The summer thermohaline structure of 2011 of the southern East China Sea shelf and its implications for the intrusion of Taiwan Warm Current and Kuroshio Current

Conductivity–temperature–depth (CTD) measurements along 5 transects including 30 hydrographic stations were carried out over the continental shelf of the southern East China Sea in July 2011. The thermohaline structure of waters and its characteristics are analyzed and the influence of the Kuroshio Branch Current and the Taiwan Warm Current on the hydrography of the shelf water discussed. Results show that shallow and deep thermoclines occur extensively. The former is present within 20 m in water depth, with weak gradients but apparent intraday evolution. The latter is present over the mid and outer shelf and the Taiwan Strait. The lower boundary of deep thermocline dwells at the water depth of ～80 m over the mid and outer shelf. It has a thickness of ～10 m, with stable and strong gradients of ～0.8 ℃/m. In contrast, deep thermocline dwells at the depths between 14～30 m in the northern Taiwan Strait. Its thickness usually varies between 6～10 m, with relatively weak gradients between 0.2～0.5 ℃/m. Salt fingering is observed around the deep thermocline due to the differences in temperature and salinity between the upper and lower waters. A cold water mass is observed below deep thermoclines at the isobaths between 90～110 m, with temperature between 16.8～17.6 ℃. The gradients of deep thermocline drop to 0.2～0.5 ℃/m over the outer shelf, their strata are synchronously lifted, and their thicknesses expanded, indicating the consequence of the Kuroshio intrusion. Therefore, once the deep thermocline gradient is lower than 0.6 ℃/m coupled with expanded thickness of its stratum, the Kuroshio intrusion will be distinguished. As a sensitive proxy, weakened thermocline gradients indicate that the Kuroshio front may reach up to the 110 m isobath over the outer shelf in summer. Disappearance of deep thermocline demonstrates that the Taiwan Warm Current prevails throughout the water column over the mid shelf at the isobaths between 50～80 m. Its influence reduces gradually from south to north. The deep water in the northern Taiwan Strait has a lower mean temperature of 22.52 ℃, which is 3 ℃ much lower than that of the deep water in the mid shelf of the southern East China Sea. Such a discrepancy suggests that the deep water of the Taiwan Warm Current may be derived from the inflow of the Kuroshio Branch Current.