Simulations of the February 1979 Stratospheric Sudden Warming: Model Comparisons and Three-Dimensional Evolution

Abstract

The evolution of the stratospheric flow during the major stratospheric sudden warming of February 1979 is studied using two primitive equation models of the stratosphere and mesosphere. The United Kingdom Meteorological Office Stratosphere-Mesosphere Model (SMM) uses log pressure as a vertical coordinate. A spectral, entropy coordinate version of the SMM (entropy coordinate model, or ECM) that has recently been developed 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 baroclinic zones. The ECM produces a more realistic recombination and recovery of the polar vortex in the midstratosphere after the warming, due mainly to better conservation properties for Rossby-Ertel potential vorticity in this model. Another advantage of the ECM is the automatic increase in vertical resolution near baroclinic zones. Comparison of SMM simulations with forecasts performed using the University of California, Los Angeles general circulation model confirms the previously noted sensitivity of stratospheric forecasts to tropospheric forecast and emphasizes the importance of adequate vertical resolution in modeling the stratosphere. The ECM simulators provide a schematic description of the three-dimensional evolution of the polar vortex and the motion of air through it. During the warming, the two cyclonic vortices till westward 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 in the upper stratosphere on the east (warm) side, strengthening the temperature gradients. After the peak of the warming, vertical velocities decrease, downward velocities move into the lower stratosphere, and upward velocities move into the upper stratosphere. Transport calculations show that air with high ozone mixing ratios is advected toward the pole from low latitudes during the warming, and air with low ozone mixing ratios is transported to the midstratosphere from both higher and lower altitudes along the baroclinic zone in the polar regions. Trajectories 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 and trajectory calculations show enhanced diabatic descent in the region between cyclone and anticyclone during the warming, consistent with the temperature structure shown.