Dynamics of wind-forced intraseasonal zonal current variations in the equatorial Indian Ocean

[1] This study examines the structure and dynamics of wind-forced intraseasonal zonal current variability in the equatorial Indian Ocean. We take advantage of a variety of satellite and in situ data sets, including unprecedented 4–8 year-long velocity time series records from the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) program. Spectral analysis reveals prominent intraseasonal zonal currents variations along the equator with periods of 30–70 days. These oscillations are vertically in phase above the thermocline and propagate eastward with the local zonal winds. In the thermocline, intraseasonal zonal velocity variations also propagate eastward across a broad range of phase speeds expected for low baroclinic equatorial Kelvin waves; amplitudes decrease with depth, with deeper levels leading those near surface. Collectively, these results suggest that the near-surface layer responds directly to intraseasonal zonal wind stress forcing and that subsequently energy radiates downward and eastward in the thermocline in the form of wind-forced equatorial Kelvin waves. In addition, intraseasonal zonal current variability on the equator is coherent with off-equatorial sea surface height fluctuations in the eastern and central of the basin. This coherence is primarily due to the fact that equatorial zonal wind variations are associated with off-equatorial wind stress curls that can generate local Ekman pumping and westward propagating Rossby waves.