Filamentation and Breakdown of the Southern Polar Vortex

Filamentation and polar vortex breakdown are major dynamical events in the stratosphere. In this presentation we examine essential features in the processes leading to filamentation and polar vortex split that occurred in the southern stratosphere during September 2002. Our approach is based on dynamical systems theory and on the insights provided by application of a Lagrangian descriptor. Accordingly, we build a kinematic model of the flow at isentropic levels in the middle stratosphere. These models have a long history in the geophysical fluid dynamics community because they allow for a detailed parametric study of the influence of identified flow structures on transport and exchange of fluid parcels. The Lagrangian descriptor we use is known as the function M, whose usefulness for studies on stratospheric dynamics has been established by several previous papers. The design of our two-dimensional kinematic model is guided by the Fourier components of the geopotential field produced by the European Centre for Medium-Range Weather Forecast (ECMWF) reanalysis data. During September 2002, the flow in the southern stratosphere at these levels was characterized by a mean axisymmetric flow disturbed mainly by waves with planetary wavenumber 1 and 2, whose amplitudes and phase speeds varied in a time-dependent fashion. We show that our kinematic model is capable of reproducing, by adjusting its parameters, Lagrangian features that are strikingly similar to others present in the reanalysis data. If only wave 1 (2) is considered, the model produces one (two) hyperbolic trajectory(ies) that can erode material either from the poleward or equatorward fkanks of the jet, depending on location of the critical level. The presence of both waves one and two produce complex Lagrangian patterns, which are remarkably similar to those obtained from the reanalysis data. The hyperbolic trajectories are the Lagrangian objects that generate filamentation phenomena and the vortex pinching that leads to its breakdown.