The Effect of Tropospheric Jet Latitude on Rossby Wave Breaking and on Coupling between the Stratospheric Polar Vortex and the Troposphere

A dry General Circulation Model is used to investigate how the latitude of the tropospheric jet affects (1) coupling between the stratospheric polar vortex and the extratropical tropospheric circulation, and (2) Rossby wave breaking. The tropospheric response to an identical stratospheric vortex configuration is shown to be strongest for a jet centered near 40° and weaker for jets near either 30° or 50° by more than a factor of three. Stratosphere-focused mechanisms based on stratospheric potential vorticity inversion, eddy phase speed, and planetary wave reflection, as well as arguments based on tropospheric eddy heat flux and zonal length scale, appear to be incapable of explaining the differences in the magnitude of the jet shift. In contrast, arguments based purely on tropospheric variability involving the strength of eddy-zonal mean flow feedbacks and jet persistence, and related changes in the synoptic eddy momentum flux, appear to explain this effect. The dependence of coupling between the stratospheric polar vortex and the troposphere on tropospheric jet latitude found here is consistent with (1) the observed variability in the North Atlantic and the North Pacific, and (2) the trend in the Southern Hemisphere as projected by comprehensive models.

Preliminary results of the distribution of Rossby wave breaking in these integrations suggest that the dependence of Rossby wave breaking on jet latitude largely follows the predictions of quasi-geostrophic linear theory. However, exceptions are found in many cases on both flanks of the jet and near the jet core. Some of these exceptions can be understood using other paradigms for the distribution of Rossby wave breaking.