Seasonal variation and dynamics of the Pearl River plume

Abstract The Pearl River plume (PRE) was highly stratified during the wet season, with a strong halocline inside the estuary. In the surface layer a plume was formed just outside the estuary. In the bottom layer a front in the axial direction was found extending from the estuary to the adjacent water, following the 6-7 m isobaths. During the dry season, vertically homogeneous high salinity coastal water occupied the eastern part of the estuary. A salinity front was found along the 5–6 m isobaths parallel to the western shoreline, similar to the bottom front found during the wet season. The circulation was different on the two sides of the salinity front during both wet and dry seasons. On the east side of the bottom salinity front, there was a two-layer gravitational circulation pattern during the wet season and it was unidirectional (seaward) in the dry season. On the west side of the salinity front, residual currents were seaward in the surface layer and westward in bottom layer during both wet and dry seasons, like the buoyant-driven coastal currents. Inside the PRE, the mean sea level slope, the slope of the halocline (in the case of summer) and the salinity gradient were all found to be larger in the cross-estuary direction than those in the along-estuary direction, during both the wet and dry seasons. Momentum balance analyses have shown that the estuary was nearly in geostrophic balance in the cross-estuary direction, indicating coastal front characteristics for the plume inside the mouth of the estuary. Because of the relatively low river runoff and relatively small tides for the PRE with such a large area, the river discharge has completed the transition from a river plume to the coastal front in the UPRE. Normally, for a typical estuary, such a transition would take place outside the mouth of the estuary. After exiting from the mouth of the estuary, the PRE plume turned towards the west during the dry season, as expected from the gravitational adjustment requirement. During the wet season, however, it turned to the east upon exiting from the PRE due to the southwesterly monsoon.