Dynamics of the surface wind field over the equatorial Indian Ocean

The annual cycle and interannual variability of the equatorial westerlies over the Indian Ocean are examined in the context of the equations of motion, using long-term surface ship observations. Eastward pressure gradient along the equator may accelerate equatorial westerlies, but the process is more intricate. The south Indian Ocean trade winds can recurve in the southern hemisphere only with an eastward pressure gradient. This is counteracted by the Coriolis acceleration directed at right angles to the left of the motion. With weak trades and strong eastward pressure gradient, the flow can recurve at relatively high southern latitudes, thus allowing a broad equatorial zone in which westerlies can develop. In the annual cycle, the southern Indian Ocean high and trade winds are closest to the equator and strongest in austral winter when, however, the eastward pressure gradient in the equatorial zone is also steepest. Accordingly, the latitude of flow recurvature in the southern trade winds stays farthest away from the equator in boreal spring and autumn. Consequently, the equatorial westerlies peak in these short transition seasons. The boreal autumn equatorial westerlies are the surface manifestation of a powerful zonal circulation cell along the Indian Ocean equator. The interannual variability of boreal autumn is characterized by weak westerlies accompanying a slack zonal pressure gradient and strong southern trade winds with recurvature near the equator. Such an ensemble of circulation departures is characteristic of abundant rainfall in East Africa and deficient precipitation in Indonesia.