THE CLIMATOLOGY OF THE STRATOSPHERIC THIN AIR MODEL

An improved two-and-a-half dimensional chemical-dynamical-radiative mo del shows remarkably realistic behaviour in a number of respects. The stratospheric/mesospheric westerly jet slopes equatorwards with height and is split at mid-latitudes in the southern hemisphere. The paramet rized orographic gravity waves exert a mid-latitude drag and are impor tant in splitting the jet, and are possibly important in pre-condition ing the stratosphere for a sudden warming. A large easterly drag is re quired in the winter high-latitude mesosphere in order to obtain a tro pical mesospheric westerly jet. The summer easterly jet maximum lies a bove the tropospheric jet, due to gravity-wave absorption in the lower stratosphere. Very realistic annual and semi-annual cycles are produc ed in the model, with a mesospheric semi-annual oscillation forced by tidal drag and gravity waves. Planetary-wave temperatures are also mod elled well, and wave amplitudes are correlated with the westerly jets. In the lower stratosphere, zonal-mean Ertel's potential vorticity gra dients strongly influence the wave activity, horizontal convergence an d heating rates, with descent being strongest equatorwards of the vort ex edge. This forms the basis for an explanation of the inter-hemisphe ric asymmetry in the ozone column. Nitrous oxide distributions indicat e too much equatorial ascent but show a 'double peak' connected with t he tropical circulation, and steep horizontal gradients near the winte r vortex edge.