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

~~Table 1 Physical chemical properties of the water masses of the Gulf of Aden asmeasured during 1984-1985.Water Masses Depth Range T"C (s x 10-3) OT O2 (ml 1-1)(m)~ ~ ~ ~ ~ ~ ~ ~ ~ _ _ _ _ ~ ~ ~ ~ ~ _ _ _SURFACE 0 24.0-30.0 36.0-36.8 22.0-24.0 4.0-5.0WATER 20- 125 16.0-25.0 35.4-36.3 24.2-25.4 1.0-4.5SUBSURFACE 70-400 13.0-28.0 35.3-35.8 26.0-26.5 0.2- 1.2WATER 200-500 13.0-16.0 35.5-36.5 26.6-27.2 0.4-1.0RED SEA 400-900 11.0-17.0 35.8-37.6 27.2-27.6 0.4- 1.2WATER 1000 and over 8.0-12.0 35.0-36.2 27.4-27.6 0.5-1.0.........................................................................................................................Subsurface Water (SSW) is distinguished by an intermediate salinity minimum, oxygenminimum and high nutrient content. It is located between the SW and a depth of 400-450 m. Thiswater originates most probably at the subtropical front in the Indian Ocean (SERIY, 1968) and comesinto the Gulf of Aden from the Arabian Sea. The general direction of its movement is towards theBab-el-Mandab Strait; it rises up a bit on its way west, and its temperature, salinity and oxygen contentincrease slightly.Seasonal changes of the SSW, though much smaller than for SW, can be still distinguished.The highest values of temperature (15-18°C) and salinity (36.0 - 36.5 x are observed inMay-June, and the lowest (13 - 15°C and 35.3 - 35.8 x 10-3 respectively) during southwest monsoonand afterwards. Oxygen content is very low, usually below 1 ml 1-1 and very often below 0.5 ml l-1.The core layer of SSW has a density of about 26.0 - 26.5.Red Sea Water (RSW) As this mixed water comes through the Strait of Bab-el-Mandab, itsinks into the Gulf of Aden and moves generally towards the east, at depths between 400 m in westdown to 1000 m towards the mouth of the Gulf. The depth of the core layer of RSW increases to theeast, and its temperature, salinity and oxygen content decrease in the same direction. Red Sea Water isdistinguished by higher temperature, salinity and oxygen content and lower nutrient values. There aresome seasonal changes of RSW indexes, partly due to seasonal variability in water exchange throughthe Strait of Bab-el-Mandab. The inflow into the Gulf of Aden increases during winter and spring,with the maximum in May, and decreases in July-August (SERIY, 1968). So higher temperatures andsalinities are observed by the end of winter and in the spring, and lower values during summer andautumn. The most probable density in the core layer of RSW is 27.2 - 27.6. Our data clearly showthis water mass (Figs. 6 and 7) in the western part of the Gulf, and much less (or not at all) in the east.Water TemperatureOther than solar heating and heat exchange with the atmosphere, the thermal structure of Gulfof Aden waters is formed by water circulation and inflow and outflow from the Red Sea and ArabianSea. Water circulation and upwelling particularly cause the redistribution of the heat in the upperlayers to such an extent that along the Arabian coast the minimal surface temperature is recorded insummer (Fig. 4),when the maximum should be expected. Generally the temperature at the surfaceduring late spring, summer and early autumn increases from the Arabian coast towards the Somalicoast. Sometimes, during May-June and September-October particularly, maximum values arerecorded in the center of the Gulf where an anticyclonic gyre is formed. During the remaining part ofthe year, October-November through April, the picture is reversed, temperature generally decreasingtowards African coast, or minimal values occurring in the center in the cyclonic gyre.The vertical distribution of temperature shows a shallow, seasonally variable surface mixedlayer: about 20-100 m. The depth of thermocline and the vertical gradient within it are changeable aswell. In spring and summer a secondary thermocline is observed near the surface. Below the main260