The Dynamics of Sudden Stratospheric Warmings

Abstract

The normal global-scale circulation of the stratosphere is dominated by a zonally (longitudinally) symmetric mean flow that is westerly (from the west) in the winter hemisphere and easterly (from the east) in the summer hemisphere. Superposed on this zonal mean flow are longitudinally varying wave perturbations referred to as planetary waves. Both the zonal mean flow and the planetary waves are in approximate geostrophic balance (i.e. the horizontal winds blow parallel to the height contours on constant-pressure surfaces, with speed proportional to the height gradient, and with low heights on the left facing downstream in the northern hemisphere). Thus, the large-scale horizontal flow can be represented by maps of the topography of constant-pressure surfaces (see Figure 1). In the summer hemisphere the wave disturbances tend to be weak and transient so that the time mean circulation is to a good approximation an axially symmetric anticyclonic (clockwise rotating) vortex centered on the pole. The height contours on constant-pressure surfaces are nearly parallel to latitude circles with height increasing toward the pole. In the winter hemisphere, on the other hand, there are substantial quasi­ stationary planetary waves of zonal wavenumbers 1 and 2 (1 and 2 wave­ lengths around a latitude circle). Thus, height contours in the cyclonic winter polar vortex depart significantly from axial symmetry due primarily to the Aleutian high formed by the planetary waves. Occasionally, the amplitudes of the planetary wave disturbances in the winter hemisphere increase dramatically over the course of several days.