Rapid alternating vertical temperature gradients in the East China Sea

Abstract Data from an array of current meters, thermistor chains, and meteorologically instrumented buoys deployed in the shallow water (depths of 97 to 115 m) of the East China Sea during AMTEX-75 (February 1985) show that the vertical temperature difference between the 20 and 70 m depths regularly alternated between∼0°C and a maximum of 0.65°C. The periodicity of change was predominantly that of the semidiurnal tide, 12.4 h, with some modulation at diurnal or near diurnal periodicity. The vertical gradient developed most often when currents were in a northwest direction, a direction parallel to the average northwest-southeast horizontal surface water temperature gradient; when the currents reversed 180°, to the southeast, the vertical gradients tended to decrease or disappear altogether. An explanation is given in terms of vertical shear in the semidiurnal tidal currents coupled with the slightly higher current speeds during the offshelf phase (southeast direction) of the current ellipse.