Seasonal and inter-annual variability of the moisture sources for Alpine precipitation during 1995-2002

This study presents a first quantitative climatology of the moisture sources for precipitation in the European Alps, covering a 7-year period from January 1995 to August 2002. Using a Lagrangian moisture source diagnostic and data from the ERA-40: European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis, the contribution of the following moisture sources to annual mean precipitation has been diagnosed: North Atlantic ocean 39.6%, Mediterranean 23.3%, North Sea and Baltic Sea 16.6%, and European land surface 20.8%. However, strong seasonal variability of the influence of various moisture sources is evident. Most notably, moisture transport to the Alps changes from an oceanic mode characterised by dominantly North Atlantic moisture sources during winter to a continental mode during summer with a marked contribution from Central European land areas. The method identifies inter-annual variability with respect to the location of the moisture sources in the North Atlantic, and the importance of precipitation recycling during summer. Despite the smoothed Alpine orography in the ERA-40 model, the Alps act as an effective barrier for meridional moisture transport, leading to distinct mean moisture source locations at their northern and southern slopes. The Northern Alps are predominantly influenced by the North Atlantic ocean and Central European land sources with a clear seasonality and limited monthly variability. In contrast, the Southern Alps receive a large fraction of precipitation from the Mediterranean with considerable month-to-month variability. Possible implications of these differences for precipitation extremes and stable isotopes in precipitation are discussed. Copyright (C) 2009 Royal Meteorological Society