Changes in the heat and freshwater forcing of the eastern Mediterranean and their influence on deep water formation

Anomalies in the air-sea heat and freshwater forcing of the eastern Mediterranean are related to observations of deep water formation in the Aegean Sea between 1987 and 1995. Fields of the buoyancy exchange (expressed as a density flux) are determined from the Southampton Oceanography Centre (SOC) flux climatology and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. A coherent picture emerges from the two data sets, in both cases the strongest losses occur in winters 1991–1992 and 1992–1993, when Aegean Sea net heat loss and net evaporation anomalies of 60 W m-2 and 0.05 m month-1, respectively, are found. These correspond to thermal and haline density fluxes of 3.3 × 10-6 and 0.6 × 10-6 kg m-2 s-1. Thus the thermal term makes the major contribution to the increase in density of the surface waters over these winters and the haline term plays only a minor role. Similar results are obtained when annual mean fields are considered as the winter forcing dominates. The NCEP/NCAR reanalysis is used to place the winters of the early 1990s in the context of variability over the longer period 1949–2002. Anomalous winter losses of similar magnitude occurred in the mid-1970s and the possibility of a related previous episode of deep water formation in the Aegean is discussed. The relationship between Aegean Sea heat loss and large-scale atmospheric pressure patterns is also investigated. Heat loss anomalies are found to be uncorrelated with the North Atlantic Oscillation. However, a strong correlation (r2 = 0.52) is found with a pattern whose main feature is anomalously high pressure over western Europe