Three-Dimensionality and Forcing of the Brewer–Dobson Circulation

Abstract

Integrations with the nonlinear primitive equations are used to study 3D diabatic structure underlying the Brewer?Dobson circulation of the middle atmosphere. Such structure reveals zonally asymmetric contributions to mean downwelling over the winter hemisphere. It is used to evaluate contributions to w* from mechanical dissipation of planetary waves, associated with irreversible eddy dispersion, and from thermal dissipation of planetary waves, associated with irreversible heat transfer. Zonal-mean downwelling follows disproportionately from those longitudes where air is deflected across contours of radiative equilibrium. This zonally asymmetric contribution to w* is pronounced at high latitudes, where the displaced vortex achieves cross-polar flow that drives air across sharply different radiative environments. Air parcels orbiting about the vortex then experience a wide swing in radiative-equilibrium temperature, driving them well out of thermal equilibrium. This renders the heat transfer experienced by them irreversible, resulting in net cooling and descent to lower ? with each orbit about the displaced vortex. By destroying anomalous potential vorticity (PV), irreversible heat transfer also leads to thermal dissipation of planetary waves and acts to resymmetrize the vortex diabatically. Integrations in which irreversible dispersion is suppressed recover much the same diabatic motion as the full integration. Downwelling is reduced at midlatitudes, where the contribution from irreversible eddy dispersion is concentrated, but it is virtually unchanged at high latitudes, where the contribution from irreversible heat transfer prevails. Lagrangian integrations show that thermal dissipation of wave activity accounts for a major fraction of the downwelling over the winter hemisphere. This is especially true at high latitudes, where cross-polar flow leads to irreversible cooling and a systematic drift of air to lower ?. Were it not for this contribution to w*, the Arctic stratosphere would be several tens of Kelvin colder.