Spatial asymmetry of nonlinear internal waves in the Lombok Strait

Abstract

Nonlinear internal waves (NLIWs) are an energetic oceanic phenomenon near the Lombok Strait; however, their generation and three-dimensional characteristics remain poorly understood. To identify the generation and evolution processes of local NLIWs, we implement high-resolution (Δx=100 m) three-dimensional numerical simulations with realistic topography and tidal forcing, and a set of two-dimensional idealized numerical simulations. Barotropic semidiurnal tides (D2 in short) are verified to predominantly determine the local NLIW generation, but barotropic diurnal tides (D1 in short) can significantly modulate it. Particularly, D1 would enhance (weaken) the amplitudes of NLIWs during the propagation processes when they are in phase (out of phase) with D2. In addition, the three-dimensional model results also suggest that internal wave energy fluxes attenuate dramatically when NLIWs are radiating away from the generation site, due to the radial spreading effects of wave energy along the crest line. However, as NLIWs propagate over the complex bottom topography, large energy fluxes mainly occur in the east-central area of the crest line, resulting in a west-east asymmetry of internal wave energy field. The three-dimensional model results indicate that the northward NLIWs are more numerous and better identified than the southward ones, which are in accordance with the satellite image (MODIS). By a series of two-dimensional idealized numerical simulations, we conclude that the northward NLIWs is 43% larger than the southward ones, due to the joint effect of north–south asymmetric slope steepness on two flanks of the sill in the Lombok Strait and a southward deeper Indian Ocean, between them, the role of asymmetric water depth is more significant (38%) than the asymmetric slope steepness (13%).