Hydrographic characteristics of the Indian sector of the Southern Ocean

Abstract

Oceanographic studies in the Indian sector of the Southern Ocean (58-61 degrees S and 30-40 degrees E) carried out during December 1995 and March 1996 indicate a 3-layer structure typical of summer in the oceanic domain south of the Antarctic Polar Front. The upper 300 m water column consists of three distinctive thermohaline characteristics. The surface layer (50 m) of summer surface water formed by seasonal warming with temperatures (0-2 degrees C) and low salinity (less than 33.8 PSU), the intermediate layer (50-150 m) of winter reminiscent water with negative temperatures (0 to 2 degrees C) and moderate salinity (33.8-34.3 PSU) and the deeper layer below 150 m) of Circumpolar Deep Watermass (CDW) with positive temperatures (0-2 degrees C) and higher salinity (34.3-34.8 PSU). The influence of the strong Antarctic Divergence between the east and west drifts is noticed at 61 degrees S, 34 degrees E marked by phytoplankton and zooplankton patches. The dissolved oxygen (DO) shows high concentrations (7.6 to greater than 8.0 ml l sup(-1)) in the upper layer. These higher levels of DO sometimes extend down to the intermediate layer and gradually decrease further downwards. Phytoplankton cell counts remain high (between 1.92 and 22.9 x 10 sup(4) per litre) while zooplankton biomass shows variations from 9.8 to 303.62 ml 100 m sup(-3). A marked drop of both phytoplankton and zooplankton in the region of krill swarm (between 60 degrees 40'-61 degrees 13'S and 33 degrees 34'-34 degrees 14'E), however, indicates active feeding by the latter on the planktonic communities.