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, SCS) 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 10 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 the SCU, MKU and OFU, driven to the first order by the wind variability. The 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 the 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 the OIV, weaker than in July, is related to the organization of this mesoscale circulation. The NCU shows a behavior different from that observed in the other areas. In the heart of summer, a large-scale circulation formed by the eastward jet and eddy dipole is well established with an alongshore current preventing the NCU development. In early and late summer, this circulation is weaker, allowing a mesoscale circulation of strongly chaotic nature to develop in the NCU area. During those periods, the OIV influence on the NCU is very strong and related to the organization of this mesoscale circulation: the NCU is favored (annihilated) by offshore-oriented (alongshore) structures.

Spurious numerical mixing under strong tidal forcing: a case study in the South East Asian Seas using the Symphonie model (v3.1.2)

“…The model employed in this study is the Symphonie ocean model (Damien et al, 2017), a three dimensional ocean circulation model solving the primitive equations under Boussinesq and hydrostatic equation. Symphonie has been used in the past over Southeast Asia at several spatial scales, from the Gulf of Tonkin shelf scale (Piton et al, 2021;Nguyen-Duy et al, 2021) and South Vietnam upwelling coastal scale (To Duy et al, 2022;Herrmann et al, 2023) to the South China Sea regional scale (Trinh et al, 2024). Turbulent closure is achieved through the k − ϵ scheme (Burchard and Bolding, 2001).…”

Modulations of the South China Sea Ocean Circulation by the Summer Monsoon Intraseasonal Oscillation Inferred from Satellite Observations

“…Intraseasonal signals of the sea surface temperature (SST) [15], the mixed-layer and upper-ocean temperatures [16], the latent heat flux [17], the sea surface height (SSH) [18], the western boundary current [19], the crossslope flow [20], the subsurface thermocline [21,22], the deep ocean temperature [23], the energy budget [24], and the deep current [25] in the SCS have been widely reported on. As for the summer upwelling area in the western SCS (Figure 1a), the related SST cooling and phytoplankton bloom often experience several intraseasonal events each summer [26][27][28][29][30]. It should be noted that the oceanic intraseasonal signals are not just passive responses to the atmospheric forcing.…”

Mechanisms and intraseasonal variability of the South Vietnam Upwelling, South China Sea: role of circulation, tides and rivers

“…Isoguchi and Kawamura (2006) and Xie et al (2007) showed that wind, itself strongly driven by Madden Jullian Oscillation at the intraseasonal summer scale, but also by tropical storms, is a major driver of SVU daily variability. Herrmann et al (2023) more specifically showed that wind is the main driver of daily variability of SEJ and of SCU, OFU and MKU. They showed that NCU variability was rather driven by the strength of the large scale circulation at the intraseasonal scale: the well established circulation and southward boundary current during the core of summer prevent the development along the northern coast of seaward currents, hence of NCU.…”

“…Recent studies showed that SVU develops over four main areas (highlighted in Fig. 1), driven by different mechanisms (Da et al, 2019;Ngo and Hsin, 2021;To Duy et al, 2022;Herrmann et al, 2023). The northeastward and southward boundary currents converge near ∼ 14°E, forming the summertime eastward jet (SEJ).…”

“…Beside those Ekman transport-driven coastal upwellings, an Ekman pumping-driven offshore upwelling (OFU) develops north of the SEJ. Last, To Duy et al (2022) and Herrmann et al (2023) revealed the existence of an upwelling that develops over the Mekong shelf (MKU), whose physical mechanisms have yet to be explained.…”