Impact of stratospheric dynamics and chemistry on northern hemisphere midlatitude ozone loss

The importance of dynamics for stratospheric ozone distribution in the northern hemisphere is investigated by using multiannual simulations of the coupled dynamic-chemical general circulation model ECHAM3/CHEM. This model includes a parameterization for heterogeneous reactions on the surfaces of polar stratospheric clouds (PSCs) and on sulfate aerosols. A warm and a cold stratospheric winter are examined to estimate the range of chemical ozone loss in the model due to heterogeneous reactions on PSCs. Ozone depletion in the model mainly occurs inside the polar vortex. An additional ozone reduction due to heterogeneous reactions on PSCs is found outside the polar vortex. Secondary vortex formation and vortex contraction after an elongation lead to a transport of air masses with chemically reduced ozone values out of the vortex. Except for such events the edge of the modeled polar vortex acts as a barrier to transport. During the formation of secondary vortices no additional heterogeneous reactions occur therein. Other dynamic events, such as the elongation of the polar vortex and its displacement to lower latitudes, lead to an intense ozone depletion. A minor stratospheric warming in the model causes a total deactivation of chlorine compounds and prevents further ozone depletion. In midlatitudes, the amplitude of short-term variations of total ozone is amplified by PSC heterogeneous chemical ozone reduction.