Internal waves in the Andaman Sea

Abstract

The phenomena of non-linear internal waves (solitons) that propagate along the pycnocline are the subject of this thesis. Past investigations have shown their extensive spatial distribution within the Andaman Sea with amplitudes up to 80m. A period of intense internal waves (IW's) were observed from the 12-17 Nov'93, as 11 successive packets within the south Andaman Sea. The aim of this research was to use the available remote sensing data to study the synoptic overview of the phenomenon and to describe the environmental conditions at the time of their generation and subsequent evolution. The ultimate aim is to provide a synergistic methodology in the absence of in situ data, to predict anomalous periods of soliton packet activity. From the available Synthetic Aperture Radar images, two methods are used to estimate internal wave dynamics e.g. amplitude, characteristic wavelengths and phase speeds. Furthermore, an aim was to explain why the anomalous period of waves occurred during a spring tide regime and not during the next spring tide condition two weeks later.

Harmonic analysis indicated that the observed surface mean advective current is westward (outflow). During the first spring tide the FFT spectra revealed predominant semidiurnal tides with reduced period compared to the subsequent spring tide 2 weeks later. The magnitude of the spectral shift is shown to be consistent with a Doppler shift by the advective mean flow. This suggests that an estimate of the mean flow variability can be inferred by measuring the variability of soliton packet spacing from SAR. Results from OCCAM Global Circulation Model simulations forced with 6-hour ECMWF winds show an outflow at 150m depth through the internal wave generation region prior to the generation of the anomalous waves.

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