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APPENDIX E 328 The Mediterranean Sea Mediterranean water forms in the northwestern part of the Mediterranean Sea in winter. Cooler winter temperatures and higher-than-normal evaporation, associated with the cold, dry Mistral winds, increases the surface water density such that vertical mixing occurs all the way to the sea floor (2,000 m). Evaporation (about 100 cm per year) exceeds precipitation plus river runoff, so there is a net loss of volume which is made up by inflow of salt water from the Atlantic Ocean (Pickard and Emery, 1990). The homogeneous Mediterranean water mass has a salinity of more than 38.4 practical salinity units (psu) and a temperature of about 12.8Â°C (Davis, 1977). Mediterranean water leaves the Straits of Gibraltar at approximately 300-500 m depth, below incoming Atlantic water. Intense mixing occurs at the interface of the Mediterranean and Atlantic waters. The least-mixed Mediterranean water has a salinity of 36.5 psu and a temperature of 11Â°C (Davis, 1977). Due to its high density, it sinks to about 1,000 m, where it becomes neutrally buoyant and spreads out. This distinctive tongue of mediterranean water can be recognized throughout much of the Atlantic Ocean by its high temperature and salinity profiles. The Mediterranean Sea's relatively long residence time (estimated at 70-100 years) makes it particularly vulnerable to pollution. North Sea The North Sea is broad and shallow; thus, it is subject to storm surges (Gross, 1982). At its southern end the North Sea is constricted at the Straits of Dover; however, there is no geographical northern boundary. The south and southeastern parts are less than 50 m deep, and the northern part is 120-145 m deep. The North Sea is not an homogeneous body of water. The residence time is approximately 0.9 year (Otto, 1983). Overall, there is an excess of precipitation over evaporation. In winter, however, the lee effect of the British Isles produces a net loss by evaporation in the western and southwestern parts of the North Sea. During summer, all parts receive an excess of water due to precipitation. As surface waters become less saline, stratification occurs between the warm, less dense water over the deeper water. During calm weather in the eastern North Sea and German Bight, a thermocline may develop, resulting in reduced oxygen concentrations in the bottom water (Clark, 1986). Persian Gulf The average depth of the Persian Gulf is 30 m, with a maximum depth of 90 m. It is so shallow that there is no significant exchange of water between it and the adjacent Gulf of Oman (Sverdrup et al., 1942), although some water does