Coastal dynamics in middle-south Vietnam during a transition period of Asian monsoon, characterized by HF radar measurements and numerical modeling

Abstract

The middle southern coasts of Vietnam (11-15ºN) are featured by a large upwelling in summer which is governed by various physical processes i.e. regional circulations, monsoonal wind, and tidal rectification. Through the HF radar measurements acquired during April-May, 2019, a transition period of the Asian monsoon, three prominent patterns of local circulation have been revealed: a northern current intrusion as a result of South China Sea Western Boundary Currents (SCSWBC); a strong southeast jet occurring at ~30km from the coast; and a seaward current appearing for several days. In order to acquire a better understanding of the regional hydrodynamics, a 3D circulation model SYMPHONIE was run covering the measurement periods of HF radar and AWAC. Sea surface current (SSC) time series obtained from HF radar and AWAC were found to be strongly correlated (R=0.58, RMSE=0.10, MAE=0.07 for u- and R=0.72, RMSE=0.13, MAE=0.11 for v-components) (Fig 1). Three subdomains (nearshore, middle-range and far-range) have been identified within the radar coverage region for detailed analysis and comparison with the model. The comparison revealed that during the first 20 days, when the SE wind prevailed, the model does not seem to capture well the variability of SSC (Fig 2). However, when the wind changed to S-SW direction, the model results show a good agreement with HF radar measurements. Besides, an eddy forming near the coast on April 16 has been well represented by both model and HF radar (Fig 3). The uncertainty of modeled velocity fields can result from the uncertainty in forcing fields. Thus, the next step of our research will be to optimize the forcing field (i.e., wind) using HF radar data with an expectation to achieve better model results of SSC variability. Figure 1 Comparison of SSC time series from  HF radar (black solid line) and AWAC (red asterisk points)Figure 2 Space averaged U- and V-velocity components of SSC from SYMPHONIE model (dash line) and HF radar measurements (solid line) in three subdomains: nearshore (a, d); middle range (b, e) ; and far range (c, f). Wind vector during the measurement period (g).Figure 3 An eddy in SYMPHONIE model simulation (left)  and HF radar measurements (right) on  April 16, 2019. Color scale represents curl of surface velocity superimposed by SSC vectors. Isobaths are shown by black lines.