European-Scale Drought: Understanding Connections between Atmospheric Circulation and Meteorological Drought Indices

Quantification of large-scale climate drivers of drought is necessary to understand better and manage these spatially extensive and often prolonged natural hazards. Here, this issue is advanced at the continental scale for Europe. Drought events are identified using two indices-the 6-month cumulative standardized precipitation and standardized precipitation evapotranspiration indices (SPI-6 and SPEI-6, respectively)-both calculated using the gridded Water and Global Change (WATCH) Forcing Dataset for 1958-2001. Correlation of monthly time series of the percentage of European area in drought with geopotential height for 1958-2001 indicates that a weakening of the prevailing westerly circulation is associated with drought onset. Such conditions are linked to variations in the eastern Atlantic/western Russia (EA/WR) and North Atlantic Oscillation (NAO) atmospheric circulation patterns. Event-based analysis of the most widespread European droughts reveals that a higher number are identified by the SPEI-6 than the SPI-6, with SPEI-6 drought events showing a greater variety of spatial locations and start dates. Atmospheric circulation drivers also vary between the two types of events, with EA/WR-type variation associated most frequently with SPEI-6 drought, and the NAO associated with SPI-6. This distinction reflects the sensitivity of these drought indices to the underlying drought type (meteorological water balance versus precipitation, respectively) and associated differences in their timing and location (Europe-wide year round versus northern Europe winter). As such, this study provides new insight into both the identification of Europe-wide drought and patterns of large-scale climate variation associated with two different drought indices.