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

Abstract

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.

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

“…The code for the SYM-PHONIE model can be downloaded freely from https: //sirocco.obs-mip.fr/ocean-models/s-model/download/ and is presented in Marsaleix et al (2008). Sea surface temperature, currents and wind stress simulated by the ensemble over the summer of 2018 are freely available on https://doi.org/10.5281/zenodo.7504087 (Herrmann et al, 2023).…”

Future Change in the Vietnam Upwelling Under a High‐Emission Scenario

“…For example, in the South China Sea, strong upwelling occurs in the coastal regions, such as at the northeastern shelf (Chen et al., 2017; Gan et al., 2009, 2015; Hu & Wang, 2016), along the coast of Hainan Island (Lin et al., 2016; Xie et al., 2017), and in the coastal and offshore regions off Vietnam (Chen et al., 2012; Da et al., 2019; Duy et al., 2022). Many previous studies investigated the dynamics, interannual variabilities, and biochemical impacts of the upwelling around the South China Sea (Da et al., 2019; Duy et al., 2022; Herrmann et al., 2023; Liu et al., 2023; Ngo & Hsin, 2021). By and large, much less attention has been paid to its response to the warming climate.…”