6.2 Cross-estuary circulations

Figure 16 shows the tidally averaged cross-estuary velocity, salinity, and vertical viscosity for the spring and neap tides across the Lantau Channel. The freshwater outlets are mainly located on the west side of the PRE, which helps to build up a density gradient in the cross-estuary direction, and therefore, a clockwise gravitational circulation (eastward current in the surface and westward current in the bottom) can be formed west of the Lantau Channel as shown in Figure 16a,b (looking into the estuary). The clockwise circulation is stronger in the neap tide than that in the spring tide. Due to the weaker westward current on the east side of the estuary, the eastward current in the surface may extend farther eastward in the spring tide (Figure 16a vs. Figure 16b). On the other hand, during the neap tide, there is a convergence in the surface layer located on the east side of Lantau Island, and the stratification is weak on the west side of estuary due to that the well-mixed freshwater reduces the vertical salinity difference (Figure 16c, d). However, in the Lantau Channel, the vertical salinity difference is greater in the neap tide, revealing that the stratification during the neap tide is stronger than that during the spring (Figure 16c vs. Figure 16d). The vertical viscosity is higher during the spring tide than that during the neap tide, especially on the west side of estuary with shallow

Figure 16.

Tidally averaged cross-estuary circulation (m s<sup>1</sup> ) (a, b), salinity (pus) (c, d), logarithm of vertical viscosity [log(m<sup>2</sup> s 1 )] (e, f) during spring (left) and neap (right) tide without wind forcing.

water, indicating that the turbulent mixing is more energetic during the spring tide and the circulation is controlled by the tidal mixing over the shoal (Figure 16e vs. Figure 16f).
