The three-dimensional structure of breaking Rossby waves in the polar wintertime stratosphere

The three-dimensional nature of breaking Rossby waves in the polar winterti me stratosphere is studied using an idealized global primitive equation mod el. The model is initialized with a well-formed polar vortex, characterized by a latitudinal band of steep potential vorticity (PV) gradients. Planeta ry-scale Rossby waves are generated by varying the topography of the bottom boundary, corresponding to undulations of the tropopause. Such topographic ally forced Rossby waves then propagate up the edge of the vortex, and thei r amplification with height leads to irreversible wave breaking. These numerical experiments highlight several nonlinear aspects of stratosp heric dynamics that are beyond the reach of both isentropic two-dimensional models and fully realistic GCM simulations. They also show that the polar vortex is contorted by the breaking Rossby waves in a surprisingly wide ran ge of shapes. With zonal wavenumber-1 forcing, wave breaking usually initiates as a deep helical tongue of PV that is extruded from the polar vortex. This tongue is often observed to roll up into deep isolated columns, which, in turn, may be stretched and tilted by horizontal and vertical shears. The wave amplitu de directly controls the depth of the wave breaking region and the amount o f vortex erosion. At large forcing amplitudes, the wave breaking in the mid dle/lower portions of the vortex destroys the PV gradients essential for ve rtical propagation, thus shielding the top of the vortex from further wave breaking. The initial vertical structure of the polar vortex is shown to play an impo rtant role in determining the characteristics of the wave breaking. Perhaps surprisingly, initially steeper PV gradients allow for stronger vertical w ave propagation and thus lead to stronger erosion. Vertical wind shear has the notable effect of tilting and stretching PV structures, and thus dramat ically accelerating the downscale stirring. An initial decrease in vortex a rea with increasing height (i.e., a conical shape) leads to focusing of wav e activity, which amplifies the wave breaking. This effect provides a geome tric interpretation of the "preconditioning" that often precedes a stratosp heric sudden warming event. The implications for stratospheric dynamics of these and other three-dimensional vortex properties are discussed.