Abstract

Extreme stratospheric events such as sudden stratospheric warming and strong vortex events associated with an anomalously weak or strong polar vortex can have downward impacts on surface weather that can last for several weeks to months. Hence, successful predictions of these stratospheric events would be beneficial for extended range weather prediction. However, the predictability limit of extreme stratospheric events is most often limited to around 2 weeks or less. The predictability also strongly differs between events, and between event types. The reasons for the observed differences in the predictability, however, are not resolved. To better understand the predictability differences between events, we expand the definitions of extreme stratospheric events to wind deceleration and acceleration events, and conduct a systematic comparison of predictability between event types in the European Centre for Medium-Range Weather Forecasts (ECMWF) prediction system for the sub-seasonal predictions. We find that wind deceleration and acceleration events follow the same predictability behaviour, that is, events of stronger magnitude are less predictable in a close to linear relationship, to the same extent for both types of events. There are however deviations from this linear behaviour for very extreme events. The difficulties of the prediction system in predicting extremely strong anomalies can be traced to a poor predictability of extreme wave activity pulses in the lower stratosphere, which impacts the prediction of deceleration events, and interestingly, also acceleration events. Improvements in the understanding of the wave amplification that is associated with extremely strong wave activity pulses and 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.

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)

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Summary

Introduction

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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