RELATIONSHIP OF HORIZONTAL EDDY DISPLACEMENTS TO MEAN MERIDIONAL MOTIONS IN THE STRATOSPHERE

Equivalent-barotropic integrations that have been carried out under high resolution reveal organized subsidence inside the polar-night vortex. The potential temperature theta along a material surface, which is a prognostic variable in the equivalent-barotropic system, drops to sharply lower values across the edge of the vortex. Resembling observed motion in the Brewer-Dobson circulation, this behavior results from diabatic effects when the vortex is driven out of radiative equilibrium by wave advection. Lagrangian analyses carried out for ensembles inside and outside the vortex elucidate specific thermodynamic processes which act on individual bodies of air and are ultimately responsible for the Brewer-Dobson circulation. When the vortex is displaced out of zonal symmetry, individual air parcels are driven out of thermodynamic equilibrium with their surroundings. As a result, they experience irreversible heat transfer that leads to a hysteresis and net change of theta with each complete orbit about the pole. Successive orbits then produce a drift of air to lower theta. Vertical motion estimated from ensemble-mean properties corresponds to an average descent rate of about 1 mm/s, which is in qualitative accord with estimates derived elsewhere. Within the Lagrangian framework, the disturbed circulation functions as a radiative refrigerator by converting work performed at its lower boundary into heat that is eventually rejected to space through longwave cooling. The Lagrangian analyses suggest a similar analog for photochemical considerations. Driven out of photochemical equilibrium, the disturbed circulation can then function as a chemical engine by producing ozone and transferring it to the extratropical lower stratosphere.