The coastal current off Western India during the Northeast monsoon

Abstract

Hydrographic data collected during December 1987 and January 1988 are used to describe the poleward coastal current along the west coast of India that flows against weak winds during the northeast monsoon. Near the southern end of the coast, at about 10°N, the current was approximately 400 km wide, 200 m deep and carried the low salinity Equatorial Surface Water. The isopycnals tilted down on approaching the coast. Near the northern end of the coast, at about 22°N, the flow was restricted mainly to the vicinity of the continental slope; the current was a narrow (100 km), 400 m deep jet with a transport of about 7 × 10⁶ m³ s⁻¹. Along most of the coastline, a southward moving undercurrent was inferred from the distribution of salinity, temperature and dynamic topography. To understand the driving mechanism of the current, the annual cycle of the contribution of the longshore pressure gradient and that of the winds to the near-surface momentum balance was examined using available climatologies. It is seen that the longshore pressure gradient overwhelms the winds during the northeast monsoon, whereas during the southwest monsoon the winds dominate. In the Leeuwin Current off western Australia, the only other known eastern boundary current that flows against the winds, the pressure gradient dominates the winds throughout the year. The overall structure of the northeast monsoon coastal current is consistent with that predicted in the analytic model proposed by McCrearyet al. (1986, Journal of Marine Research, 44, 71-92) to explain the Leeuwin Current. In the model a poleward baroclinic pressure gradient is generated by a density gradient along the coast. The latter is known to exist along the west coast of India. However, it seems likely that the current is also influenced by a barotropic pressure gradient.