Turbulence in the East China Sea: The summertime stratification

Abstract

A series of 134 microstructure profiles were taken in the central East China Sea from a drifting ship, covering more than 12 miles over the sloping bottom. The water depth z varied in the range 52-62 m; the tidal elevation of semidiurnal component was ±1 m. Prior to the onset of 25 h drift, a hydrographic section consisting of 9 stations, 15 miles apart, was taken in the area. A shallow mixed surface layer (SL) underlain by a diurnal pycnocline (z < 13 m), a sharp thermohalocline (13 < z < 30 m), and about 20 m tall bottom boundary layer (BBL) were the main features of summertime stratification. The splitting of upper pycnocline by a warmer, saltier intrusion led to the generation of turbulent patches at the lower, but not at the upper, density interface of this water body, suggesting asymmetry of mixing across the intrusion. In a limited range of the normalized squared buoyancy frequency, 2 <6.3, the turbulent kinetic energy dissipation rate ε decreased sharply as ε/εm=(N2/N02)-3, where εm=4×10-6 W/kg and N0=5×10-3 s-1. Measurements in the sharp pycnocline and in lesser-stratified transitional layers between the pycnocline and the SL and BBL show that the cumulative distribution functions CDF(log10ε) can be modeled either by the generalized extreme value (GEV) distribution or the lognormal distribution.