The role of cyclones and potential vorticity cutoffs for the occurrence of unusually long wet spells in Europe

Abstract

The synoptic dynamics leading to the longest wet spells in Europe are so far poorly investigated despite these events' potentially large societal impacts. Here we examine the role of cyclones and potential vorticity (PV) cutoffs for unusually long wet spells in Europe, defined as the 20 longest uninterrupted periods with at least 5 mm daily accumulated precipitation at each ERA-Interim grid point in Europe (this set of spells is hereafter referred to as S20). The S20 occur predominantly in summer over the eastern continent, in winter over the North Atlantic, in winter or fall over the Atlantic coast, and in fall over the Mediterranean and European inland seas. Four case studies reveal distinct archetypal synoptic storylines for long wet spells: (a) a 7 d wet spell near Moscow, Russia, is associated with a single slow-moving cutoff–cyclone couple; (b) a 15 d wet spell in Norway features a total of nine rapidly passing extratropical cyclones and illustrates serial cyclone clustering as a second storyline; (c) a 12 d wet spell in Tuscany, Italy, is associated with a single but very large cutoff complex, which is replenished multiple times by a sequence of recurrent anticyclonic wave breaking events over the North Atlantic and western Europe; and (d) a 17 d wet spell in the Balkans features intermittent periods of diurnal convection in an environment of weak synoptic forcing and recurrent passages of cutoffs and thus also highlights the role of diurnal convection for long wet spells over land. A systematic analysis of cyclone and cutoff occurrences during the S20 across Europe reveals considerable spatial variability in their respective role for the S20. For instance, cyclones are present anywhere between 10 % and 90 % and cutoffs between 20 % and 70 % of the S20 time steps, depending on the geographical region. However, overall both cyclones and cutoffs appear in a larger number and at a higher rate during the S20 compared to climatology. Furthermore, in the Mediterranean, cutoffs and cyclones are significantly more persistent during the S20 compared to climatology. Our study thus documents for the first time the palette of synoptic storylines accompanying unusually long wet spells across Europe, which is a prerequisite for developing an understanding of how these events might change in a warming climate and for evaluating the ability of climate models to realistically simulate the synoptic processes relevant to these events.