Utilizing in situ and remote sensing measurements as well as the results from a three-dimensional numerical model, this research identifies the variability and the dynamical mechanisms that lead to the formation of prominent upwelling in the steep shelf to the east of Hainan Island (EHI) and in the ambient shallow Gulf of Zhanjiang (GOZ). According to observational study, distinct cold water bands exist within 20 km to 30 km to EHI and in the GOZ. Strengthened upslope advections of cold deep waters occur around a submerged valley off EHI and around a diverging isobaths off GOZ that lead to the upwelling. A three-dimensional numerical ocean model has been utilized to investigate the upwelling processes and underlying dynamics. Forced by an idealized upwelling favorable southwesterly mons...[ Read more ]

Utilizing in situ and remote sensing measurements as well as the results from a three-dimensional numerical model, this research identifies the variability and the dynamical mechanisms that lead to the formation of prominent upwelling in the steep shelf to the east of Hainan Island (EHI) and in the ambient shallow Gulf of Zhanjiang (GOZ). According to observational study, distinct cold water bands exist within 20 km to 30 km to EHI and in the GOZ. Strengthened upslope advections of cold deep waters occur around a submerged valley off EHI and around a diverging isobaths off GOZ that lead to the upwelling. A three-dimensional numerical ocean model has been utilized to investigate the upwelling processes and underlying dynamics. Forced by an idealized upwelling favorable southwesterly monsoonal wind stress, the model well captures the observed upwelling. The coastal waters off Hainan and in the Gulf are distinctly shown by the presence of strong upslope shoreward transport over the steep shelf and along the widening shelf with concaving isobaths, respectively, as found in the measurements. The dynamic analyses show that the shoreward cross-isobath transports in both EHI and GOZ are formed mainly by cross-isobath geostrophic current as a result of a negative along–isobath pressure gradient force (PGF), and partly by the bottom frictional Ekman transport. From the depth-integrated vorticity dynamics, it is found that the sources of this negative along-isobath PGF arise from the Modified Joint Effect Baroclinicity and Relief (MJEBAR) in EHI and from the vorticity advection and the net stress curl in GOZ. The shelf current that maintains the flow-topography interaction for the formation of the negative PGF and bottom Ekman transport in both EHI and GOZ are largely geostrophic. This research also found that the bottom cross-isobath pressure gradient force (BPGF) is not a main contributor to cross-isobath PGF in a steep region like EHI, but it is in the relatively flat region like GOZ. This study provides new understanding of coastal circulation in the region.