Nonlinear internal waves in the South China Sea

Author

Qiang Li, University of Rhode Island

Date of Award

2010

Degree Type

Dissertation

First Advisor

David M. Farmer

Abstract

This thesis investigates the generation and propagation of large-amplitude nonlinear internal waves observed in the South China Sea. Internal tides generated in Luzon Strait are transformed into high-frequency nonlinear internal waves due to nonlinear steepening and non-hydrostatic dispersion. The observations are acquired from modified high-frequency inverted echo sounders. The performance of these instruments is examined in order to understand the measurements. Their generation is first investigated with KdV-type weakly nonlinear theories, next with a composite 2-layer model which includes linear internal tide generation and nonlinear internal wave evolution, and finally with a regional ocean model, the MITgcm. Comparisons of these model calculations with our observations confirm the role of rotation. The relationship between internal tide generation and the subsequent nonlinear evolution is established. Internal tide generation in Luzon Strait appears to be reasonably well approximated by a linear hydrostatic model. In the central part of Luzon Strait, the eastern ridge dominates internal tide generation with the western ridge intensifying semidiurnal internal tide generation due to the ridge separation approximately matching the semidiurnal internal tide wavelength. This double-ridge topography contributes semidiurnal harmonics to the internal tide generation, and rotation disperses the diurnal internal tide component. As a result of these effects, the semidiurnal internal tide is dominant in the middle of the basin, whereas diurnal barotropic forcing is dominant in Luzon Strait. Nonlinear internal waves are seldom observed by satellite imagery in winter. This appears to be due to suppression of internal tide generation in the South China Sea resulting from westward intrusion of the Kuroshio. In addition, nonlinear internal waves are rarely observed east of Luzon Strait in the Pacific Ocean. This intriguing phenomenon results from the joint effect of topography, rotation and the Kuroshio: the deeper thermocline in the Pacific Ocean associated with geostrophic balance of the Kuroshio which reduces the nonlinearity, diminishing its role relative to rotational dispersion, and the double-ridge topography does not intensify the internal tides east of the strait.

Recommended Citation

Li, Qiang, "Nonlinear internal waves in the South China Sea" (2010). Open Access Dissertations. Paper 2353.
https://digitalcommons.uri.edu/oa_diss/2353