Effects of large-scale atmospheric circulation on precipitation and river stream flows in Europe during cold seasons

Winter precipitation remarkably affects river flooding risk in Europe. This study focuses on linkages between large-scale atmospheric circulation, precipitation and river discharge in western and northern Europe during extended winter period (October – April). Compared to other seasons, the effects of large-scale atmospheric circulation on precipitation are strongest in winter, when moisture transport is a major source of moisture for precipitation, and large-scale updraft is an effective mechanism to produce water condensation. This study shows that winter precipitation amount correlates with interannual variation of river stream flow in the month when the annual maximum in river streamflow occurs.

We applied Self-Organizing Maps (SOM) to categorize circulation patterns. The results show that variation of circulation patterns, based on clusters of daily 500-hPa circulation patterns produced by a SOM analysis, were able to explain a relatively large fraction of interannual variation of monthly mean precipitation in western and northern Europe. However, the explained fraction decreases towards eastern Europe. Based on the SOM analysis, four main circulation types, which strongly affect regional interannual variation of precipitation in western and northern Europe, were recognized: (i) “Westerly flow” characterized by an almost zonal circulation pattern in the 500-hPa geopotential field causes the strongest positive precipitation anomalies in a large part of western Europe. (ii) “Wester European blocking” decreased precipitation over a large part of western Europe but increased it in Scandinavia due to a northward moisture transport from the northern North Atlantic. (iii) “North Atlantic blocking” together with an upper-level trough over the Baltic Sea region allows moisture transport from the North Sea to central Europe, causing positive anomalies in precipitation in central Europe. (iv) “Azorean low” centred over the northern side of Azores allows moisture transport from the North Atlantic to reach Iberian Peninsula and the Mediterranean region, increasing precipitation in the region. Even though these circulation types are not able to exhaustively explain the entire interannual variation of precipitation in western and northern Europe, a frequent occurrence of some of these circulation types increases probability of enhanced river stream flow in the region where the circulation type causes positive precipitation anomalies.