James H. Stagge

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

AbstractQuantification 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 SPE...

Observed drought indices show increasing divergence across Europe

Recent severe European droughts raise the vital question: are we already experiencing measurable changes in drought likelihood that agree with climate change projections? The plethora of drought definitions compounds this question, requiring instead that we ask: how have various types of drought changed, how do these changes compare with climate projections, and what are the causes of observed differences? To our knowledge, this study is the first to reveal a regional divergence in drought likelihood as measured by the two most prominent meteorological drought indices: the Standardized Precipitation Index (SPI) and the Standardized Precipitation-Evapotranspiration Index (SPEI) across Europe over the period 1958–2014. This divergence is driven primarily by an increase in temperature from 1970–2014, which in turn increased reference evapotranspiration (ET0) and thereby drought area measured by the SPEI. For both indices, Europe-wide analysis shows increasing drought frequencies in southern Europe and decreasing frequencies in northern Europe. Notably, increases in temperature and ET0 have enhanced droughts in southern Europe while counteracting increased precipitation in northern Europe. This is consistent with projections under climate change, indicating that climate change impacts on European drought may already be observable and highlighting the potential for discrepancies among standardized drought indices in a non-stationary climate.