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

Summer monsoon southwest wind induces the South Vietnam Upwelling (SVU) over four main areas along the southern and central Vietnamese coast: offshore the Mekong shelf (MKU), along the Southern and Northern coasts (SCU and NCU) and offshore (OFU). Previous studies have highlighted the roles of wind and Ocean Intrinsic Variability (OIV) in the SVU intraseasonal to interannual variability. The present study complements these results by examining the influence of tides and river discharges and investigating the physical mechanisms involved in MKU functioning.

MKU is driven by non chaotic processes, explaining its negligible intrinsic variability. It is triggered first by the interactions of currents over a marked topography. The surface convergence of currents over the southwestern slope of the Mekong shelf induces a downwelling of the warm northeastward alongshore current. It flows over the shelf and encounters a cold northwestward bottom current when reaching the northeastern slope. The associated bottom convergence and surface divergence lead to an upwelling of cold water which is entrained further north by the surface alongshore current.

Tides and rivers do not modify the chronology of upwelling for the four areas. Tides do not significantly influence OFU and NCU intensity. They strengthen the circulation-topography-induced MKU through two processes. First, tidal currents weaken the current over the shallow coastal shelf by enhancing the bottom friction. This increases the horizontal velocity gradient hence the resulting surface convergence and divergence and the associated downwelling and upwelling. Second, they reinforce the surface cooling upstream and downstream the shelf through lateral and vertical tidal mixing. This tidal reinforcement explains 72 % of MKU intensity on average over the summer, and is partly transmitted to SCU through advection. River discharges do not significantly influence OFU, NCU and SCU intensity. Mekong waters slightly weaken MKU (by 9 % on the annual average) by strengthening the stratification.