SIMULATIONS OF THE FEBRUARY 1979 STRATOSPHERIC SUDDEN WARMING - MODELCOMPARISONS AND 3-DIMENSIONAL EVOLUTION

The evolution of the stratospheric flow during the major stratospheric sudden warming of February 1979 is studied using two primitive equati on models of the stratosphere and mesosphere. The United Kingdom Meteo rological Office Stratosphere-Mesosphere Model (SMM) uses log pressure as a vertical coordinate. A spectral, entropy coordinate version of t he SMM (entropy coordinate model, or ECM) that has recently been devel oped is also used. Both models produce similar successful simulations through the peak of the warming, capturing the splitting of the vortex and the development of small-scale structures, such as narrow barocli nic zones. The ECM produces a more realistic recombination and recover y of the polar vortex in the midstratosphere after the warming, due ma inly to better conservation properties for Rossby-Ertel potential vort icity in this model. Another advantage of the ECM is the automatic inc rease in vertical resolution near baroclinic zones. Comparison of SMM simulations with forecasts performed using the University of Californi a, Los Angeles general circulation model confirms the previously noted sensitivity of stratospheric forecasts to tropospheric forecast and e mphasizes the importance of adequate vertical resolution in modeling t he stratosphere. The ECM simulations provide a schematic description o f the three-dimensional evolution of the polar vortex and the motion o f air through it. During the warming, the two cyclonic vortices tilt w estward and equatorward with height. Strong upward velocities develop in the lower stratosphere on the west (cold) side of a baroclinic zone as it forms over Europe and Asia. Strong downward velocities appear i n the upper stratosphere on the east (warm) side, strengthening the te mperature gradients. After the peak of the warming, vertical velocitie s decrease, downward velocities move into the lower stratosphere, and upward velocities move into the upper stratosphere. Transport calculat ions show that air with high ozone mixing ratios is advected toward th e pole from low latitudes during the warming, and air with low ozone m ixing ratios is transported to the midstratosphere from both higher an d lower altitudes along the baroclinic zone in the polar regions. Traj ectories of parcels moving around the vortex oscillate up and down as they move through regions of ascending and descending motions, with an overall increase in pressure in the polar regions. Tracer transport a nd trajectory calculations show enhanced diabatic descent in the regio n between cyclone and anticyclone during the warming, consistent with the temperature structure shown.