A Preliminary Study Of Infragravity Wave Mixing In The South Of The Bohai Sea

An introduction to history and present situation of researches on ocean infragravity waves is made in this paper. Infragravity waves are low frequency waves with frequency between 0.005 and 0.05Hz, which are generally weak in the deep ocean but can be very energetic in the shallow water. Infragravity energy levels are highly associated with swell-sea energy and depended on the water depth and the general geographic surroundings. Infragravity motions could be separated into two categories, a bound or forced part, and a free part. Bound infragravity waves are forced by wave-wave nonlinear interaction of two wind waves, depended only on the swell energy and water depth. The observed total infragravity levels are between 3 and 1500 times higher than the forced infragravity levels, indicating that the infragravity band was often dominated by free, uncoupled motions, but the forced waves also contributes a significant fraction of the total infragravity energy with high energy swell or in very shallow water. Free infragravity waves are of two main forms: The edge waves, obliquely incident, and refractively trapped on the shore or on the shelf, and the shore normal leaky wave.In terms of the observed data and by using the k-εmodel of turbulence, we calculate the dissipation ratesεof turbulent kinetic energy produced by infragravity waves and tidal flow respectively in the south of the Bohai Sea, and analyse and compare the results ofε. The turbulent mixing due to infragravity waves is dominating, and the dissipation rateεof infragravity waves is higher than that of tidal flow about 2 orders. This result prove that infragravity waves play a key role in the interior mixing in the south of the Bohai Sea. Among 4 tidal durations the differences of values and vertical distributions of the dissipation ratesεare obvious. In the high tide duration and the low tide duration the vertical averages ofεare higher than those in the flood tide duration and the ebb tide duration over 1 order. And the dissipation rateεin the low tide duration is maximal.