The Final Warming of the Stratosphere

Abstract

The dramatic changes during spring in the shape of the polar vortex in the stratosphere of the two hemispheres are illustrated using perspective plots of the three-dimensional structure of the isotach and potential vorticity fields. In the southern hemisphere, the vortex shape evolves from a cone, slightly expanding with height and nearly symmetric about the pole, to an inverted cone, distorted and displaced from the pole. At 10 mb, in particular, the flow reverses to easterlies some time after the temperature gradient at high latitudes reverses. In the northern hemisphere, there is much more variability in both the structure of the flow and the time of the breakdown of the westerly vortex. At 10 mb, in particular, the reversal of the flow may precede or follow that of the temperature gradient. There are also important interhemispheric differences in the location and magnitude of the largest temperature increases over the polar regions during spring. Those in the southern hemisphere are in the lower stratosphere whereas those in the northern hemisphere are in the upper stratosphere, the former being almost twice as large as the latter. The values of minimum temperatures in the lower stratosphere suggest that in early spring conditions suitable for the formation of polar stratospheric clouds, thought to play a key role in ozone destruction, are the rule in the southern hemisphere and the exception in the northern hemisphere.