Equatorial Velocity Profiles. Part II: Zonal Component

Abstract

Vertical profiles of horizontal velocity made along 53°E in the western Indian Ocean, during and after he onset of the southwest monsoon in 1976, show features in zonal velocity of relatively small vertical scale. Persistence of the features over the month-long observation period and over 2½ degrees of longitude indicates long temporal and zonal scales. The vertical structure is common to those profiles close to the equator, with no appreciable variation in amplitude or depth. Between 1°30?N and 3°N the phase of the features reverses. There is no evidence of similar features at 5°N. The data suggest meridionally trapped equatorial waves. Similarities between the observed phenomena and the linear theory of equatorial waves are striking but quantitative comparisons lead us to question its naïve application here. Equatorial intensification is apparent but it does not scale with the Rossby radius of deformation, indicating that Kelvin waves are not dominant at any vertical scale. The phase change and corresponding amplitudes of zonal velocity do not fit a low-frequency, first meridional-mode long Rossby wave. Higher meridional modes produce further inconsistencies. There is no evidence of vertical propagation of the larger scale features. Although the small vertical scale variability is ubiquitous in the equatorial regions, no satisfactory theory exists at present. Our conclusion is that although there are similarities between observations and linear theory, there are serious discrepancies which may be related to the proximity of these observations to the slanting coast of East Africa.