129 Organization of extreme windstorms by the large-scale circulation over the North Atlantic

Thursday, 20 June 2013
Bellevue Ballroom (The Hotel Viking)
Rodrigo Caballero, Stockholm Univ., Stockholm, Sweden; and J. Hanley

Extreme windstorms over Europe can inflict damage comparable to that of a major US hurricanes. Their typically small spatial scale and sensitivity to moist processes makes it challenging to adequately simulate such storms using general circulation models and to predict changes in their statistics in altered climates. A better understanding of how large-scale, low-frequency atmospheric variability controls the behavior of such storms would facilitate more robust prediction of these storms on both weather and climate timescales. Here we present an observational study of the top 25 windstorms affecting various regions in Europe over the 40 years covered by the ERA-40 reanalysis dataset. We show that these storms typically occur during particularly strong and persistent positive NAO anomalies which peak approximately 2 days before the storms' peak intensity; the NAO pattern then shifts eastward to a position over the European continent when the storms strike Europe. A temporal composite of potential temperature on the 2-PVU surface suggests that this NAO shift is the result of simultaneous cyclonic and anticyclonic wave breaking penetrating further to the east than during a typical high-NAO event. This creates an extremely intense, zonally-orientated jet over the North Atlantic whose baroclinicity favours explosive intensification of storms while steering them into continental Europe.
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