On the distribution of cold air near the vortex edge in the lower stratosphere

Abstract

The horizontal distributions of air in the temperature ranges −70° > T > −77°C, −77° > T > −85°C and T < −85° C have been examined during the Arctic winter of 1988/1989 and the Antarctic winter of 1987, using T106 European Centre for Medium‐range Weather Forecasts (ECMWF) analyses of standard meteorological variables and potential vorticity. ER‐2 airborne data were used to define, using water vapor and nitrous oxide observations, a potential vorticity (PV) contour corresponding to a chemically defined vortex edge. The jet stream axis was used to select a PV contour corresponding to the maximum winds, or a dynamically defined vortex edge. Air in the range −70° > T > −77° C was confined within the dynamically defined Arctic vortex, with only small fractions appearing outside the chemically defined vortex. In the Antarctic, up to 14% of this air was outside the chemically defined vortex but very little was outside the dynamically defined vortex. After late January (Arctic) and late August (Antarctic), respectively, the chemically defined vortices shrank, being 27% and 20% smaller in February and September. The dynamically defined vortices remained essentially constant in area. There were no occurrences of nominal type II polar stratospheric clouds (PSCs) outside either vortex, however defined. One implication of these results is that heterogeneous conversion of ClONO2 and HCl to reactive forms of chlorine is confined almost entirely to the Arctic vortex, while some such conversion may occur at the edge of the Antarctic vortex. However, small and infrequent areas of air in the range −77° > T > −85°C did occur immediately outside both vortices, so separating the effects of peel‐off, type I PSCs and sulfuric acid aerosol processing may be difficult on occasions. What is clear is that all three processes are intimately associated with the polar night jet stream.