The generation of non-linear internal waves in the Gulf of Oman

Abstract

The generation of non-linear internal waves evident in two Synthetic Aperture Radar (SAR) images of the Gulf of Oman has been studied. The internal waves form packets, propagating towards the Musandam Peninsula from the eastern shelf-edge, travelling right up to the coast. The internal waves on the shelf have the classic appearance of rank ordered non-linear waves that depress the pycnocline. The images suggest a tidal generation mechanism. Under this assumption, estimates of the phase speeds of the waves were derived and found to vary from 0.5 m s −1 near the coast to 0.7 m s −1 on the shelf. The possible generation of the internal waves at the Gulf of Oman shelf-break has been investigated using numerical models. A 2D barotropic tidal model has been used to assess the tidal flux across the continental slope in the region of interest. The model indicates that tidal flow at the shelf-edge is mainly across-slope. The tidal flux information is used to drive a two-layer non-linear internal tide model, which simulates the internal tide along a vertical slice perpendicular to the shelf-edge. Numerical results for a tidal flux corresponding to the spring tide (corresponding to one of the SAR images) indicate that this is strong enough to produce high-frequency waves for all layer depths considered. It is found that for the weaker modelled value of tidal flux corresponding to the time of the other SAR image, using two-layer parameters derived from a climatological density profile, the forcing is just too weak to force high-frequency internal waves. However, a sensitivity study investigating the effects of different layer depths suggests that a reduction in the top layer thickness by 10 m is sufficient to produce waves similar to those observed. The model predicts internal waves of depression with peak to trough amplitudes up to 25 m, surface currents up to 0.6 m s −1, and strain rates of O(10 −3 s −1). The phase speeds of the internal waves from the model (0.9 m s −1) are slightly higher than those observed, possibly due to approximations of smooth bathymetry and climatological density profiles used in the model, or due to the approximate nature of inferring observed phase speeds from two SAR images from different years. As the qualitative features of the observed internal waves are reproduced by the internal tide model, it is concluded that tidal flow over the shelf-edge is the likely generation mechanism for the internal waves imaged by SAR.