Tidal and Subtidal Flow along a Cross-Shelf Transect on the East China Sea

Abstract

Fourteen repetitions of a cross-shelf transect in the Kuroshio current edge in the East China Sea allowed the description of a) the semidiurnal tidal properties of the transect and b) the subduction of low salinity water at the edge of a Kuroshio meander intrusion onto the shelf. Water velocity and density profiles were obtained along the transect throughout a seven-day period of May 2000 in which warm Kuroshio waters moved onto the shelf at typical speeds of 0.1 to 0.2 m/s. Irregular lengths of the transect (maximum of ≈75 km) and the non-uniform frequency of sampling permitted elucidation only of the semidiurnal contributions to the tidal currents. The semidiurnal tidal currents in the transect exhibited typical amplitudes of 0.5 m/s, decreasing offshore. Locally, the tidal current amplitudes decreased and for the most part rotated clockwise with depth. Orientation of the tidal currents was essentially in the NW-SE direction throughout the transect and the tidal phase progressed onshore. This information contributes to the scarce observational evidence on the tidal properties of the East China Sea's continental shelf. A Kuroshio meander was identified by the gradual increase of Sea Surface Temperatures on the outer shelf of the East China Sea during the period of observation. The meander exhibited strong vertical shears and flow convergences that allowed subduction of low salinity waters at its edge. Ageostrophic traits in the meander were denoted by the development of a) cross-shelf flows and b) marked discrepancies between geostrophically derived along-shelf flow estimates and detided along-shelf flows. Similarities between geostrophic estimates and detided along-shelf flows during the first two days of observations provide confidence in the effective separation of tidal and non-tidal signals with the method employed.