Equatorial Currents at Semi-Annual Period in the Indian Ocean

Abstract Moored records of zonal velocity in the western equatorial Indian Ocean are dominated by motion of semi-annual period. The motion is coherent over an array spanning more than 1000 km along the equator, 160 km across the equator, and 500 m in the vertical The signal has an amplitude of 0.15 m s−1. It has zonal, vertical and meridional length scales which are long compared to the dimensions of the array, and it shows upward propagation of phase. Behavior is characteristic of an eastward propagating equatorial Kelvin wave and a westward long equatorial Rossby wave. The vertical wavelength is an estimated 5000 m, implying an equatorial trapping scale of 400 km. Zonal wavelengths are ∼8000 km for the equatorial Rossby wave and 24000 km for the equatorial Kelvin wave. A downward energy flux, estimated to be 3 × 1016 erg s−1, most likely represents propagation away from surface forcing by the zonal wind.