A Numerical Experiment on the Breakdown of a Polar Vortex due to Forced Rossby Waves

Abstract

In order to get a deeper insight into the stratospheric circulation during sudden warming events, evolution of a circumpolar vortex perturbed by forced Rossby waves is investigated for a wide range of external parameters by using a high-resolution barotropic model in a spherical domain. If the amplitude of the wave forcing is large, the main polar vortex breaks down and the absolute vorticity is mixed irreversibly over the hemisphere. On the other hand, the main vortex migrates off the pole during the forcing period but returns to the pole afterwards without much erosion, if the amplitude of the forcing is small. The evolution of the polar vortex depends not only on the amplitude of wave forcing but also on other parameters such as latitudinal and longitudinal scales of the forcing, intensity of the polar vortex and latitudinal configuration of the vortex. The polar vortex breaks easily, if latitudinal extent of the forcing is wide or zonal wavenumber of the forcing is not 1 but 2. If the initial polar vortex is strong enough, there is a clear separation between the two types of the response at a critical amplitude of the wave forcing, i. e., breakdown or migration of the main polar vortex. Moreover, the evolution is highly dependent on the latitudinal configuration of the initial zonal wind over the globe : If the initial zonal wind is symmetric with respect to the equator, the vortex breaks easily. On the other hand, it is very robust if the zonal wind is anti-symmetric.