Anomalous moisture sources of the Rhine basin during the extremely dry summers of 2003 and 2018

Droughts can be studied from an atmospheric perspective by analysing large-scale dynamics and thermodynamics, and from a hydrological perspective by analysing interaction of precipitation, evaporation, soil moisture and temperature at the land-surface. Here, we study it from both perspectives, and assess the moisture (evaporative) sources of precipitation in the Rhine basin during the exceptionally dry summers of 2003 and 2018. We use ERA5 re-analysis data (1979–2018) and the Eulerian moisture tracking model WAM-2layers in order to determine the moisture sources of the Rhine basin. During an average summer, these evaporative sources are mostly located over the Atlantic Ocean, and there is a large contribution from continental evaporation, mostly from regions west of the Rhine basin. Both in 2003 and 2018 the absolute moisture source contribution declined over the ocean. In both years the anomalous moisture fluxes over the boundaries of the Rhine basin are mainly a result of anomalous wind and not because of anomalous moisture advection by the mean wind. Due to high pressure (blocking) over Europe, moisture is transported from the ocean with anticyclonic flow around the Rhine basin, but not into the basin. In 2018, unlike 2003, moisture is transported from the east towards the basin as a result of the anticyclonic flow around the Scandinavian blocking. The large-scale synoptic situation during the summer of 2018 was exceptional, and very favourable for dry conditions over the Rhine basin. Although blocking also occurred in 2003, the exact synoptic conditions were less favourable to dryness over the Rhine basin. In 2003 however, the recycling of moisture within the basin was much lower than the climatology and 2018, especially in August, possibly indicating the drying out of the soil resulting in the second heatwave in August 2003. To conclude, although the summer of 2003 and 2018 were both exceptionally dry, their characteristics in terms of moisture sources and recycling, and thereby their dependence on the large-scale circulation and land-atmosphere interactions, were found to be very different. It is therefore imperative that droughts are also studied as individual events to advance understanding of complex interactions between the large-scale atmospheric processes and the land surface.