Oceanography of the northern Arabian Sea

Abstract

The euphotic zone of the northern Arabian Sea ranges from 20-60 m with an average depth of about 40 m. Surface temperatures range from 22.5 to 28.5 degrees C, increasing from north to south. At 1000 m, the temperature is about 15 degrees C lower than at the surface. Maximum difference in temperature is found during the SW monsoon. Salinity also decreases from north to south. High salinity in the northern region is probably due to the excess of evaporation over precipitation and runoff, and to high salinity water coming from the Persian Gulf. There is little seasonal effect on the salinity, but there are large differences in the dissolved oxygen at the surface during the SW monsoon. There are two oxygen minima, the first between 100 and 400 m and second between 800 and 1500 m. The formation of the first oxygen minimum is probably due to high organic production in the euphotic zone, sinking of a large amount of organic matter, the lack of horizontal advection due to the land-locked nature of the sea, and the presence of high salinity water in the upper layers. High oxygen at intermediate depths and the second oxygen minimum in the range 800 to 1500 m probably occur as a result of physical processes peculiar to this part of the Arabian Sea. The flow pattern consists of several eddies and meanders. Inorganic phosphorus is high in the surface layer and still higher at greater depths. Nitrate-nitrogen is low at the surface and increases with depth. Ratios between apparent oxygen utilization (AOU), and changes in carbon, silicon, nitrogen, and phosphorus by atoms were 280:108:40:16:1. Using this relationship it was possible to estimate nitrate reduction at the intermediate depth range and the residence time of water in the oxygen-deficient layer. The rate of primary production in the northern Arabian Sea was 835 mg C m-2 day-1 or 530 x 106 tonnes of carbon each year. The average chlorophyll in the euphotic zone was also high. About 25% of the total photosynthetic production of the Arabian Sea occurs in the northern part in about 8% of the area. Zooplankton biomass was also several time that in the rest of the Arabian Sea, and was highly confined to the upper 200 m. Benthic biomass, on the other hand, was lower than in the other regions of the Indian Ocean