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. An average buoyancy frequency profile was used to reduce each velocity profile to the form which would have been observed in a uniformly stratified ocean. Removal of the variations of amplitude and scale resulting from the background density stratification makes it possible to discuss vertical wavenumbers throughout the temporal and spatial array, rather than merely local vertical wavenumbers.; Autospectral estimates reveal equatorial intensification that varies with vertical wavenumber. The hypothesis is that equatorially-trapped waves of one kind or another are the basis for the observed motion. To examine vertical propagation, dropped lagged coherences have been computed. Pairs of drops separated by the same temporal lag were grouped together to estimate cross-spectra. 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.; If linear equatorial wave theory is applicable, zonal wavelengths can be determined from the dispersion relation, and possible forcing mechanisms can be examined. Linear theory gives a zonal wavelength of 300-400 km for the 1200 stretched meter (sm) oscillation. The rms velocity is the same order as the phase speed, however, so that nonlinearities may not be completely negligible. Indications are of multiple processes within the same wavenumber band, a longer-period Kelvin or Rossby wave coexisting with the above-mentioned mixed Rossby-gravity wave.