Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> Extreme stratospheric events such as sudden stratospheric warming (SSW) and strong vortex events can have downward impacts on surface weather that can last for several weeks to months. Hence, successful predictions of these stratospheric events can be beneficial for extended-range weather prediction. However, the predictability of extreme stratospheric events is most often limited to around 2Â weeks or less. The predictability strongly differs within events of the same type and also between event types. The reasons for the observed differences in the predictability, however, are not resolved. We extend the analysis of the predictability of stratospheric extreme events to include wind deceleration and acceleration events, with SSW and strong vortex events as subsets, to conduct a systematic comparison of sub-seasonal predictability between events in the European Centre for Medium-Range Weather Forecasts (ECMWF) prediction system. Events of stronger magnitude are found to be less predictable than weaker events for both wind deceleration and acceleration events, with both types of events showing a close to linear dependence of predictability on event magnitude. There are, however, deviations from this linear behaviour for very strong magnitude events. The difficulties of the prediction system in predicting extremely strong anomalies can be traced to a poor predictability of extreme wave activity fluxes in the lower stratosphere, which impacts the prediction of deceleration events and, interestingly, also acceleration events. Our study suggests that improvements in the understanding of the wave amplification that is associated with extremely strong wave activity fluxes and accurately representing these processes in the model are expected to enhance the predictability of stratospheric extreme events and, by extension, their impacts on surface weather and climate.
Highlights
The stratospheric polar vortex (SPV) is a band of strong westerly winds over the polar region at the height of around 20-50km 20 during winter
Improvements in the understanding of the wave amplification that is associated with extremely strong wave activity pulses and 15 accurately representing these processes in the model is expected to enhance the predictability of stratospheric extreme events and, by extension, their impacts on surface weather and climate
The magnitude of the events identified in reanalysis (∆u), measured by the wind difference between day 9 and day 0, predicted by the model hindcasts is compared against the same value in reanalysis for all lead time groups (Fig. 1)
Summary
The stratospheric polar vortex (SPV) is a band of strong westerly winds over the polar region at the height of around 20-50km 20 during winter. These circumpolar winds result from a strong temperature gradient in the stratosphere between the polar and subtropical regions during winter due to reduced solar heating over the polar regions. Upwards into the stratosphere (e.g. Polvani and Waugh, 2004; Sjoberg and Birner, 2012). Depending on the wave activity and the state of the vortex, the SPV can undergo periods of weakening or strengthening, largely varying in strength during the 25 wintertime
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