Relationship of horizontal eddy displacements to mean meridional motions in the stratosphere

Equivalent-barotropic integrations that have been carriedout under high resolution reveal organized subsidence inside the polar-nightvortex. The potential temperature θ along a material surface, which is aprognostic variable in the equivalent-barotropic system, drops to sharply lowervalues across the edge of the vortex. Resembling observed motion in theBrewer-Dobson circulation, this behavior results from diabatic effects when thevortex is driven out of radiative equilibrium by wave advection. Lagrangiananalyses carried out for ensembles inside and outside the vortex elucidatespecific thermodynamic processes which act on individual bodies of air and areultimately responsible for the Brewer-Dobson circulation. When the vortex isdisplaced out of zonal symmetry, individual air parcels are driven out ofthermodynamic equilibrium with their surroundings. As a result, they experienceirreversible heat transfer that leads to a hysteresis and net change of θ witheach complete orbit about the pole. Successive orbits then produce a drift ofair to lower θ. Vertical motion estimated from ensemble-mean propertiescorresponds to an average descent rate of about 1 mm/s, which is in qualitativeaccord with estimates derived elsewhere. Within the Lagrangian framework, thedisturbed circulation functions as a radiative refrigerator by converting workperformed at its lower boundary into heat that is eventually rejected to spacethrough longwave cooling. The Lagrangian analyses suggest a similar analog forphotochemical considerations. Driven out of photochemical equilibrium, thedisturbed circulation can then function as a chemical engine by producing ozoneand transferring it to the extratropical lower stratosphere.