6.1 Along-estuary circulations

Figure 15 illustrates the tidally averaged along-estuary velocity, salinity, and vertical viscosity for the spring and neap tides along the Lantau Channel section. A prominent two-layer exchange flow is shown, with seaward current in the surface and landward current in the bottom (Figure 15a, b). The maximum surface seaward current speed is 0.37 m s<sup>1</sup> during the neap tide about 42% stronger than that (0.26 m s<sup>1</sup> ) during the spring tide in the lower estuary. The bottom landward current is weaker in the spring tide than that in the neap tide. This suggests that the exchange circulation is stronger in the neap tide than that in the spring tide. However, although the bottom landward current is weaker in the spring tide, it may extend farther landward toward the estuary head (Figure 15a vs. Figure 15b).

Vertical salinity difference of 2–4 psu appears in the estuary during the spring, whereas it is 4–10 psu during the neap tide, revealing that the stratification during the neap tide is stronger than that during the spring (Figure 15c vs. Figure 15d). The maximum stratification is located in the middle of the estuary, and the wellmixed freshwater appears in the upper estuary from 22.5°N during both the spring and the neap tide. It is obvious that the surface salinity is higher in the spring tide

#### Figure 14.

The Pearl River estuary with bathymetry contours (m). The along-estuary and cross-estuary sections are shown in blue lines. The intersection is marked as A.

Figure 15.

Tidally averaged along-estuary circulation (m s<sup>1</sup> ) (a, b), salinity (pus) (c, d), logarithm of vertical viscosity [log(m2 s 1 )] (e, f) during spring tide (left) and neap tide (right) without wind forcing.

than that during the neap tide; however, in the bottom, the salinity intrusion during the neap tide is stronger than that during the spring tide. Without the wind forcing, the estuarine exchange flow pattern is more controlled by the mixing inside the estuary.

As shown in Figure 15e and f, the bottom vertical viscosity is higher than that on the surface both during the spring and neap tides, suggesting that the mixing is generated by the bottom friction that applies to the tidal current inside the PRE [6]. The vertical viscosity is higher during the spring tide than that during the neap tide, indicating that the turbulent mixing is more energetic during the spring tide. The averaged viscosity over the along-channel section is 0.0041 m<sup>2</sup> s <sup>1</sup> during the spring and is 0.0026 m<sup>2</sup> s <sup>1</sup> during the neap tide. The PRE shows a classical pattern of the spring-neap cycle of the along-estuary circulation which is mainly controlled by the mixing inside the estuary. The higher mixing dissipates more kinetic energy of the residual current, resulting in a reduced exchange flow in the PRE. Although the horizontal density gradient is stronger during the spring tide, the vertical diffusion

may overwhelm the forcing aroused by the density gradient, leading to the weaker longitudinal circulation on the spring tide than on the neap tide.
