Isotope composition of air moisture over the Mediterranean Sea: an index of the air–sea interaction pattern

The isotope composition of atmospheric moisture over the Mediterranean Sea, collected during the cruise of the research vessel meteor in January 1995, confirmed that the intensive air–sea interaction near the coast under conditions of a large humidity deficit labels the resultant atmospheric waters with a large deuterium-excess parameter. The present data set shows this effect to result both when cold air from the European continent moves over the sea as well as when warm and dry air from North Africa is involved. The situation in the eastern and western Mediterranean differ in the vertical structure of the isotope composition further away from the coast, as expressed by the gradients of the dexcess values with altitude over the sea surface, i.e. increasing with altitude in the eastern Mediterranean, whereas the opposite effect is noted in the western section and near the coast. A comparison of the isotopic composition of the samples with the expected buildup of moisture over the sea, based on the Craig–Gordon model, suggests that up to one half of the added moisture may have resulted at times from the evaporation of sea-spray droplets, without any significant isotope fractionation, in addition to the vapour-mediated transport from the sea surface, which favors the lighter isotopic species.