École Nationale Supérieure de
Techniques Avancées

Résumé

Reliably forecasting European weather and climate is challenging because of the complexity of the climate system. To improve predictions, an improved understanding of the large-scale atmospheric processes is necessary. One key driver of European winter weather is the stratospheric polar vortex. It is a stream of fast westerly winds at approximately 30 km height above the Arctic. Extreme events of the polar vortex, socalled sudden stratospheric warmings (SSW), can impact the tropospheric circulation below, and thereby the surface weather. Although SSWs are generally known to affect European weather, the surface response varies greatly between events, and it is not well understood which factors modulate this response. Here I address the role of the timing of SSWs within the cold season (December to March). Given the limited sample size of SSWs in the observational record, hindcasts of the seasonal forecasting model SEAS5 from the European Centre for Medium-Range Weather Forecasts (ECMWF) were used. I show that early winter (December and January) events are followed by much stronger temperature and precipitation anomalies than late winter events (February and March). This difference is not directly related to the number of downward propagating SSWs but seems to be linked to the prevailing state of the atmospheric circulation during the onset of the SSWs, consistent with previous studies focusing on the role of the onset regime of SSWs. The information provided by the role of timing of SSWs has the potential to be also used to better forecast European weather on sub-seasonal to seasonal timescales.