Observational evidence for tidal straining over a sloping continental shelf

Straining of a horizontal density gradient by tidal currents acts to periodically produce and destroy near-bottom stratification, which has been shown to modulate turbulence in the bottom boundary layer (BBL). Previous observations of such periodic variations have been limited to the coastal ocean and estuaries, where horizontal density gradients are maintained by river runoff or differential heating. In the present study, we show evidence for the existence of tidal straining over the continental shelf, outside any regions of freshwater influence, where horizontal density gradients are likely to result from the projection of the interior vertical stratification onto sloping topography. Based on microstructure data obtained in the East China Sea, we demonstrate that the tidal current shear interacting with the cross-isobath density gradient results in semidiurnal switching between unstable and stable stratification in the lower part of the BBL. The cycle of turbulent dissipation is quarter-diurnal, corresponding to the semidiurnal variation of tidal current shear. In addition, a noticeable diurnal modulation in stratification as well as a significant diurnal cycle of turbulent dissipation are observed in the upper part of the BBL, where the time evolution of stratification is dominated by tidal advection, rather than tidal straining.