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[1] Dense water formation processes in the Aegean Sea (eastern Mediterranean) are studied using a three-dimensional numerical ocean model. The simulations cover the period 1979-1994 during which major changes that affected the thermohaline circulation of the whole Mediterranean Sea were recorded. Sensitivity studies that focus on the role of freshwater budget are presented, and the results are evaluated against available hydrological data of the same period. The very cold winters of 1987, 1992, and 1993 and the extended dry period 1989-1993 that affected the whole eastern Mediterranean Sea are the main driving mechanisms, corresponding to 50% and 32%, respectively, of the excessive deepwater volume formed in the Aegean after 1987. The reduced Black Sea Water outflow during the same dry period was another important forcing mechanism, contributing 18% to the total formation, while the increased inflow of saline waters from the Levantine Sea after 1992 was an additional preconditioning factor. The locations and mechanisms of water formation processes are identified with combined analysis of data from the March 1987 oceanographic cruise in the Aegean Sea and the respective model results for that period. Deep water is found to be formed mainly through open ocean convection in the central and north Aegean Sea, while the contribution of shelf areas is limited. Intermediate water is also formed through open ocean convection in the southern Aegean Sea during cold winters as well as in the central and northern Aegean during mild winters. The total volume of dense water formed during 1979-1994 corresponds to an annual formation rate of 0.24 Sv for deep water and 0.34 Sv for intermediate water.

The Aegean Sea is a region of special interest for the Mediterranean oceanographic community, as one of the densewater formation sites of the Mediterranean, driving its thermohaline circulation. Early oceanographic literature exhibits significantly varying opinions regarding the role of the Aegean as a contributor to the water masses of the eastern Mediterranean. The higher temporal and spatial resolution studies that followed the introduction of ConductivityTemperature-Depth (CTD) profilers in the 1980s, revealed that the various scenarios were within the interannual variability of dense water formation in the region. A peak in this variability was the appearance of the Eastern Mediterranean Transient event in the early 1990s. This phenomenon showed that the Aegean Sea has the potential to function as a source of dense water for the eastern Mediterranean; however, it takes over this role only sporadically, depending on the meteorological conditions over the eastern Mediterranean and, possibly, central/eastern Europe. The North Atlantic oscillation appears to be a contributor to this bimodal behaviour. Palaeoceanographic information has confirmed the large sensitivity of the Aegean Sea to climatic variability. Based on the available information, possible scenarios are examined for the response of the Aegean to the current climatic trends.

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In winter 1995 the last major field work of the Physical Oceanography of the Eastern Mediterranean (POEM) program was carried out, the Levantine Intermediate Water Experiment (LIWEX). In this study a thorough analysis is presented of the data set collected during three successive surveys in January, February, and March–April 1995. The major overall result is that the Levantine basin is shown to be the site for multiple, and different, water mass formation processes. Levantine Deep Water (LDW) was formed in the Rhodes gyre, with the preconditioning phase starting in December 1994. In late January the chimney was ventilating to the atmosphere. In February the strong mixing phase is documented by the convective cell vertically homogeneous to 900 m depth. In March–April, recapping has occurred in the upper 200 m. LDW remained confined inside the Rhodes gyre cyclonic circulation. Levantine Intermediate Water (LIW) is instead deeply affected by the upper thermocline circulation. Shallow ventilating chimneys with the LIW thermostads were found in the January survey inside the cyclonic region of the northern Levantine. The formation process of LIW does not involve deep penetrative convection. Winter surface cooling and evaporation are sufficient to produce a surface mixed layer 100 m deep with LIW characteristics. At the mixed layer basis, LIW subducts and spreads along isopycnal surfaces along pathways determined by the cyclonic/anticyclonic structures of the upper thermocline circulation.