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

BASIC OCEANOGRAPHIC FEATURES OF THE GULF OF ADENPrevious pertinent knowledge has been summarized by the late K. Grasshoff, Y. Halim, S.Morcos and G. Siedler (in HALIM et al., 1980); we loosely quote as follows:The temperature-salinity-density structure of the area is mainly controlled by themonsoon-related wind and atmospheric pressure field and its seasonal variations, and by the waterexchange through Bab-al-Mandab due to the thermohaline circulation caused by intensive evaporation(approximately 2 m) in the Red Sea. As a result, there exists a two-layer structure in Bab-al-Mandab.The high salinity bottom water (up to 40 x flows over the sill and through the Strait down slopeinto the Gulf of Aden, spreading horizontally at a depth of about 500 m.The hydrographic structure of the Gulf of Aden is characterized by a surface layer of warm,fairly saline water between 20 m (winter) and 120 m (summer) in thickness, below which is a decreasein temperature and salinity (maximum 36 x and a reduced vertical temperature gradient, causedby the spreading Red Sea water. At greater depths down to 2,500 m is the upper Indian Ocean deepwater (YC, 35 x 10-3).Average surface currents are directed into the Gulf of Aden and the Red Sea during winter andout of the Gulf during summer, Gyre-type motions in the Gulf of Aden are indicated by availablegeostrophic current data.Upwelling is significant off the Yemeni coast during the SW Monsoon (summer) whereEkman transport tends to carry coastal surface waters away from the shelf, thus causing convectivemotions on the shelf. CURRIE et al. (1973) discussed upwelling in Omani shelf waters, but did notprovide any evidence of the upwelling in the Gulf of Aden itself. Upwelling also may be significantoff Socotra where major changes in the circulation pattern result from the seasonal reversal of themonsoon. Though it is more intensive along the coast, a significant feature of the south Arabianupwelling is its width and the much greater depth of the water supplied than is usual in other coastalupwelling regions.Short-term atmospheric events may cause considerable deviations from the mean conditions,particularly close to the coast. These variations in the hydrographic structure have a strong effect onthe distribution of nutrients, oxygen and chlorophyll (WYRTKI, 1971; KREY and BABENARD,1976) and, therefore on the primary production and the fish population.The hydrochemical regime of the Gulf of Aden is marked by a number of essential featureswhich also have a significant influence on the concentration and distribution of nutrients. Togetherwith the distribution of oxygen, they certainly influence the abundance of fish, especially the planktonfeeders. Very little is known about the variations of oxygen and nutrients in the euphotic layer, both intime and space.The general water circulation in the upper layers, particularly the upwelling processes thatoccur along the Yemeni coast and south of Socotra, is strongly dependent on the monsoon, itstemporal variations, and on the topography of the coast line. Compilation of data from theInternational Indian Ocean Expedition (IIOE) suggests that the depth of the mixed layer in the Gulfregion may be as shallow as 20 m or less during the period of the SW monsoon in April throughSeptember, whereas it is rather deep (>lo0 m) south of Socotra during this period. From October toFebruary the situation changes completely, creating a deeper mixed layer along the Yemeni coast (>60m) and a shallower one (>40 m) during March through June. It must be stressed, however, that thenumber of observations and occupied stations in the region are too few to allow any firm conclusionsor detailed analysis.In the upper thermocline, nutrients have high gradients (0.3 - 2.0 ug-at 1-1 for phosphatephosphorus,