The monsoonal current regimes of the tropical Indian Ocean: Observed surface flow fields and their geostrophic and wind‐driven components

Abstract

The annual cycle of current regimes in the tropical Indian Ocean (to 30°S and 120°E) is studied on the basis of long‐term observations of the surface wind field, ship drift measurements of surface currents, and subsurface temperature and salinity casts to 400 dbar depth. For the major current domains, the Ekman surface current is computed from wind observations under assumption of a drag coefficient and of an eddy viscosity coefficient; the geostrophic surface flow is calculated from the field of the geopotential anomaly of the ocean surface relative to 400 dbar, constructed from subsurface temperature and salinity casts; the observed current is compiled from ship drift measurements; the Ekman volume transport is estimated using the assumed drag coefficient; and the geostrophic volume transport is obtained from the field of mass transport function between zero and 400 m relative to 400 dbar, constructed from subsurface temperature and salinity casts. The monsoonal reversals of wind stress forcing, which are most dramatic in the northern Indian Ocean and the equatorial zone and more moderate in the southern part of the basin, have diverse consequences for the various current systems. Under the steady southeast trades the westward directed South Equatorial Current (SEC) is basically geostrophic, with an additional strong Ekman contribution in boreal summer. The eastward directed South Equatorial Countercurrent (SCC) is mainly geostrophic, but in boreal summer it is virtually eliminated by the then westward directed Ekman component. The westward/eastward flowing Northeast Monsoon Current/Southwest Monsoon Current (NEM and SWM) of boreal winter/summer are Ekman flows. The Eastward Equatorial Jets (EEJ) during the monsoon transitions have larger geostrophic than Ekman components, while in the northward East African Coastal Current (EAC) and the northeastward Somali Current (SCN) of boreal summer the Ekman prevails over the geostrophic flow. In the latter three current systems the Ekman flow creates mass distributions that in turn entail sympathetic geostrophic currents. The prevailing geostrophic annual mean transports are of order 5 sverdrups (Sv) for the SCC, NEM, and SWM and about 13 Sv for the SEC. A transport performance comparable to that of the perennial SEC is attained by the EEJ during the monsoon transitions.