Detection of the Equatorial Roll in the Mixed Layer of the Indian Ocean

The “equatorial roll” in the mixed layer of the Indian Ocean was first identified over 20 years ago in model simulations of the large-scale ocean circulation. There has been scant observational evidence for this feature however because of its relatively small scale and the lack of suitable direct velocity measurements. This study investigates the meridional circulation in the upper layers of central equatorial Indian Ocean using acoustic Doppler current profiler (ADCP) data and other data sets along 80.5°E for the period 2004-13. The ADCP data set is the most comprehensive collection of direct velocity measurements in the central Indian Ocean to date, allowing us to confirm the existence of a shallow equatorial roll in observations. The roll results from a northward wind-driven current in the upper 25 m near the equator overlaying a southward subsurface flow whose transport is in Sverdrup balance with the wind stress curl. This circulation is narrowly confined to within ±1° of the equator and is most strongly developed seasonally during July-October.. Other unique features of the flow field in this region are also identified, such as a mean downwelling favorable meridional circulation in response to westerly wind forcing near the equator and a seasonally varying cross equatorial volume transport from the summer to the winter hemisphere in quasi-steady Sverdrup balance with the seasonally varying winds.