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

position in time and according to changes in the wind field. The alongshore Ekman transport of thewater has a shoreward component, and accumulation of surface water on the shelf is usually observed.During the summer monsoon with southwest winds, the surface water moves generally in theopposite direction, e.g. towards the Arabian Sea. The alongshore wind stress, with the coast to theleft (northern Hemisphere), causes a net offshore transport in the surface Ekman layer. To preservethe water balance the deeper subsurface water replaces surface waters which have been pushed away.Our observations, made in August 1984 on RIV Dr. F. Nansen, show a picture of upwelling along theArabian coast of the eastern Gulf of Aden. It is known (SERIY, 1968) that in the western part of theGulf the southwest monsoon wind is much less stable and that it ceases earlier. The presence of muchcolder and low oxygenated waters in the vicinity of Aden (Figs. 3 and 4) in the subsurface layer below20 m indicates upwelling, even if not observed at the surface. Surface temperature distribution (Fig.5) shows three partly separated sources of coastal upwelling, with the lowest temperature below 17°Cin the vicinity of Mukalla.Such “patchiness” in coastal upwelling has been noted in other upwelling regions (BOJE andTOMCZAK, 1978). The temporal and spatial scale of the circulation is usually of smaller magnitudethan can be resolved by the observations (CURRIE et al., 1973). A striking feature of this upwellingis its extent and influence over a very wide area of the Gulf, as seen in the temperature and oxygendistributions at a depth of 20 m (Figs. 5 and 10). In the main coastal upwelling regions, the width ofthe active upwelling is roughly equal to the Rossby radius, which is tens of kilometers (HUYER,1983). Such an offshore extent of upwelling has been described further eastward, along the southernArabian peninsula. BOTTERO (1969) confirmed that upwelling extends at least 400 km offshore andparallels the Arabian coast for a distance over 1,000 km. This phenomenon also was mentioned bySWALLOW (1984) and CURRIE et al. (1973). Because of this, the upwelled water must be suppliedfrom much greater depths than is usual in other coastal upwelling regions (CURRIE et al., 1973).Our data, density particularly, indicate the upwelling from as deep as 500 m.Usually in coastal upwelling regions there are strong undercurrents flowing opposite to thesurface current direction. However, in our case the distribution of physical properties does notintiicate an undercurrent.Some indications of weak and restricted upwellings are also observed during the rest of theyear. This can be caused by short-term fluctuations (periods of several days) in wind stress, evenwhen the long-term wind stress is not favourable.NUTRIENTSInorganic PhosphateAs is generally typical for the North Arabian Sea (SEN GUPTA and NAQVI, 1984) theintermediate and deeper waters of the entire Gulf of Aden contain almost inexhaustible reserves ofphosphate. Throughout the year the whole column from 200-1,000 m generally contains phosphateconcentrations above 2.0 pM dm-3 P-P04-3, reaching maxima of about 2.5 pM dm-3 in the westernand 3.0 pM dm-3 in the eastern parts of the Gulf. Even at 100 m concentrations are rarely below 1.0pM dm-3. The lower part of euphotic layer (seasonally between 30 and 50 m) consistently shows highlevels of 0.4 - 0.6 pM dm-3, and very rarely minima of 0.1 - 0.2 pM dm-3 are recorded in inshorewaters. Almost the same levels are found in uppermost layer and at the very surface; a few records fallbelow 0.2 pM dm-3, and in one localized inshore area concentrations fall below 0.05 pM dm-3.During the summer the upwelled intermediate waters enrich the very surface waters withexcessive phosphate concentrations of 1.15 - 2.50 pM dm-3, probably the highest surface concentrationsrecorded in the tropical oceanic environment. Due to lateral and offshore circulation, thesephosphate-rich surface waters seem to spread over the Gulf, hence supplying sufficient quantities forprimary productivity. Therefore it seems clear that the phosphorus is never a limiting factor.262