Intraseasonal variability of the South Vietnam Upwelling, South China Sea: influence of atmospheric forcing and ocean intrinsic variability

Abstract. The South Vietnam Upwelling (SVU) develops off the Vietnamese coast (South China Sea) during the southwest summer monsoon over four main areas: the northern coastal upwelling (NCU), the southern coastal upwelling (SCU), the offshore upwelling (OFU) and the shelf off the Mekong River mouth (MKU). An ensemble of ten simulations with perturbed initial conditions were run with the fine-resolution SYMPHONIE model (1 km inshore) to investigate the daily to intraseasonal variability of the SVU and the influence of the ocean intrinsic variability (OIV) during the strong SVU of summer 2018.

The intraseasonal variability is similar for SCU, MKU and OFU, driven to the first order by the wind variability. MKU and SCU are induced by stable ocean dynamics (the northeastward then eastward boundary current) and have very little chaotic variability. The OIV has a stronger influence on OFU. In July, OFU mainly develops along the northern flank of the eastward jet. The influence of OIV is strongest and related to the chaotic variability of the meridional position of the jet. In August this position is stable and OFU develops mainly in the area of positive wind curl and cyclonic eddies north of the jet. The influence of OIV, weaker than in July, is related to the organization of this mesoscale circulation. NCU shows a completely different behavior from the other areas. In the heart of summer, the large-scale circulation formed by the eastward jet and eddy dipole is well established with an alongshore current preventing NCU. In early and late summer, the large-scale circulation is weaker, allowing a mesoscale circulation of strongly chaotic nature to develop in the NCU area. During those periods, the OIV influence on NCU is very strong and related to the organization of this mesoscale circulation: NCU is favored (annihilated) by offshore-oriented (alongshore) structures.