Enhanced mixing induced by internal solitary waves in the South China Sea

In this paper, based on the limited observational data of the internal solitary waves (ISWs) near the Dongsha Islands in the northern South China Sea (SCS), the temporal and spatial enhanced mixing during the passage of ISWs is discussed. It is found that, when ISWs pass, the mixing rate increases largely. The variation of the time-averaged vertical current shear agrees well with the KdV equation theory, indicating a minimum shear near the background main thermocline. In the upper layer, when the ISWs arrive, the maximum turbulent dissipation rate and the diapycnal mixing rate can increase by three orders of magnitude; after the passage of ISWs, the mixing reduces sharply to a normal magnitude as that before the arrival of the ISWs. In the upper layer, two types of the instantaneous enhanced mixing versus depth are categorized, one type has several enhanced mixing peaks at different depths when the ISW is about to arrive or to leave, and the other type has only one enhanced mixing peak. In general, the maximum instantaneous enhanced mixing occurs at depths about 10–50 m above the disturbed thermocline. The mixing rate increases largely when the gradient Richardson number is less than 1/4, indicating that the shear instability might be a vital mechanism to the local mixing; and it is interesting to find that, the enhanced mixing occurs when the Froude number approximately equals critical value of unity.