Reduced stratospheric variability following sudden stratospheric warming events

Sudden stratospheric warming (SSW) events can form a window of forecast opportunity for polar vortex predictions on subseasonal-to-seasonal time scales. Analysing numerical ensemble simulations, we show that negative wind anomalies in the polar stratosphere following SSWs lead to a reduction in upward planetary wave propagation and hence a reduction in the dynamical variability of the polar vortex. Ensembles that predict an SSW show reduced ensemble spread in terms of polar vortex strength for several weeks to follow, as well as a corresponding reduction in forecast errors. The associated increase in predictability is particularly pronounced for strong SSWs and even occurs if not all ensemble members predict a major SSW. The decrease in upward wave fluxes and polar vortex variability following the event then manifests in a delay of the final warming during years that experience an SSW and potentially has further implications for the tropospheric or mesospheric circulation.