The vertical Advection of angular Momentum during the austral polar Vortex Splitting of September 2002

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

It is found that the AAM tendency for the mean polar cap (90° - 65° S) oscillates with a period of 5 days until the break-up of the polar vortex. The polar friction torque induced by katabatic winds is weakly positive and nearly constant in comparison with that of mid-latitudes which is strongly negative. For the polar cap average, the mountain torque is mainly positive, strongly variable and larger than the friction torque. The changes of the mountain torque are determined by shifts of surface pressure in relation to the orography of Antarctica. The fluxes of AAM are northwards and reduce the AAM of the polar cap. 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. During the polar vortex break-up there is a negative imbalance between mountain torque and the convergence of relative AAM fluxes.

The vortex splitting is associated with a strong decrease of the amount of mountain torque and an increase in the convergence of relative AAM fluxes. The changing torques are related to a Rossby wave train extending from the western coast of Australia southeastward over the Southern Pacific to the Antarctic Peninsula. The vertical advection of atmospheric angular momentum and its role for the vortex splitting will be discussed.