1. Introduction

The Pearl River Estuary (PRE) is located along the coast of Guangdong Province in China between 113°E and 115°E and 21°N and 23°N, in a trumpet-like shape with a width of 5 km at the northern (upper) end and 35 km at the southern (lower) end (Figure 1). The Pearl River freshwater flows into the northern South China Sea (NSCS) through eight inlets, named Modaomen, Humen, Hongqili, Jiaomen, Jitimen, Hengmen, Yamen, and Hutiaomen, with an annual average discharge of 10,000 m<sup>3</sup> s <sup>1</sup> [1]. The river discharge reaches a maximum of 20,000 m<sup>3</sup> s <sup>1</sup> in the wet summer and has a minimum of 3600 m<sup>3</sup> s <sup>1</sup> in the dry winter, combining the discharge of all eight inlets together. In particular, four major inlets (Humen, Jiaomen, Hongqimen, and Hengmen) located along the west shore of the PRE contribute more than half of the total discharge [2]. There are two longitudinal channels in the central region and on the eastern side, connecting the Pearl River Estuary with the NSCS. The east channel connects the coastal water through the

Figure 1. The northern South China Sea region (a) and the Pearl River estuary (b) with bathymetry contours (meter).

Urmston Road near the east bank of the PRE and the west one through the Lantau Channel along the middle estuary. Except for these channels where the water depth varies between 5 and 15 m, most of the PRE is shallow with a water depth between 2 and 10 m; outside the estuary, the water depth increases to more than 20 m, and the isobaths are approximately parallel with the coastline with a strong cross-shelf gradient on the shelf. Furthermore, the coastline is complex around the PRE and its adjacent shelf, with many small islands scattered around the estuary mouth and coastal bays. These geometric and topographic features can greatly affect the circulation in both the estuary and the coastal region.

Generally, the circulations of the PRE consist of the buoyant gravitational circulation associated with the freshwater discharge and the complicated bathymetry, the tidal currents, and the wind-driven currents [1, 3]. The PRE is a micro-tidal estuary, and the M2 and K1 constituents dominate the tidal currents, which have a 1.0 m tidal range inside the PRE [4]. The mean tidal range just outside the PRE is 0.85–0.95 m [2]. The tidal currents also can change stratification structures in the PRE and play a vital role in the vertical mixing, especially in the shallow near-shore region [5, 6]. Using the Princeton Ocean Model (POM) with curvilinear orthogonal grid to study the Pearl River plume, Wong et al. [3] found that the water column near the head of the estuary becomes well mixing owing to tidal mixing and the moderate southwesterly wind has limited effects on the circulation inside the PRE [3, 7]. The circulation in the upper estuary and northern part of the lower PRE is characterized by the seaward gravitational current. However, the southern part of the lower estuary is governed by the competition between gravitational current and geostrophic intrusive currents from the shelf [5]. Based on cruise survey data and numerical modeling of the PRE for the summer time, Pan et al. [6] revealed the role that wind and tidal forcing play in determining plume dynamical properties and concluded that the density structure in the PRE can be modulated by the wind with strong stratification under the southwesterly wind and weak stratification under the southeasterly wind [8]. Under the southeasterly wind during spring tide, an anticyclonic circulation bulge appeared, with a Kelvin number of 0.93 and a supercritical Froude number of 1.45. Under southwesterly wind during a neap tide, the plume displayed a diffusive front, with a Kelvin number of 2.9 and a Froude number of only 0.07 [8]. The turbulent mixing in PRE is governed by the tidal strength, and the wind stirring tends to increase the internal shear instability mixing in the recirculating plume bulge [6]. Observation and modeling reveal that

Circulations in the Pearl River Estuary: Observation and Modeling DOI: http://dx.doi.org/10.5772/intechopen.91058

strong mixing appeared in the bottom layer on the larger ebb, caused by the vigorous bottom stress.

The spreading of the plume in the surrounding coastal waters is controlled by the wind stress and ambient currents. The easterly or southeasterly wind drives the plume westward, and southerly or southwesterly wind forces the plume eastward [3, 8, 9]. The wind-driven coastal current closely interacts with the estuarine circulation in the lower part of the PRE [1]. Ou used the field measurement data to investigate the dynamics of the buoyant plume near the PRE, suggesting four major horizontal plume types in summer [9]. These types are offshore bulge spreading, westward alongshore spreading, eastward offshore spreading, and symmetrical alongshore spreading. In this chapter, we report detailed cruise observations on the PRE water current and the salinity structure. A numerical model of high spatial resolution is developed in the PRE, which can well simulate the circulations of the PRE.
