Thermohaline circulation in the Central Indian Basin (CIB) during austral summer and winter periods of 1997

As a part of Indian Deep Sea Environment Experiment (INDEX) aimed at assessing the environmental impact of manganese nodule mining in the Central Indian Basin (CIB), a study on baseline physical conditions of water column viz. potential temperature (θ), salinity and potential density together with geostrophic circulation regime in the deeper depths of the basin was conducted. The hydrography data used in the present analysis were collected over a wide area of the western part of CIB (71°–79°E; 9°–14°S) during austral summer (January 1997) from the Indian research vessel ORV Sagar Kanya, while during the austral winter season (June–July 1997), hydrographical stations were occupied by Russian research vessel RV Yuzhmorgeologia in the central part of CIB (75°–77°E; 9°–11°S) where a benthic disturbance on experimental scale was carried out. The spatial variations in the physical parameters decreased below 3500 m, inferring a restricted basin-scale deep circulation. The dynamic topography field at 5000 m relative to 2000 db surface in the central part of CIB, representing the abyssal circulation, was generally characterized by a southwestward weak flow around 10°S flanked by cyclonic and anti-cyclonic eddies on its right and left sides, respectively. This flow regime agreed with the earlier one inferred by Warren (J. Mar. Res. 40 (1982) 823) linking the source of deep water in CIB to a saddle overflow across Ninetyeast Ridge from West Australian Basin around 10°S.