Daily variability of upwellings ? The case study of South Vietnam upwelling (South China Sea)

The South Vietnam upwelling (SVU) is one of the major processes involved in the South China Sea (SCS) ocean dynamics and planktonic ecosystem. Several numerical modelling studies examined its variability, revealing the leading role of wind and ENSO, but also, more recently, of ocean intrinsic variability (OIV) related to chaotic eddies. However, the spatial resolution of the models used in these studies did not allow to fully consider and understand the role of small scale dynamics. Our objective is therefore to implement a very high resolution model over the SCS in order to investigate the contribution of fine scale dynamics to the daily to interannual variability of the SVU.

We developed a configuration of the SYMPHONIE regional ocean model, using a curvilinear orthogonal grid over most of the SCS with a horizontal resolution increasing linearly from ~1.0 km along the Vietnamese coast to ~4.5 km offshore, and 50 layers. The surface forcing is prescribed using the 3-hourly output of the ECMWF, tidal forcing by FES2014, the initial and lateral ocean boundary conditions by the daily outputs of the global ocean 1/12° COPERNICUS analysis; monthly climatology and daily of freshwater river runoff are used for the 35 main rivers of the modeled domain.

We first evaluate the realism of the model by comparing a simulation performed over the period 2008-19 with in-situ measurements and satellite data. This multiannual simulation moreover confirms the leading role of wind on the daily to interannual variability of upwelling that develops in the coastal and offshore region. It also suggests, as already demonstrated by Da et al. (2019) at the interannual scale, that other processes are involved in this development. We thus explore the impact of ocean intrinsic variability, tides and rivers at the daily scale on the upwelling development by studying in details the intense upwelling that develops during summer 2018. For that, we perform several sensitivity experiments including ensemble simulations with perturbated initial conditions. We will present a synthesis of the results that reveal the strong impact of ocean background mesoscale circulation on the upwelling intensity at the daily scale, and its evolution during the summer.