On the Analysis of Mean Downward Velocities around the Antarctic Polar Vortex

The Lagrangian characteristics of vertical motions around the Antarctic polar vortex are investigated using a general circulation model (GCM) and various analysis methods. A trace analysis that estimated the vertical velocity from the vertical displacement of tracer isopleths confirmed that using zonal means at geographical latitudes gives a Lagrangian mean circulation around the Antarctic polar vortex similar to that computed using equivalent latitudes. In the mass-weighted isentropic zonal means, the mean vertical velocity dynamically estimated from the meridional velocity shows strong downward motion outside the Antarctic polar vortex around 45°–55°S in the lower stratosphere, which is consistent with the thermodynamically estimated values from the diabatic heating rate. In comparison, the transformed Eulerian mean analysis tends to overestimate the downward velocity outside the Antarctic polar vortex and underestimate it inside the Antarctic polar vortex. Trace analysis produces a good approximation of the dynamical estimate inside the Antarctic polar vortex, but it does not capture the strong downward velocity outside the vortex because of active horizontal mixing. If eddy mixing effects are included in the mean-meridional transport equation, the trace analysis agrees well with the dynamical estimate. The mean downward motion outside the Antarctic polar vortex causes adiabatic heating and contributes to the formation of the polar night jet stream from the lower to middle stratosphere through the thermal wind balance. Analysis of the mean vertical velocities in reanalysis products (assimilations) is very noisy compared to that from free running models because of dynamical inconsistencies caused by the assimilation process.