Modeling Rossby Wave Breaking in the Southern Spring Stratosphere

Rossby wave breaking (RWB) plays a central role in the evolution of stratospheric flows. The generation and evolution of RWB is examined in the simple dynamical framework of a one-layer shallow-water system on a sphere. The initial condition represents a realistic, zonally symmetric velocity profile corresponding to the springtime southern stratosphere. Single zonal wavenumber Rossby waves, which are either stationary or traveling zonally with realistic speeds, are superimposed on the initial velocity profile. Particular attention is placed on the Lagrangian structures associated with RWB. The Lagrangian analysis is based on the calculation of trajectories and the application of a diagnostic tool known as the "M" function. Hyperbolic trajectories (HTs), produced by the transverse intersections of stable and unstable invariant manifolds, may yield chaotic saddles in M. Previous studies associated HTs with "cat's eyes" generated by planetary wave breaking at the critical levels. HTs, and hence RWB, are found both outside and inside the stratospheric polar vortex (SPV). Significant findings are as follows: (i) stationary forcing produces HTs only outside of the SPV and (ii) eastward-traveling wave forcing can produce HTs both outside and inside of the SPV. In either case, HTs appear at or near the critical latitudes. RWB was found to occur inside the SPV even when the forcing was located completely outside. In all cases, the westerly jet remained impermeable throughout the simulations. The results suggest that the HT inside the SPV observed by de la Camara et al. during the southern spring 2005 was due to RWB of an eastward-traveling wave of wavenumber 1.