Atmospheric angular momentum balance for the southern hemisphere during the polar vortex break-up of September 2002

Abstract

The evolution of the axial component of the atmospheric angular momentum vector (AAM) is investigated for September 2002, using ECMWF analyses. The components of the conservation equation of AAM for southern polar cap volume are calculated in order to examine the importance of their contributions to the AAM tendency during the unexpected break-up of the southern polar vortex in the last week of September.

The AAM tendency for the mean polar cap (90◦–65◦ S) oscillates with a period of 5 d until the break-up of the polar vortex, after that it is negative for about 9 d. The polar friction torque induced by katabatic winds is weakly positive and nearly constant. For this mean polar cap, the mountain torque at the surface is mainly positive and larger than the friction torque. The strong changes of the mountain torque are caused by shifts of surface pressure patterns induced by a Rossby wave train touching the orography of Antarctica. The fluxes of AAM are northwards and reduce the AAM of the polar cap volume. The changes of AAM fluxes are mainly determined by the evolution of momentum fluxes due to transient Rossby waves in the upper troposphere and lower stratosphere. The vortex break-up is associated with a strong decrease of the amount of mountain torque and an increase in the convergence of relative AAM fluxes.