Klaus – an exceptional winter storm over northern Iberia and southern France

Extratropical cyclones are one of the most important features of the mid-latitude climate and represent a primary mechanism for poleward transport of heat and moisture. They typically develop as a result of the interaction between warm subtropical air and cold polar air masses over the mid-latitudes of both hemispheres. Over the North Atlantic (NA) extratropical cyclones (depressions) often undergo a strong intensification phase over the ocean, move eastwards and reach Europe where they are one of the main factors influencing local weather. Intense depressions are often associated with very strong winds and large precipitation totals (Raible et al., 2007) and are among the most severe natural hazards affecting Europe. Such systems appear less often over southern Europe (Trigo, 2006; Pinto et al., 2009), but do occur sometimes when the jet stream pattern allows, and this paper describes an extreme example: storm Klaus (see Box 1 for storm names). On 21 January 2009 the jet stream extended across the NA Basin into Western Europe, and Klaus formed and moved, broadly, eastwards at a relatively low latitude (between 35°N and 45°N) on the edge of the dominant Atlantic storm-track identified in most climatological studies (e.g. Hoskins and Hodges, 2002; Pinto et al., 2005; Trigo, 2006, and Figure 1). It swept by northern Iberia and southern France late on 23 January and during 24 January, and was considered the most damaging wind storm to affect these areas since the devastating storm Martin in late December 1999 (Ulbrich et al., 2001).