Observational evidence for remote forcing of the West India coastal current

Abstract

Circulation in the north Indian Ocean is influenced by both local and remote wind forcing. So far, however, determining the contribution of these two forcing mechanisms at a location has been possible only in numerical experiments. Here we separate remote and local forcing in observations. Using field measurements (current, sea level, and wind) for a month during March-April 2003 off Goa in the near-coast regime of the West India Coastal Current (WICC), we show that the current was driven by local winds only at periods less than ∼10 days, with remote forcing contributing at longer periods. The high-passed (HP; period less than ~10 days) component of the along-shore current was strongly correlated with the HP component of the along-shore wind, the current lagging the wind by half a day. The low-passed (LP) components of the wind and current were not correlated: the former was unidirectional, but the latter reversed during the period of observation. The relationship between the HP wind and current was used to estimate the locally forced LP current, permitting an estimate of the remote current, the LP residual. This separation of locally forced and remotely forced currents showed that remote forcing contributed as much as local forcing to the WICC. The local current behaved like a classical eastern boundary current forced by local winds. The reversal in the remote current was due to winds 700 km farther south along the coast; frictional damping had an impact only at periods less than 10 days, there being no remotely forced HP current.