Observations of Vertically Propagating Equatorially-Trapped Waves in the Deep Western Indian Ocean

摘要

An hypothesis of equatorially-trapped waves is found to be consistent with time series of vertical profiles of horizontal velocity and CTD data from the western Indian Ocean. The profiles were collected using an acoustic dropsonde, the White Horse, along the 53 degrees E meridian. The temporal coverage is a month-long period spanning the onset of the southwest monsoon in 1976; the latitudinal coverage is 3/4 degrees S to 5 degrees N. To examine the composition of the velocity field, a WKB stretching procedure was applied to the depth and a WKB normalizing procedure to the velocity. Autospectral estimates reveal equatorial intensification that varies with vertical wavenumber. To examine vertical propagation, dropped lagged coherences have been computed. The results indicate the presence of a mixed Rossby-gravity wave of 60-77 day period with phase propagation downward at a vertical wavelength of 1200 in the stretched coordinate, which is equivalent to approximately three wavelengths between the thermocline and the bottom. Vertical propagation is indicated in other wavenumber bands as well, also corresponding to autospectral peaks. (Author)