Rossby wave phase speeds and mixing barriers in the stratosphere. Part I: Observations

Abstract

Lagrangian trajectories are used to calculate isentropic mixing properties for unfiltered and filtered Southern Hemisphere stratospheric winds. In wintertime significant mixing is confined to the surf zone between the Tropics and the edge of the polar vortex. The mixing barrier at the edge of the vortex is located near the core of the polar jet stream (where the maximum wind speeds occur), which is also approximately where the meridional potential vorticity gradient is largest. In summer there is significant mixing throughout the hemisphere, and no high-latitude mixing barrier exists. When the winds are filtered by zonal wavenumber to retain either the planetary-scale waves (1-3) or the smaller-scale waves (4-12), mixing in the surf zone is generally reduced but not eliminated. When the winds are filtered by phase speed, however, mixing is significantly reduced in restricted latitude zones where the phase speeds of the filtered waves are close to the speed of the local zonal-mean zonal wind. These results indicate that mixing primarily occurs near the critical lines for Rossby waves, where the waves would be expected to break. The presence of the mixing barrier around the polar vortex can be interpreted as a result of the lack of waves with fast phase speeds comparable to the speed of the jet. Artificially amplifying the fast-moving waves can destroy the mixing barrier around the vortex. In summer, when winds are weaker, waves break throughout the hemisphere and the mixing barrier disappears.