A numerical model of the sudden stratospheric warming mechanism

A simple numerical model is developed to study the planetary wave and zonal flow interaction that is thought to be responsible for a sudden stratospheric warming. The wave is generated by switch-on forcing at the base of an isothermal atmosphere and propagates vertically into an initial zonal flow profile that subsequently evolves in time in response to vertically structured horizontal heat flux by the wave. The principal model assumptions are β plane geometry and neglect of latitudinal dependence of the evolving zonal flow in the wave equation. Results indicate that the mechanism responsible for the warming event in a numerical treatment by Matsuno that is not based on these simplifying assumptions is contained in this model. Integrations for different values of model parameters show the dependence of certain features of the warming on dissipation and on the nature, amplitude, and duration of the wave forcing. We then use the model to analyze in detail the behavior of oscillating potential vorticity transport arising from linear superposition of the forced wave and transient free modes and establish this as the mechanism that triggers the nonlinear feedback that produces the warming event. The amplitude of this oscillation is the decisive factor in whether or not a warming occurs. A further set of model runs shows the dependence of the time it takes for a warming to occur on the initial zonal flow profile. These show that resonant forcing of the wave, occurring when the initial zonal flow configuration is such that one of the free modes is stationary, is not particularly favorable for the production of a warming.