Semiannual Variations in 1,000‐dbar Equatorial Indian Ocean Velocity and Isotherm Displacements from Argo Data

Abstract

AbstractThe semiannual Rossby wave in the equatorial Indian Ocean is analyzed using 1,000‐dbar horizontal drift and 3‐D isotherm displacement data from Argo floats. The strong semiannual variations in zonal velocity associated with this wave that have been previously studied are confirmed here, and a more complete picture of the wave is provided with the addition of the 1,000‐dbar meridional velocities and 0‐ to 2,000‐dbar isotherm displacements. At 1,000 dbar, semiannual variations in each field are consistent with Rossby wave physics: Zonal velocity is maximal on the equator, isotherm displacement and meridional velocity extrema are colocated with equatorially symmetric zonal velocity phase reversals near ±2.5° in latitude, and all three fields propagate westward over the year. The latitudinal structure of the meridional velocities in the central and eastern Indian Ocean hints at Rossby wave energy in higher meridional modes than the first, but that signature is not apparent in the isotherm displacements or the zonal velocities. With upward and westward phase propagation and downward and westward energy propagation, isotherm displacement vertical sections confirm the presence of a vertically propagating wave that reaches to at least 2,000 dbar, the deepest extent of the Argo floats.