139736eo.pdf (20MB) - Japan Oceanographic Data Center

thermocline temperature decreases slowly towards the bottom. In the western part of the Gulf, close tothe Bab-el-Mandab Strait, a small increase of temperature can be observed at the depth where RSW isspreading.Seasonal changes of water temperature, due to incoming solar energy variability and tomonsoon wind-induced circulation, are usually restricted to the surface layers. Surface temperatureusually reaches its highest values, over 30°C by the end of May - beginning of June, when there ishigh input of solar energy, weak and variable winds and a shallow secondary thermocline (Fig. 3).In July-August, when surface heating is even greater, the temperature is a bit lower, onaverage about OS'C, than in May-June, due to strong SW monsoon winds, intensive mixing andcooling by upwelled waters. In areas wherc coastal upwelling is strong and reaches the surface,temperature can fall as low as 17°C (Fig. 5). The upper mixed layer remains thin; the thermocline ispushed up by upwelling and sometimes destroyed. Vertical gradients in the thermocline increase andin some regions can reach maximum values (Fig. 2c). In autumn the surface temperatures firstincrease slightly to about 30"C, and then from October they decrease to a minimum in February (about2525°C). At the same time the surface mixed layer becomes thicker due to surface cooling andconvection, reaching down to about 100 m or more (Fig. 2c). The thermocline deepens to a depth ofabout 80-150 m, and in some cases the vertical gradient within it increases. Below the thermoclinemuch smaller seasonal changes of temperature can occur, particularly in the RSW due to seasonalchanges of volume and temperature of incoming waters. In the areas with a strong upwelling, summerdecreases in temperature also can be observed in the deep waters down to a depth of 500-700 m andmore (Fig. 3).SalinityThe vertical and temporal changes of salinity in the Gulf of Aden are small compared tohorizontal variability. The main factors which increase salinity are evaporation and inflow of highlysaline RSW. Water circulation further complicates the picture, but at all depths salinity generallyincreases towards the Bab-el-Mandab Strait and towards the African coast. On the surface thedifferences in salinity between the far northeast region and the vicinity of Bab-el-Mandab can reach1.5 x but usually they are 0.5 - 0.8 x Sometimes during the winter monsoon particularly,the maximum values are observed in the central part of the Gulf where the centers of anticyclonic gyresare located.Vertical profiles show two layers with distinctly higher salinity and two layers with lowervalues. The surface layer has higher salinity (36.0 to over 37.0 x 10-3) due to intensive evaporationthroughout the year and nearly non-existent precipitation and river inflow. Below this, to about 400m, salinities less than 36 x are observed, as described above. Below this depth salinity increasessignificantly, in some regions to over 37 x within the layer of the RSW, and then decreases in thebottom water.Seasonal changes in salinity usually do not exceed 0.5 x 10-3, depending on intensity ofevaporation, water exchange with adjacent seas and water circulation (KHIMITSA, 1968b). In thesurface layer high salinities are observed during winter, spring and in the early autumn, and the lowones in the summer and in early winter (Figs. 5 and 6). In the deeper layers, except in the RSW,seasonal variations of salinity are very small, usually less than 0.2 x 10-3. At about 500-800 m (RSW)the annual variations, particularly in the western part of the Gulf, can exceed 1.5 x 10-3, with highervalues in the winter and in the early spring and lower in the summer (Fig. 5). To the northeast annualchanges in this layer are much smaller (Fig. 6).Water Circulation and UpwellingThe main driving force of surface currents in the Gulf of Aden, the wind, has a pronouncedseasonal pattern which is manifested in the strong and quite steady southwest monsoon wind, aweaker northeast monsoon and two intermediate periods with the weak and variable winds. Thesouthwest monsoon is usually restricted to July-August and northeast monsoon to November throughFebruary-March, so twice a year the surface water circulation is restructured by the prevailing winds.Winter northeast winds cause surface waters to move generally towards Bab-el-Mandab, particularlyalong the Arabian coast (SERIY, 1968). Along the African coast there is a weak current in theopposite direction, and a cyclonic eddy is observed in the central part of the Gulf. Its center changes261