On the flow and thermohaline structure off Cochin during pre-monsoon season

The pre-monsoon characteristics of thermohaline and current structure off Cochin (9.75°N and 75.75°E) in the Arabian Sea suggest the occurrence of wind induced upwelling. A zone of high temperature (>30°C) noticed is believed to be the part of the Indian Ocean warm pool. The surface flow is predominantly southward and is baroclinic. The presence of a northward undercurrent, a characteristic feature associated with coastal upwelling systems, is clearly established with direct current measurements. This zone is associated with intense vertical mixing. Oxygen analysis and T—S confirmed that this current is from the equatorial region. A water mass of local origin (T ∼- 30°C, S ⋍ 35.5 PSU, oxygen ⋍ 3.5–4 ml l−1, sigma-t22 kg m−3) is formed in the surface layers due to pre-monsoon heating and southward advection of Arabian Sea high saline water (ASHSW) over the remnants of low saline Bay of Bengal/Equatorial Indian Ocean water. Mixing characteristics indicated that the instability is mainly controlled by shear rather than buoyancy.