[1] This study examines the dynamics of the Wyrtki jets, which are strong equatorial zonal flows that occur typically during boreal spring and fall in the Indian Ocean. Our diagnosis relies primarily on a continuously stratified linear longwave ocean model driven by QuikSCAT zonal winds. Model results, which compare well with satellite altimetry and in situ current observations, indicate that the zonal currents propagate westward along the equator at semiannual periods with an average speed of −1.5 m s −1 . This propagation speed is three times faster than the propagation speed of the dominant wave mode in model zonal velocity, namely the first meridional, second baroclinic mode Rossby wave. We interpret this result in terms of a superposition of Rossby waves on a wind-forced jet, with the jet stronger than the waves by a factor of 2. Sea surface height (SSH), on the other hand, shows propagating features that vary in both speed and direction from region to region. This contrasting behavior between SSH and zonal velocity results from differing influences of Kelvin and Rossby wave dynamics on the variability. These results are in many respects analogous to the distinction between SSH and zonal current behavior found in previous studies of the equatorial Pacific and Atlantic oceans on seasonal time scales.

[1] This study examines interannual variability in the equatorial Indian Ocean using observations and a continuously stratified linear long-wave ocean model driven by European Centre for Medium-Range Weather Forecasts winds. Our focus is on the relationship between wind stress, zonal velocity, and sea surface height (SSH) in association with the Indian Ocean dipole (IOD). The model correctly simulates the dominant pattern of variability associated with the IOD in which SSH anomalies near the equator tend to tilt zonally in phase with zonal wind forcing. Both observations and the model also show that surface zonal velocity on the equator tends to lead zonal wind stress by about 1 month on interannual time scales. This phasing occurs because velocity anomalies reverse before the wind anomalies reverse during the decay of IOD events. The model simulations indicate that this reversal of velocity earlier than winds is caused by reflected Rossby waves radiating from the eastern boundary. These results have important implications for understanding the evolution of IOD events because of the role of zonal advection in determining interannual variations in equatorial Indian Ocean sea surface temperature anomalies.Citation: Nagura, M., and M. J. , Dynamics of zonal current variations associated with the Indian Ocean dipole,