Abstract. The South Vietnam Upwelling (SVU) develops in the South China Sea (SCS) under the influence of southwest monsoon winds. To study the role of small spatiotemporal scales on the SVU functioning and variability, a simulation was performed over 2009–2018 with a high-resolution configuration (1 km at the coast) of the SYMPHONIE model implemented over the western region of the SCS. Its capability to represent ocean dynamics and water masses from daily to interannual scales and from coastal to regional areas is quantitatively demonstrated by comparison with available satellite data and four in situ datasets. The SVU interannual variability is examined for the three development areas already known: the southern (SCU) and northern (NCU) coastal upwelling areas and the offshore upwelling area (OFU). Our high-resolution model, together with in situ observations and high-resolution satellite data, moreover shows for the first time that upwelling develops over the Sunda Shelf off the Mekong Delta (MKU). Our results confirm for the SCU and OFU and show for the MKU the role of the mean summer intensity of wind and cyclonic circulation over the offshore area in driving the interannual variability of the upwelling intensity. They further reveal that other factors contribute to SCU and OFU variability. First, the intraseasonal wind chronology strengthens (in the case of regular wind peaks occurring throughout the summer for SCU or of stronger winds in July–August for OFU) or weakens (in the case of intermittent wind peaks for SCU) the summer average upwelling intensity. Second, the mesoscale circulation influences this intensity (multiple dipole eddies and associated eastward jets developing along the coast enhance the SCU intensity). The NCU interannual variability is less driven by the regional-scale wind (with weaker monsoon favoring stronger NCU) and more by the mesoscale circulation in the NCU area: the NCU is prevented (favored) when alongshore (offshore) currents prevail.

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.

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.

Abstract. The South Vietnam Upwelling (SVU) develops in the South China Sea off the Vietnamese coast under the influence of southwest monsoon winds. A very high resolution configuration (1 km at the coast) of the SYMPHONIE model was developed over the western coastal region of the South China Sea. A simulation was performed over the period 2009–2018 to study the functioning, variability and influence of oceanic circulation and hydrology in the coastal region, in particular of the SVU. The realism of the simulation in terms of representation of ocean dynamics and water masses, from daily to interannual and coastal to regional scales, is assessed here in detail by comparison with available satellite data and 4 sets of in-situ observations. The interannual variability of the SVU is examined for its 4 main development areas: the southern (SCU) and northern (NCU) coasts, the offshore area (OFU), and the Sunda Shelf area off the Mekong Delta (MKU). For the SCU and OFU, our results confirm the driving role of the summer mean wind and the summer circulation over the offshore area in the interannual variability of the upwelling intensity. They moreover reveal the impact of the spatial and temporal organization of mesoscale ocean structures and high frequency atmospheric forcing. For the NCU, the upwelling interannual variability does not seem to be related to regional scale forcing and dynamics, but is mainly determined by coastal mesoscale structures and circulation: similar summer wind conditions can be associated with very contrasting NCU intensities, and vice versa, depending on the circulation in the NCU area. Finally, our study reveals that upwelling also develops off the Mekong Delta, with an interannual variability mostly determined by the summer wind and the wind-driven circulation over the SVU region.