Variability of the chemical hydrography at the frontal region between the East China Sea and the Kuroshio north-east of Taiwan

Abstract

The hydrography across the frontal region between the East China Sea and the Okinawa Trough north-east of Taiwan, observed during the summer of 1985 and 1988 and the early spring of 1987, was governed mainly by mixing across the front and the topographically induced upwelling of the modified Kuroshio water in the Okinawa Trough during the periodic shelfward migration of the Kuroshio. The location of the front relative to the shelf break seemed to be temporally variable. Topographically induced upwelling was evident during the summer of 1988 and the early spring of 1987 when the front was located close to the shelf break. It might not have occurred in the summer of 1985 when the front was further offshore. The end-member composition of the upwelling water was similar in both seasons. It originated from about 300 m with a temperature and salinity of 13 °C and 34·4 psu. It was rich in nutrients and poor in oxygen with concentrations of nitrate, phosphate, silicate and oxygen of 16, 1, 18 and 160 μM respectively. This upwelling water is potentially a major source of nutrients to the East China Sea. The deep water in the Okinawa Trough at temperatures below 15 C did not participate in cross-shelf mixing. Its chemical characteristics did not change significantly from year to year.