Strain-induced periodic stratification observed in the bottom boundary layer over the continental shelf of the East China Sea

The straining of horizontal density gradients by tidal motions acts to produce periodic stratification that modulates turbulence generated at the seabed. This strain-induced periodic stratification (SIPS) has been so far observed in regions of freshwater influence (ROFI). Here we show the existence of the SIPS in the bottom boundary layer (BBL) in the open ocean by analyzing a 37-hour series of simultaneous measurements of the dissipation rate of turbulent kinetic energy, density, and horizontal velocity over the continental shelf of the East China Sea. As observed in the ROFI, there is a clear semi-diurnal signal in the density fluctuations, whose amplitude increases toward the top of the BBL. However, a diurnal inequality in the semi-diurnal tides produces a significant diurnal cycle of stratification in the upper part of the BBL, which has not been observed in the ROFI. During the period when the upper part of the BBL becomes stratified, strong dissipation is confined to the lower part of the BBL. By contrast, during the period when the straining acts to reduce stratification, high values of dissipation extend throughout the BBL. The cycle of dissipation is therefore predominantly diurnal in the upper part of the BBL, whereas the principal variation near the seabed occurs at the quarter-diurnal frequency.