An investigation of resonant waves in a numerical model of an observed sudden stratospheric warming

A quasi-linear time-dependent model is used to investigate several questions about the role of resonance in the 1979 warming. The model is global and has boundaries at the earth's surface and at 100 km to minimize the impact of artificial boundaries on resonance. The basic state is initialized with observations based on the LIMS data for February 1979. A vorticity perturbation with a constant phase speed is applied to the upper troposphere. Eastward phase progression of the wave 2 forcing is necessary to simulate the warming (for a realistic amplitude of wave forcing), although the model results are not sensitive to the exact frequency. The forcing frequencies for which warmings develop in these tests are close to the resonant frequencies found in steady state calculations using the observed winds. The range of wave frequencies that lead to a sudden warming shifts toward lower frequences with progressive initialization days. -from Author