Regional hydrological drought in north-western Europe: linking a new Regional Drought Area Index with weather types

Links between severe hydrological droughts and weather types (WTs) were explored to improve the understanding of hydroclimatological processes involved in the development of regional hydrological drought in north-western Europe. A new Regional Drought Area Index (RDAI) was developed, using daily streamflow, to represent the drought-affected area. Daily RDAI series (1964-2001) were created for two regions with homogeneous drought behaviour in Denmark and four regions in Great Britain. An objective version of the Hess-Brezowsky Grosswetterlagen yielding 29 WTs was used. Regional drought characteristics, including duration and frequency, were found to vary considerably between regions. However, in 1976 and 1996, all regions experienced severe events, and these years were found to be the most severe drought years across the study region as a whole. The hydrological response time (i.e. the time over which WTs influence drought development) was found to vary markedly (45-210 days) between regions according to basin storage properties. WT-frequency anomalies (FAs) before and during the onset of the five most severe droughts were identified for each region. The dominant drought-yielding WTs changed between regions and between events within each region. High-pressure systems centred over the respective region were most frequently associated with droughts as well as WTs with a northern (N, NE or NW) or a southern (S, SE or SW) airflow over the Danish and British regions. Five of the six WTs associated with drought for all regions represented a northern high-pressure system (i.e. over Great Britain, Fennoscandia or the Norwegian Sea). This article demonstrates (1) hydrological response time to be fundamental in moderating drought response to mesoscale climatic drivers and (2) severe hydrological droughts may be caused by a complex set of hydroclimatological processes that vary between regions and events. Copyright. (c) 2010 John Wiley & Sons, Ltd.