Deep Velocity Measurements in the Western Equatorial Indian Ocean

Abstract

Vertical profiles of horizontal current collected in April and June 1979 in the western Indian Ocean revealed the presence of short vertical scale (150?300 m) deep zonal jets, trapped to within 1° of the equator. Meridional velocity records displayed in general higher temporal and spatial variability and significantly less energy at the larger vertical scales as compared with zonal records. Vertical wavenumber spectra of velocity showed no statistically significant peaks. The conspicuous alternating zonal jets were identified with the slightly more energetic (as compared to background levels) 500?429 stretched meter (sm) wavelengths. For, most vertical wavenumbers, zonal currents were not significantly coherent at distances greater than 400 km downstream. At shorter zonal separations, phase lags were indistinguishable from zero. On the equator, there is an indication of upward shift of approximately 100 sm over the 500?429 sm band. Also on the equator, zonal current (u) and vertical displacement (?) were significantly coherent for the 429 sm wavelength with ? leading u with increasing depth by approximately /2, For the same wavelength. zonal kinetic energy decayed away from the equator on scales comparable to the theoretical Kelvin wave scaling. Both these results suggested the possible presence of Kelvin waves in the records, at the 429 sm band. Comparison of these findings with previous studies from the Indian and Pacific oceans indicates different temporal and spatial scales for the deep jets in the two basins.