Impact of North Atlantic evaporation hot spots on southern Alpine heavy precipitation events

This study investigates transient events of intense ocean evaporation with an amplitude exceeding 250 W m-2, a duration of a few days, and a spatial extent of about 10(6) km2 over the eastern North Atlantic (referred to as evaporation hot spots) and their impact on southern Alpine heavy precipitation. First, moisture sources for a heavy precipitation event in the Piedmont in November 2002 are studied using a water-tagging simulation with a regional model. The results reveal three main moisture sources: land evapotranspiration, and evaporation from the Mediterranean and the North Atlantic, with the last source contributing the most. This was partly due to an evaporation hot spot that appeared along the western edge of a prominent upper-level trough about two days prior to the onset of heavy precipitation. In the hot spot area strong surface winds induced by the upper-level trough led to intense evaporation of water that was transported around the trough to the Piedmont region during subsequent days, where it contributed to the heavy precipitation. Secondly, analyses by the European Centre for Medium-Range Weather Forecasts (ECMWF) are used to investigate climatologically the potential relationship between eastern North Atlantic evaporation hot spots and southern Alpine precipitation. During a 10-year time period, 42 hot spots have been identified in the eastern North Atlantic. It is shown that they typically occur along the western flank of prominent upper-level troughs, and that the evaporating moisture is transported to Europe within one to four days. A climatological analysis of southern Alpine heavy precipitation events shows that they are frequently preceded by intense North Atlantic evaporation. Hence the climatological analysis further supports the conclusion from the Piedmont 2002 tagging experiment that intense evaporation over the North Atlantic and the subsequent moisture transport, both induced by the upper-level trough, are potential key factors for the development of southern Alpine heavy precipitation events. Copyright (c) 2011 Royal Meteorological Society