Field-observation for an anticyclonic mesoscale eddy consisted of twelve gliders and sixty-two expendable probes in the northern South China Sea during summer 2017

An intensive field observation experiment using 12 Chinese gliders equipped with conductivity-temperature-depth (CTD) sensors and 62 expendable CTD probes (XCTDs) was performed to investigate the 3-D structure and time evolution of an anticyclonic eddy in the northern South China Sea (NSCS). The observed results showed that the anticyclonic eddy had a horizontal radius of about 80 km at surface and a vertical depth of impact of more than 1000 m. The largest temperature and salinity anomalies compared with the averaged values of the temperature and salinity profiles were 3.5°C and 0.4 psu at 120 m depth, respectively. Combined analysis of altimeter sea level and water mass properties indicated that the anticyclonic eddy was shed from the Kuroshio loop current. The vertical axis of the anticyclonic eddy tilted from surface to the observed maximum depth (1000 m) along its translation direction against the 2000 m isobath. The center of the anticyclonic eddy remained in the region east of Dongsha Island for more than half a month. During this time, the long axis direction of the eddy changed from across the slope to along the slope. Then, the eddy moved southward along the 2000 m isobaths. Both the geostrophic current and temperature distribution revealed that the eddy intensity weakened during the observation period gradually. These observations indicated strong interaction between the anticyclonic eddy and the slope topography of Dongsha Island.