Realistische Modellierung der tiefreichenden Zirkulation des Mittelmeers durch Simulation von Tracerverteilungen

Abstract

The thermohaline circulation of the Mediterranean is simulated by means of the OGCM (ocean general circulation model) MOMA. Special focus lies on the eastern basin, where simulated concentrations of transient tracers, especially the chlorofluorocarbon CFC-12, is compared with observations. The inner oceanic concentrations of these tracers provide a good means to identify pathways of newly ventilated water in the ocean and therefore allow to assess the model results. After 1987 the deep circulation in the eastern Mediterranean changed dramatically. The Adriatic was replaced by the Aegean as the main source of deep water. This process is simulated in the model via a decrease of sea surface temperature and an increase of sea surface salinity over the Aegean during winter. Realistic ventilation rates of the deeper parts of the Levantine and Ionian basin can be obtained by adjusting the rate of this extra buoyancy forcing. The resulting transient model circulation is compared with observational data and an explanation for the remarkable increase of salt content in the eastern basin of the Mediterranean is given.The concepts of age and age distributions are applied within the model for the old and new circulation state of the Mediterranean. Recently ventilated waters are characterized by young ages close to zero. During the spreading of the water into the ocean interior the ideal age increases, so it gives information on the circulation similar to those derived from transient tracers. The oldest water in the eastern Mediterranean is present in the Levantine basin, where upwelling of the deep water takes place, which reaches close to the surface.The ideal age is compared with concentration and ratio ages obtained from the modelled concentrations of transient tracers.The relation between ideal and tracer derived ages as well as between tracer concentrations and age distribution is shown.