Dynamics of a Coastal Upwelling and the Pearl River Plume in Guangdong Coastal Waters

The upwelling event that occurred in Guangdong coastal water during 14-16 July, 2003 is observed by using satellite multi-sensor data including the Moderate Resolution Imaging Spectroradiometer (MODIS) sea surface temperature (SST) and QuikSCAT ocean surface winds. Successive MODIS SST images reveal a jet-like upwelling cold water body in surface layer under the forcing of southwesterly winds. The ROMS is used to simulate the upwelling process and explore its dynamics. The model successfully reproduces the jet-like shape of the surface upwelling water as well as the upwelling-developing process by comparisons of 28 °C isotherms between the modeling and MODIS SSTs. Analyses of modeled momentums reveal that the large offshore transport appeared on the west side of Honghai Bay as results of high alongshore pressure gradient and nonlinear advections, and in addition to the offshore-ward Ekman transport generated by the southwesterly winds, the enhanced horizontal advection also played an important role in developing the prominent upwelling in Honghai Bay.;As testified by a numerical experiment, it is the wind-driven upwelling not the wind-induced vertical turbulent mixing that induced the surface cold water. Further numerical analyses reveal strong internal tides occurring in Honghai Bay caused by the local bottom topography. The interaction between the upwelling and internal tides enhances the bottom water uplifting. The offshore expansion of the upwelling water is controlled by the cross-shore topography slope: a gentle and offshore-extended slope helps the bottom water to climb up to the surface in a wide range in cross-shore direction, whereas a steep and narrow slope restricts the expansion of the upwelling water and confines the cold water in a narrow band along the shore. A sea cruise was carried out to capture Pearl River plume structure in the Pearl River Estuary (PRE) and its adjacent coastal waters from 4 June to 14 June, 2012. The cruise data are analyzed to unveil the plume dynamics. A nested model is used to simulate the plume expansion process as well. Model results are compared with cruise observations and tidal gauge sea level data. Modeling results suggest that there is a sub-tidal, anti-cyclonic bulge on the west side out of the river mouth under southeasterly winds, which constitutes a plume re-circulating. When the wind changes to the southwesterly, however, the plume re-circulating vanishes and a plume far-field appears.;The distinct, supercritical plume front occurs with southeasterly winds prevailing in spring tide. The tidal salt deficit flux can reach as high as 12.5% of the mean current flux, and indicates an interaction between the wind forcing and tides. The variation of plume stratification is studied by a scalar parameter. It is found that the stratification of the plume is sensitive to the wind forcing: The southeasterly winds can enhance vertical mixing in the whole water column and restrict seaward expansion of the plume water. Under the southwesterly winds, the strong stratification acts as a barrier separating wind-induced surface vertical mixing and bottom mixing. The plume water in the surface layer maintains its stratification and spreads horizontally.