Features of Internal Waves in a Shoaling Thermocline

Vadim V. Navrotsky,Valeriy Yu. Liapidevskii,Elena P. Pavlova

doi:10.4236/ijg.2013.45081

Abstract

Observations and numeric modeling of internal wave generation and transformation in the shelf zone of sea show that the main part of tidal energy is transported to shores in form of internal gravitational waves. Long-term measurements of temperature and current velocity fluctuations at many levels in the near-bottom thermocline were carried out during the periods when stable seasonal thermocline was present. Analysis of the measurements permits us to understand mechanisms of internal wave destruction with turbulent motion generation and corresponding rebuilding of velocity and density mean fields in the stratified near-bottom layer. Spectral analysis of temperature fluctuations shows that in shoaling internal waves the low-frequency maxima disappear, maxima at higher frequencies appear, and the spectra slope in the high frequency range changes with depth. Taking into account the concurrent analysis of near-bottom pressure fluctuations and current velocity fluctuations from surface till bottom we come to the conclusion that breaking internal waves in a near-bottom thermocline generate not only small-scale three-dimensional turbulence, but also quasi-horizontal turbulence of larger scales, which considerably contributes into mixing and sediments, alluvium, and nutrients transport in the shelf zone of sea.

Highlights

Huge energy of tides and inertial motions is dissipated in oceans’ coastal waters, but in stably stratified flows the initial stage of the energy transformation is generation of internal waves (IW) over the continental slope and shelf break
Taking into account the concurrent analysis of near-bottom pressure fluctuations and current velocity fluctuations from surface till bottom we come to the conclusion that breaking internal waves in a near-bottom thermocline generate small-scale three-dimensional turbulence, and quasi-horizontal turbulence of larger scales, which considerably contributes into mixing and sediments, alluvium, and nutrients transport in the shelf zone of sea
That process is intensified nearer to the shore because nonlinearity of IW becomes stronger in a shoaling thermocline

Summary

Introduction

Huge energy of tides and inertial motions is dissipated in oceans’ coastal waters, but in stably stratified flows the initial stage of the energy transformation is generation of internal waves (IW) over the continental slope and shelf break. Far from shores, where the lower boundary of a thermocline is far from bottom, energy of IW serves to change mean density structure and form vertical fine structure These phenomena were explained as a result of internal wave-induced mixing within the thermocline that can be effective mechanism for generation of multi-layered structures (vertical fine structure) in a coastal ocean [2,3]. The changed vertical structure parametrically changes IW properties—dispersion relations, lengths, phase and group velocities, and energy spectrum These nonlinear transformations can go on practically without IW breaking till the zones, where thermocline begins to feel bottom. Our main goals are to see how and where waves become “not waves” and what phenomena accompany that process in the near-bottom thermocline

Instrumentation and Observations

Results and Discussion

Conclusions