Modulation of Atmospheric River Occurrence and Associated Precipitation Extremes in the North Atlantic Region by European Weather Regimes

The variability of large-scale moisture transport by atmospheric rivers (AR) and its linkage to precipitation extremes in the North Atlantic-European region is studied. A weather regime approach is adopted to describe the variability of the large-scale circulation. Weather regimes modulate the climatologically mean westerly flow into Europe, in which ARs are commonly embedded. In cyclonic regimes, AR landfall is enhanced in wide parts of Iberia, Western Europe, the British Isles, and southern Scandinavia. In blocked regimes, ARs are deviated around the blocking anticyclone enhancing AR landfall at high latitudes or in the western Mediterranean and North Africa. The likelihood of precipitation extremes increases locally more than twofold in regions of enhanced AR occurrence during the different weather regimes. These results suggest that specific weather regimes establish favorable conditions for AR landfall in Europe. Therefore, accurate forecasts of weather regimes can give guidance for predicting also large-scale precipitation extremes. Plain Language Summary Atmospheric rivers (ARs) are elongated bands of strong water vapor transport, which are embedded in the large-scale atmospheric circulation. ARs resemble rivers on land except that, instead of liquid water, ARs transport water vapor which is important for the formation of precipitation. The amount of water transported by an AR can be similar to the one of the largest rivers on land. These ARs frequently trigger precipitation extremes along the west coasts of the midlatitude continents. In this study, we investigate how the large-scale circulation influences AR's landfall location and subsequent precipitation extremes in Europe. To represent the natural variability of the westerly flow, we use seven typical persistent flow patterns over the North Atlantic-European region, called weather regimes (WRs). WRs characterized by a strong zonal flow enhance AR landfall in Iberia, Western Europe, the British Isles, and southern Scandinavia. WRs characterized by an anticyclone over Europe deviate ARs around the blocking anticyclone enhancing AR landfall at high latitudes or in the western Mediterranean and North Africa. The likelihood of precipitation extremes increases in regions of enhanced AR occurrence during the different WRs. Therefore, accurate WRs forecasts can give guidance for predicting also large-scale precipitation extremes in Europe.