Enhanced Polar Vortex Predictability Following Sudden Stratospheric Warming Events

Abstract

<title xmlns:mml="http://www.w3.org/1998/Math/MathML">Abstract</title><p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en">Sudden stratospheric warming (SSW) events can form a window of forecast opportunity for polar vortex predictions on subseasonal‐to‐seasonal time scales. Analyzing numerical ensemble simulations, we quantify the associated enhanced predictability due to reduced upward planetary wave fluxes during the mostly radiatively driven recovery phase following SSWs. 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. This increased predictability is particularly pronounced for strong SSWs and even occurs if not all ensemble members predict a major SSW. Furthermore, we found a direct impact of the occurrence of SSWs on the date of the final warming (FW): the decrease in upward wave fluxes delays the FW significantly. The reduced spread after SSWs and the delay in FW date have potentially further implications for (subseasonal) predictions of the tropospheric and mesospheric circulations.</p>

Plain Language Summary: The polar vortex is a large scale circulation active during winter in the higher levels of the polar atmosphere. Changes in the strength of the polar vortex can have an impact on the weather over mid‐latitude regions like Europe. This is the case especially for the period after so‐called sudden stratospheric warming (SSW) events, where the polar vortex breaks down very abruptly and then slowly recovers over several weeks. Such a break‐down of the polar vortex tends to suppress wave activity and hence reduces the dynamical variability in the polar stratosphere, leading to a more predictable evolution of the circulation. We quantify the strength and timescale of this increase in predictability of the polar vortex after an SSW using a large set of winter time model forecasts.</p>

Key Points: <list list-type="bullet"> <list-item> <p xml:lang="en">Sudden stratospheric warmings (SSWs) lead to reduced forecast spread in the polar stratosphere for several weeks after the event</p></list-item> <list-item> <p xml:lang="en">Reduced forecast spread after SSWs is driven by suppressed vertical planetary wave propagation due to persistent negative wind anomalies</p></list-item> <list-item> <p xml:lang="en">Final warmings are delayed for winters with SSW, consistent with reduced upward wave fluxes following the SSW</p></list-item> </list> </p>