/acpd-8-16555-2008 http://www.atmos-chem-phys-discuss.net/8/16555/2008/ http://www.atmos-chem-phys-discuss.net/8/16555/2008/acpd-8-16555-2008.html http://www.atmos-chem-phys-discuss.net/8/16555/2008/acpd-8-16555-2008.pdf 16555 16583 2008-09-01 Past and future conditions for polar stratospheric cloud formation simulated by the Canadian Middle Atmosphere Model P. Hitchcock peterh@atmosp.physics.utoronto.ca T. G. Shepherd C. McLandress Department of Physics, University of Toronto, Toronto, ON, Canada Observations of the Arctic winter lower stratosphere over the past four decades suggest that the thermodynamic conditions required for the formation of polar stratospheric clouds (PSCs) have become increasingly widespread in the Northern Hemisphere. The trend is apparent only in the coldest winters during which the Arctic stratosphere is minimally disturbed by upwelling wave activity from the troposphere. The mechanism responsible for this increase remains unclear. In an effort to evaluate possible mechanisms, we analyze here the polar stratospheric temperatures in an ensemble of three 150-year integrations of the Canadian Middle Atmosphere Model (CMAM), an interactive chemistry-climate model which simulates ozone depletion and recovery, as well as climate change. <br><br> We find that in the Antarctic winter lower stratosphere, the low temperature extremes required for PSC formation increase 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 Antarctic response. <br><br> The model trend in low temperature extremes in the Arctic 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 extremes. In the future, radiative cooling in the Arctic winter due to climate change is more than compensated by an increase in dynamically driven downwelling over the pole. Austin, J., Shindell, D., Beagley, S R., Brühl, C., Dameris, M., Manzini, E., Nagashima, T., Newman, P., Pawson, S., Pitari, G., Rozanov, E., Schnadt, C., and Shepherd, T G.: Uncertainties and assessments of chemistry-climate models of the stratosphere, Atmos. Chem. 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