Observational and model studies of the circulation in the Gulf of Tonkin, South China Sea

Abstract

[1] Moored current measurements were made at one mooring site in the northern Gulf of Tonkin for about 1 year during [1988][1989]. Analyses were performed to examine characteristics and variability of tidal and subtidal flows. Rotary spectra showed two peaks at diurnal and semidiurnal periods, with higher diurnal energy. Complex demodulations of diurnal and semidiurnal tidal currents indicated that the tidal current magnitudes varied significantly with seasons: more energetic in the stratified summer than in the vertically well-mixed winter. The observed subtidal currents were highly correlated with the surface wind in winter but not in summer; challenging the conceptual summertime anticyclonic circulation pattern derived using wind-driven homogenous circulation theory. The computed currents from a global ocean model were in good agreement with the observed currents. Similar to the current observations, the model-computed flow patterns were consistent with the conceptual wind-driven circulation pattern in winter but opposite in summer. Process-oriented experiments suggest that the summertime cyclonic circulation in the northern Gulf of Tonkin forms as a result of the combination of stratified wind-driven circulation and tidal-rectified inflow from Qiongzhou Strait. The interaction between the southwest monsoon and buoyancy-driven flow from Hong River can significantly intensify the cyclonic circulation near the surface, but its contribution to the vertically averaged flow of the cyclonic circulation is limited.

Variability of the Red River Plume in the Gulf of Tonkin as Revealed by Numerical Modeling and Clustering Analysis

“…It is a meso-tidal region, dominated by diurnal constituents (K1, O1) as described for instance by Nguyen et al (2014) and Piton et al (2020). The ocean circulation of the Gulf of Tonkin (hereafter GOT) has been examined by several authors (e.g., Ding et al, 2013;Gao et al, 2014;Rogowski et al, 2019; for a recent review see Piton et al, 2021). Using model results and observational data, they all agree that the winter circulation at the gulf scale is cyclonic and driven by wind.…”

Sensitivity study on the main tidal constituents of the Gulf of Tonkin by using the frequency-domain tidal solver in T-UGOm

“…In the first parameterization, a constant profile of the speed is assumed over the whole water height, leading to quadratic bottom stress and a constant C D that depends on the Chézy coefficient C and on the acceleration due to gravity g (Dronkers, 1964) as follows:…”

Marine Environmental Regionalization for the Beibu Gulf Based on a Physical-Biological Model

“…Therefore, the environmental conditions of the gulf are likely diverse in time and space. In addition, physical conditions, such as temperature, salinity, and current, also have non-negligible impacts on the temporal and spatial distributions of environmental variables [16,[25][26][27][28]. Clustering both physical and biochemical variables using an unsupervised machine learning algorithm for the Beibu Gulf could help to define the impacted geographic range of specific dynamic and environmental conditions.…”