ON THE SUBTROPICAL EDGE OF THE STRATOSPHERIC SURF ZONE

The formation of a subtropical ''transport barrier'' in the wintertime stratosphere is investigated in the context of a high-resolution shal low-water model in which Rossby waves are topographically forced on a zonally symmetric basic state. Two sets of experiments are performed: in the first ''adiabatic'' set, no dissipation or forcing of the mean state is imposed; in the second set, the layer thickness is relaxed to an equilibrium state taken to be representative of middle stratospher ic radiative equilibrium temperatures. It is found that in the adiabat ic case only a very weak subtropical barrier forms for forcing amplitu des that generate realistically steep potential vorticity gradients at the edge of the polar vortex; the vigorous wave breaking in the surf zone generates secondary waves that spread and, in turn, break well in to the summer hemisphere. In contrast, the inclusion of relaxation to a realistic thermal equilibrium leads to the formation of a subtropica l region of steep PV gradients. The strong subtropical shear induced b y the diabatic relaxation is shown to be an important factor for the f ormation of the subtropical edge of the surf zone. Furthermore, the au thors demonstrate that a simple one-layer shallow-water model can capt ure the full process of the formation of a surf zone with both polar a nd tropical edges starting from conditions typical of the early fall-t hat is, with a flow in which the polar vortex is not initially present . Finally, the authors quantify the mixing of polar and subtropical ai r into the midlatitude surf zone with the help of the contour advectio n technique. Although the quantitative estimates depend sensitively on how the edges of the surf zone are defined, our results indicate that more tropical than polar air is entrained into the surf zone.