Three-dimensional circulation in northern South China Sea during early summer of 2015

Using the hydrographic data obtained during two nearly simultaneous surveys in June 2015, we carried out semi-diagnostic calculations with the help of a finite element model and a modified inverse method, to study the circulation in the northern South China Sea (NSCS) during the early summer of 2015. A number of new circulation features were found. (1) In most of the observation region, a large, basin-scale anticyclonic gyre appeared south of the 50-m isobath, which contained anticyclonic eddies. One anticyclonic eddy existed from the sea surface to 50-m depth, whose center showed no tilt, while the center of another eddy tilted eastward from the sea surface to 500-m depth. In the eastern part of the observation region, which is west of the Dongsha Islands, there was a sub-basin-scale cyclonic gyre containing a cyclonic eddy whose center tilted southward from the sea surface to 200-m depth. (2) There was a cross-continental slope current (CCSC) in the area southwest of the Dongsha Islands. Its volume transport was about 2.0×106 m3/s. (3) From the estimated order of magnitude of the stream function equation, the joint effect term of the baroclinity and relief (JEBAR) and β-effect term are two important dynamic mechanisms affecting the variation of the circulation in the NSCS. (4) The JEBAR, as a transport-generating term, resulted in the dynamic mechanism determining the pattern of the depth-averaged flow across the contours of potential vorticity fH−1. Furthermore, we show that the negative values of the JEBAR were the most dominant dynamic mechanism, causing the CCSC southwest of the Dongsha Islands to deflect from the isobaths and veer toward the deep water. The CCSC around the Dongsha Islands was located further southwest during the early summer of 2015 than during the fall of 2005 (revealed by a published study), which suggests that the location of the CCSC around the Dongsha Islands may vary with season.