Isotopic composition of single rain events in the central Mediterranean

Abstract

The ratios of stable isotopes of single rain events were investigated during the period October 2005 to September 2006 in the central Mediterranean. Clear seasonal trends were identified in both oxygen isotope ratios and the deuterium-excess parameter, and these were ascribed to the dominant circulation systems during both cold and hot intraannual periods. Rain events were classified on the basis of the origin of rain-bearing systems. Air masses coming from the south usually give rise to rainwater with a low deuterium excess. Air masses coming from the north and the northeast are often dry and cold, and are associated with high evaporation from the Mediterranean Sea that occurs under isotopic nonequilibrium conditions. Kinetic fractionation enhances lighter isotopomers in the vapor phase, increasing the deuterium excess. During cold periods large vapor fluxes from the Mediterranean Sea, as estimated by the ECMWF (European Centre for Medium-Range Weather Forecasts), usually precede rain events with a high deuterium excess. However, the isotope signatures of the Mediterranean moisture contribution may be masked by the original isotope content of the circulating air masses and/or by secondary evaporation effects.