The Three Dimensional Circulation in the Central Equatorial Indian Ocean

The wind driven circulation of the tropical oceans was one of Mark Cane's early scientific interests. In his 1980 publication, "On the dynamics of equatorial currents, with application to the Indian Ocean", he used a simple dynamical ocean model to interpret some of the earliest equatorial time series measurements collected in 1973-75 from the Maldive Islands. The Indian Ocean is dominated by the monsoons, which result in equatorial winds that "...are predominantly westerly, resulting in downwelling at the equator" as Cane notes in the abstract of his paper. Surprisingly in the more than four decades since this foundational publication, there have been no suitable observational estimates of vertical velocity along the equator in the Indian Ocean to quantify this downwelling circulation. Here, we present the first long-term estimates of vertical velocity in the Indian Ocean using five years of data from an array of acoustic Doppler current profilers (ADCPs) centered at 0‚ 80.5°E. Our estimates, based on the continuity equation using horizontal velocities from the ADCP array, indicate mean downwelling with a maximum of about 5 m per day in the upper 80 m, below which there is upwelling of similar magnitude. We compare our derived estimates with those from a high spatial resolution regional wind-forced ocean general circulation model, which simulates the observed velocities with a high degree of fidelity. We then use the data and the model to describe the dynamics of the three-dimensional circulation in the central equatorial Indian Ocean, how near-surface downwelling and deeper upwelling are consistent with this flow field, and how the monsoon dominated circulation in the Indian Ocean differs from the circulation observed in the trade wind driven Pacific and Atlantic Oceans.