A multi-year high-resolution climatology of the moisture sources of the European Alps: linking model results with observations of stable isotopes in precipitation

The water cycle of the European Alps is vitally important for large parts of Central Europe. A number of aspects of the hydrological cycle in that region are however not fully understood, in particular with respect to the origin of precipitation water. Better knowledge of the processes of the Alpine water cycle is also key to the interpretation of the numerous paleoclimate archives in the area, such as ice cores, peat bogs, and tree rings. This study presents a climatology of the moisture source regions of the Alpine mountain range, and initial attempts to create a link between model results and data from routine observations of stable isotopes in precipitation. A climatology of the Alpine moisture sources covering the years 1995-2002 was created, using a quantitative Lagrangian moisture source diagnostic based on backward trajectories, and ECMWF's ERA-40 reanalysis data. Calculations were performed on a 0.5° x 0.5° grid at 6-hourly time resolution. Monthly observational data of oxygen-18 and deuterium stable isotopes in precipitation were acquired from the Swiss National Network for the Observation of Isotopes in the Water Cycle (NISOT). The main moisture sources of the Alpine mountain range are the eastern North Atlantic, the western Mediterranean, and the central European land mass. However, a pronounced seasonal cycle is present in the in the moisture origin. Winter precipitation has a strong contribution from long-range transport of moisture originating in the North Atlantic, while during summer precipitation sources are considerably more local, and indicate recycling over the European continent. In addition, moisture sources at the northern and southern slope of the Alpine main crest show substantial differences, which also indicate differences in the frequency of extreme precipitation events. Precipitation stable isotope observations along a north-south transect in the western Alps generally confirm the influence of different moisture sources to precipitation in the Alps. Despite being limited by the temporal resolution and spatial extent of the stable isotope data, this study allows for a closer connection between the interpretation of stable isotopes in paleoarchives and physical climate processes.