Abstract:
Summer monsoon southwest wind induces the
South Vietnam Upwelling (SVU) over four main areas along
the southern and central Vietnamese coast: upwelling offshore of 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 intraseasonal to interannual variability in the SVU. The present study complements these results by examining the influence of tides and river discharges
and investigates 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 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 strengthen this 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
and hence the resulting surface convergence and divergence
and the associated downwelling and upwelling. Second, they
reinforce the surface cooling upstream and downstream of
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. Tides do not significantly influence OFU and
NCU intensity. Mekong waters slightly weaken MKU (by
9 % of the annual average) by strengthening the stratification
but do not significantly influence OFU, NCU, and SCU.
Last, tides and rivers do not modify the chronology of
upwelling in the four areas.