Mechanism Analysis of the Strong Coastal Current Zone and Abrupt Strong–Current Phenomena in Spring and Summer in the Yangjiang Sea Area of Western Guangdong in the Northwest of the South China Sea

Abstract

Residual current analysis of multiple stations’ periodic observational data for sea currents, and multiple voyages, multiple seasons from 2018 to 2022, revealed the existence of a strong southwest current zone, 20–30 m underwater and within the coastal current area of Yangjiang, western Guangdong, in the northwest region of the South China Sea. The velocity of the residual currents in the strong–current zone was 38.2–100.3 cm/s. Observational data for wind, sea currents, salinity and tide from multiple coastal stations in the spring (from March 2019 to May 2019) and summer (from June 2019 to August 2019) of 2019 demonstrated the existence of abrupt strong currents in the coastal current sea area of Yangjiang, western Guangdong. Analyses of continuous sequence hydro meteorological data for the Yangjiang area indicated that wind stress was the main factor determining the direction of sea currents; increases in the near-shore water level produce a westward geostrophic current and a wind current in the shallow sea area; their joint effect was the key factor determining the velocity of the coastal current in this sea area. In spring and summer, when Pearl River runoff into the sea reaches a peak, and under the action of the northeast wind, the water level on the west coast of Guangdong rose. This created a barotropic pressure–gradient force from the shore to the outer sea, generating a southwestward geostrophic current in the same direction as the wind-driven current. The joint action of the wind-driven and geostrophic currents then generated a sudden southwestward coastal current.

1. Introduction

The Yangjiang sea area is located in the coastal area of Western Guangdong in the northwest of the South China Sea, with a geographical location of 111°10’ E~112°10’ E, 20°30’ N~21°40’ N, and a water depth range of 10~50 m (Figure 1). “Coastal current” refers to the shallow sea current, the main part of which is located on the continental shelf and moves along the coast [1]. The coastal current in Western Guangdong is affected by complex environmental factors such as monsoon forcing, topographic action, local sea-surface buoyancy forcing and tidal action, and has long been the focus of South China Sea circulation research [2]. Wu (1990) concluded that the coastal currents of Western Guangdong are formed by the geostrophic effect and easterly flow [3]. Ying (1999) suggested that the coastal currents of Western Guangdong remain westward all year round [4]. However, Yang et al. (2003) found that upper currents exist in the continental shelf area in the north of the South China Sea: in summer, as the temperature of the near-shore water body increases, the coastal runoff flows into the sea, causing the near-shore sea level to increase and the off-coast sea level to decrease. The off-coast gradient force of stress results in the near-shore seawater moving westward. Moreover, in summer, the rainfall in the southern area of China increases, and the diluted water of the Pearl River moves southwestward, which is another reason for the formation of the coastal currents in western Guangdong province [5]. Based on wind circulation theory, Yan et al. (2005) believe that the northwest coastal waters of the South China Sea are mainly westerly currents; however, when the southwest monsoon is strong, the northwest coastal waters of the South China Sea appear to be northeasterly currents. The currents in the northwest coastal area of the South China Sea are mainly affected by meteorological conditions, having a large magnitude but not ideal periodicity [6]. Xie et al. (2012) considered that, influenced by the cyclonic circulation of outer sea waters in summer, the coastal currents of Western Guangdong travel westward throughout the year [7]. Using a numerical simulation of the Pearl River Estuary area, Wong et al. (2003) further supported the premise that the freshwater of the Pearl River is the dominant factor in the formation of the coastal current in western Guangdong in summer [8]. In addition, several studies have indicated that the existence of cyclonic circulation in western Guangdong is the main influencing factor on its coastal current in summer [9]. In general, scholars believe that there is a typical coastal current in western Guangdong, in the northwest region of the South China Sea, but they have different views on the flow frequency and formation mechanism of the westward coastal current [10].

Figure 1. Coastal continuous hydrology climate observation stations (1#, 2#, 3#, 4#, 5# and 6# were continuous observation stations, the observation factors and time period for each continuous station were shown in Table 1).

Figure 1. Coastal continuous hydrology climate observation stations (1#, 2#, 3#, 4#, 5# and 6# were continuous observation stations, the observation factors and time period for each continuous station were shown in Table 1).

Rainfall around the South China Sea is high, with an annual rainfall of more than 2000 mm. The runoff into the sea affects coastal sea levels [11]. The northern sea region of the South China Sea is controlled by the East Asian monsoons, with obvious seasonal changes. The southwest monsoon prevails in summer and the northeast monsoon prevails in winter [12]. The coastal current is a seasonal current and responds quickly to the monsoon field [13]. In the gale process, any wind drift near the 10 m isobath normally disappears within 24 h; wind drift near the 50 m isobath is not normally observed after 3 days [14]. The direction and amplitude of the coastal current in western Guangdong are unstable [15]. In addition, the strong–current zone along the northwest coast of the South China Sea and the sudden strong–current phenomenon in spring and summer have not been studied by scholars. Field survey data verification is often used in Marine Science research [16]. Due to the lack of sufficient observational data to support research, the local phenomena and mechanisms of the coastal currents in western Guangdong still require investigation [17].

The Yangjiang shallow sea area is located in a typical sea area in the southwest region of the South China Sea. The temporal and spatial variation features of its sea currents and its dynamic mechanism have an important significance when researching the coastal currents of the western Guangdong area [18]. This work adopts one-day observational data for currents from multiple voyages and near-shore continuous hydro meteorology observational data from multiple stations, emphasizing the detailed analyses of the features of the coastal strong–current zone, and the abrupt strong–current phenomenon of spring and summer and its mechanisms.

Table 1. The observation factors and time period for each continuous observation station.

Name of the Station	Observation Factors	Observation Time
Station 1	sea current, water level	1 October 2018~30 September 2019
Station 2	salinity, water level	1 March 2019~31 August 2019
Station 3	salinity	1 March 2019~31 August 2019
Station 4	wind, salinity, water level	1 March 2019~31 August 2019
Station 5	salinity, water level	1 March 2019~31 August 2019
Station 6	salinity, water level	1 March 2019~31 August 2019

Table 1. The observation factors and time period for each continuous observation station.

Name of the Station	Observation Factors	Observation Time
Station 1	sea current, water level	1 October 2018~30 September 2019
Station 2	salinity, water level	1 March 2019~31 August 2019
Station 3	salinity	1 March 2019~31 August 2019
Station 4	wind, salinity, water level	1 March 2019~31 August 2019
Station 5	salinity, water level	1 March 2019~31 August 2019
Station 6	salinity, water level	1 March 2019~31 August 2019

2. Data Resource

2.1. Observation of One-Day Period Sea Currents in Multiple Voyages

Sea current observations, taken over a one-day period, can reflect the moving status of the water body in the sea area during a tidal period. Continuous observations of fixed-point sea currents, for a one-day period, were conducted on multiple voyages; the observation time, number of stations, and range of water level at each station are listed in

Table 2. Observation status of sea currents for one-day periods on multiple voyages.

Voyage	Observation Date	Number of Stations	Range of Water Level
1	4:00 on 14 July 2018~5:00 on 15 July 2018	10	22 m to 29 m
2	7:00 on 5 December 2018~8:00 on 6 December 2018	9	23 m to 34 m
3	6:00 on 26 April 2020~7:00 on 27 April 2020	10	8 m to 45 m
4	13:00 on 12 August 2022~14:00 on 13 August 2022	10	6 m to 28 m

below. Each station was at a different water level and distance from the coast, reflecting the differences in sea current and the seawater transport features in the sea area under consideration. Each station used a Seaguard RCM current meter for current observations over a 26-h period, as shown in Table 2. The Seaguard RCM current meter is a self-contained current meter, and its battery capacity can meet the needs of voyage observations. The six-point method was used for observations, meaning that the water level was divided into six sections, i.e., the surface, 0.2 H, 0.4 H, 0.6 H, 0.8 H, and the bottom layer. (“H” indicates the actual water level in the observation; the surface layer is 2.0 m below the water surface after removing the instrument observation blind area; 0.2 H layer means that H indicates the water level multiplied by 0.2, producing the result of the water level under the sea surface. The calculation is repeated for the 0.4 H, 0.6 H, and 0.8 H layers. The bottom layer indicates the water level 1.0 m above the seabed.) The sea surface current represents the interaction between the sea and air; the seabed current represents the friction effect at the seabed; and other intermediate currents reflect the transition between the surface and bottom layers. The observation elements of the Seaguard RCM current meter are flow rate, flow direction, pressure, and water depth. In order to continuously observe the current meter during the whole voyage, the water depth of each layer was observed for 3 min per hour. The sampling frequency of the current meter was set to 10 s, and the time interval was set to 1 min; that is, the data collected every 10 s were averaged over 1 min, and a group of output data was obtained every 1 min.

2.2. Coastal Continuous Hydrology Climate Observation

Data for continuous actual wind measurements, sea currents, salinity, and tide level from multiple near-shore stations were adopted in the analysis of the dynamic process of the coastal strong–current phenomenon in the northwest area. The positions of the stations, which cover the northwest coastal sea area of the South China Sea, are shown in Figure 1. Station 1 is located in the sea area where strong currents occur; Station 2 is located in the area representing the west coast area on the Pearl River Estuary; and Stations 3–6 are in regional positions in the northwest coastal area of the South China Sea. Stations 2–6 are offshore fixed marine hydrometeorological observation stations. No wind observation was conducted at Station 1; Station 4 is about 38 km away from Station 1. The wind observation data from nearby Station 4 were, therefore, used for regional wind characteristic analysis. Station 1 employs an acoustic Doppler current profiler instrument to observe sea current once every 30 min and collects the average of the samples every 60 s; a pneumatic water gauge is used to observe the tide level once every 5 min, and collects the average of the samples every 60 s. The water depth of Station 1 is 28 m. Stations 2, 3, 4, 5, and 6 are solid sea stations near the shore. The observation factors and time period for each continuous observation station are displayed in Table 1. Station 1 collects the sea-current data 2 m underwater, as sea surface current data. Salinity data were obtained at the surface level on the shoreline.

3. Coastal Strong–Current Zone in the Yangjiang Sea Area

3.1. Location of the Strong–Current Zone in the Yangjiang Sea Area

The residual current usually means the residual current (Euler residual current) in the measured data after deducting the influence of periodic (astronomical tide) currents, including the non-regular currents for residual tidal currents, wind and sea currents, and density currents, and which can objectively reflect the transport direction of seawater in the observation sea area. The residual current is the low-frequency portion of the total current at sub-tidal periods. The observed sea current comprises the following:

$$V_{total}=V_{tidal}+V_r+V’$$

(1)

In this formula, $V_{total}$ represents the observed sea current; $V_{tidal}$ represents the tide current; $V’$ represents the random high-frequency signals of the observed sea current; $V_r$ represents the residual current, indicating the low-frequency flow acquired after the deduction of random signals, and tides that represent regular variations in the sea current in the observed sea current, all of which reflect the features of seawater transport and material diffusion [18].

The data from our one-day-long sea-current surveys, on multiple voyages, revealed that (see Figure 2) there was an obvious strong–current zone 20–30 m underwater in the sea area of the coastal current and that the velocity of the current in the strong–current zone was apparently greater than the velocity of the sea current on the north and south sides of the sea area. The position of the strong–current zone is not fixed: it moves south–north in the sea area at a depth of 20 m to 30 m. Figure 2 demonstrates that the velocity of the surface residual current during the first voyage at the A2 station was 100.3 cm/s, while the velocity on the second voyage at the B8 station was 39.7 cm/s, the velocity on the third voyage at the C2 station was 38.2 cm/s and the velocity on the fourth voyage at the D7 station was 56.8 cm/s; the current velocities at the remaining stations in the strong–current zone were about 20–40 cm/s. The current-survey stations used during the multiple voyages in this study are located less than 60 km offshore and all are located in typical coastal current waters.

Due to a lack of actual sea measurements during its early stage, the present research was unable to uncover the structure of the current field in the northwest area of the coastal current, nor that of the coastal strong–current zone. Besides, the coastal strong–current zone does not always exist in time, but is characterized by instability; as a consequence, during other investigation voyages, the coastal strong–current zone did not occur.

3.2. Analysis of Residual Current in the Strong–Current Zone during the Observation Period of the First Voyage

From the above analysis, it can be concluded that the maximum residual current velocity in the strong current zone of each voyage is 100.3 cm/s, which occurred during the first one-day period voyage from 4:00 on 14 July 2018 to 5:00 on 15 July 2018. Cao et al. (2022) analyzed the data of the first voyage of current observation and pointed out that the wind direction during the first voyage of current observation was northeast wind, and the direction of residual current was basically consistent with the wind direction. The residual current had the property of a wind-driven current; as well, it is believed that the large residual current in the coastal waters of western Guangdong is related to the concentration of seawater in the coastal waters [19]. Figure 3 shows the sea surface level anomaly (SLA) and sea surface geostrophic current of the western Guangdong sea area on 15 July 2018 as a schematic diagram of the AVISO satellite altimeter, from which it can be seen that there was an obvious westward geostrophic current in the coastal waters of western Guangdong during the first voyage.

4. Seasonal Variation Features of the Coastal Current in the Yangjiang Sea Area

We divided the direction of the sea current into 16 branches and performed statistical analyses regarding the frequency of the sea currents in each direction, from which we created the Station 1 current rose map (Figure 4 below) of the sea-surface currents in spring, summer, autumn, and winter. Figure 4 shows that the southwest current plays a leading role in the observation areas in spring, autumn, and winter, but in summer, the frequency of the sea current deviating eastward is higher than that deviating westward in the observation areas. Therefore, the coastal sea area of Yangjiang, Guangdong displays the typical wind current and sea current action areas, in which the coastal sea current deviates west under the influence of the northeast monsoon, and east under the influence of the southwest monsoon. The maximum velocity of the sea surface current reaches 107.4 cm/s in autumn, and 96.4 cm/s in winter. The velocity of the surface coastal current that deviates southeastward in spring and summer is rather high, while in winter, when the northeast monsoon is the strongest, the frequency of the southwest coastal current is rather high with a small current velocity. This means the coastal sea current in the sea area of Yangjiang, Guangdong is influenced by other seasonal environmental factors, in addition to responding to the monsoon field.

5. Abrupt Strong–Current Phenomena in the Yangjiang Sea Area in Spring and Summer

An internal abrupt strong current is a sudden abnormal flow of high velocity observed in the sea [20]. Xiu et al. (2004) have undertaken a vast amount of research on the abrupt strong–current phenomena in Chinese coastal waters: they think that the convergence of flow fields is the key factor in strong current formation. A strong sea current has the attributes of tremendous velocity, short duration, small spatial range, and great randomness [21]. The abrupt strong current in the Yangjiang Sea area occurs in spring and summer. Xiu et al. (2004) did not mention the seasonality of abrupt strong–current phenomena. To date, no scholars have considered abrupt strong–current phenomena in spring and summer, in the Yangjiang sea area.

At 12:00 on 24 March 2019, the surface velocity at Station 1 increased sharply to 138.0 cm/s, while the velocity before the strong current was only 51.5 cm/s, and the ratio of the strong current to the velocity before the strong current was 2.7. The strong current lasted for 3 h, then the velocity dropped sharply and returned to normal. At 06:00 on 5 May 2019, the surface velocity at Station 1 increased sharply to 164.7 cm/s. At 0:00 on 8 May 2019, the surface velocity at Station 1 increased sharply to 161.1 cm/s. At 05:00 o’clock on 1 August 2019, the surface velocity at Station 1 increased sharply to 161.8 cm/s. As can be seen from Figure 5, the phenomena observed at Station 1 were similar to the state of the strong current in the sea [9], i.e., a sudden strong–current phenomenon. The strong–current phenomena at Station 1 occurred on the surface of the sea, and then the velocity of the subsurface and bottom increased gradually. The duration of the strong–current phenomena was short, lasting about 2~4 h, with discontinuity and randomness.

6. Hydrodynamic Characteristics and Influence Mechanism of Westward Current in the Coastal Sea Area of Western Guangdong

6.1. Maximum Monthly Surface Current Velocity at Station 1

The monthly maximum velocity was obtained by calculating the surface velocity at Station 1 (Figure 6). The maximum velocity in spring and summer was significantly higher than that in autumn and winter. In November 2018, the observed sea area was affected by typhoon No. 201826—Yutu—(severe tropical storm), with a maximum velocity of 107.4 cm/s. The maximum velocity was 138.0 cm/s in March 2019, 164.7 cm/s in May 2019, and 161.8 cm/s in August 2019.

6.2. Time Variation of Sea Current in Spring and Summer at Station 1#

The residual current velocity was set as $U$ and the degree measurement of residual current direction was set as $D_r$. The east component of residual currents was $u$ and the north component of current velocity was set as $v$.

Current velocity component expression of relation:

$$U^2=u^2+v^2$$

(2)

The upper formulation can be divided into branch formulas as follows:

$$u=U\times \sin \left(\frac{(90-D_r)\times \pi }{180}\right)\mathrm{and}v=U\times \cos \left(\frac{(90-D_r)\times \pi }{180}\right)$$

(3)

The east and north components of wind speed below were calculated by the same method.

Figure 7 shows large fluctuations in current velocity during observations. The velocities of the east and north components of residual currents were negative in spring, and the southwestward currents played the leading role in the observed sea area. In summer, the positive value of the north component of the residual current velocity was higher, the size of the eastern component of the residual current velocity was close to 0, and the water body of the observation sea area mainly moved in the direction of the northerly wind. It can be seen that when there was a high current velocity on the surface of the sea, such as on 7 March, 5 May, and 1 August, the eastern and northern components of the residual current velocity were obviously negative; that is, the strong currents in the observed area were all southwesterly currents.

Station 4 did not experience unusually strong winds in spring and summer. In spring, the north component of the wind speed at Station 4 was positive, while the east component fluctuated between positive and negative values, and the positive value of the east component was relatively high. In other words, in spring, there was a southwest wind with high wind speed and a moderate southeast wind at Station 4. In summer, the north component of wind speed had positive and negative values, and the north component of the basic wind speed was also negative when the east value of the wind speed was negative. At Station 4, the southwest wind and northeast wind alternated (Figure 8). By comparing the time series changes in velocity in Figure 7 and the wind component in Figure 8, it becomes apparent that, in summer, the southwesterly wind dominated the northward current in the sea area at Station 1 Before the occurrence of strong–current phenomena, such as on 7 March, 23 March, 1 April, 1 May, 5 May and 1 August, the eastern component of wind speed was negative for a period of time; however, when the strong–current phenomenon occurred, the eastern component of wind speed was positive. The strong–current phenomenon only occurred when the southwest monsoon was relaxed in spring and summer, and the wind direction in the coastal current area changed to a strong northeast wind. It can be seen from Figure 7 and Figure 8 that when the current speed had a typical peak, such as on 7 March, 23 March, 1 April, 1 May, 5 May, and 1 August, the corresponding wind speed component also exhibited an obvious peak; however, when the wind speed experienced a large peak, such as on 16 June, 2 July and 29 August, the peak velocity was not obvious. This demonstrates that the observed sea area was a wind-current sea area, and wind stress is the main factor that determines the current direction, but it is not the only dynamic factor that affects current velocity.

In summer, the northern shelf area of the South China Sea is an upwelling area. The upwelling water comes from the highly saline seawater of the offshore sea. The offshore baroclinic pressure–gradient force is formed in the shelf area, and the northward geospheric current is generated. In addition, in summer, the northwest coast of the South China Sea is affected by the diluted water of the Pearl River. The upper part of the sea has a high water temperature and low salinity, while the lower part has a low water temperature and high salinity. The low-salinity water is mainly near the shore, while the highly saline water is mainly out to sea, forming a trend in the sea level decreasing from shore to outer sea [2]. The pressure–gradient force vector points from high pressure to low pressure. Without the Coriolis force, water flows from high pressure to low pressure. Therefore, when the salinity of the surface water in the northwest of the South China Sea decreases, it contributes to producing the westward coastal current. Consequently, the decrease in the salinity of surface seawater in the northwest of the South China Sea is conducive to the generation of the westward coastal current. The salinity time series for each station in the northwest of the South China Sea (Figure 9) reflects the density structure of the offshore waters in the northwest of the South China Sea. Station 3 represents the influence of the Pearl River diluted westward water. Station 4 is close to Station 1, and the salinity time change process is more consistent with Station 6. Compared with the time series of salinity changes in Figure 7 and Figure 9, when strong outflows occurred on 5 May and 1 August, the salinity at Station 3 showed an increasing trend, and that at Station 4 showed a decreasing trend. However, no strong–current phenomenon was observed when the salinity dropped sharply at Station 4 on 28 May.

Analysis of the water level data for each station was performed by averaging the tide level data (Figure 10). The time sequence for the water levels at the observation stations in western Guangdong shows that the trend in water level fluctuation processes occurring at Stations 1, 2, 4, and 5 were similar to each other, along with the same timing of their peak. The time sequence for the water level at Station 6 is different from that of other stations. Station 2 is located in the Pearl River Estuary, which reflects the runoff status of the Pearl River. Stations 1, 4, and 5 are located in the northwest coastal area of the South China Sea, and the variation trends in the water level from the Pearl Estuary to Station 5 are essentially the same. Therefore, the main influencing factors on variations in water level are the same. The Station 2 water level variations are influenced by the Pearl River runoff, so it can be deduced that the Station 5 water level variations are mainly influenced by the diluted water of the Pearl River in spring and summer. Comparing the trends in water level variation at each station with the current velocity variation line in Figure 8, it can be seen that when strong–current phenomena occurred from 5 May to 1 August, water levels at Stations 1, 2, 4 and 5 had apparently reached their peak. When the strong–current phenomena occurred on 7 March, 23 March and 1 April, water levels at Stations 1, 2, 4 and 5 had also reached a peak. Therefore, the strong current phenomenon of the westward coastal current in the northwest of the South China Sea is relatively consistent with the growth process of the nearshore water level.

6.3. Strong–Current Phenomenon Dynamic Mechanism Analysis

Based on the above analysis, it can be seen that the strength of the coastal current in the western part of Guangdong is closely related to local wind stress and water level changes. The flow direction of the coastal current is singular in the west direction, so the role of various dynamic factors can be analyzed by establishing a simple velocity control equation. This section analyzes the radial dynamic mechanism of the strong–current phenomenon. If we assume that in the y direction, the movement of seawater is sufficient to achieve a balance among the Coriolis force parameter, gradient force of stress, and friction [22], then we can conclude a governing equation of flow velocity, thus:

$$\rho fu=-\frac{\partial p}{\partial y}+\frac{\partial {\tau }_{by}}{\partial z}$$

(4)

In the upper formula, $\rho$ represents the density of seawater, and $p$ represents intensity of pressure; $u$ is the east component of seawater; ${\tau }_{by}$, respectively, represents the friction stress of the seabed and the north component; and $f$ represents the Coriolis force parameter. It is known that, when the friction stress of the seabed is ignored, the stronger the pressure–gradient force from north to south, and the greater the velocity of the westward current.

According to Formula (5), without considering the effect of wind stress, the water level difference in the north–south direction caused by the change in sea level can generate an east–west direction of seawater flow. In the coastal waters of western Guangdong, the effect of the mixing of coastal waters on convection velocity is not obvious. Monsoon action can cause a difference in sea level height from the coast to the outside in the western Guangdong Sea area, forming a north–south horizontal pressure–gradient force, thereby affecting the east–west coastal current. In spring and summer, runoff from the Pearl Estuary reaches a peak. Under the influence of the northeast wind, the coastal water level of the northwest region of the South China Sea forms a gradient force of positive pressure in the direction from the shore to open waters and produces south-westward geostrophic currents in the same direction as the wind currents. In addition, under the joint effect of wind currents and geostrophic currents, it forms the abrupt strong southwestward coastal currents. These abrupt strong southwestward coastal currents phenomenon may be related to the episodic occurrence of very strong northeasterly currents.

7. Conclusions

Analysis of the continuous observation information on hydro meteorology factors at multiple positions, over a long period of time, in the northwest coastal area, as well as research on sea currents and the relationship of time changes to local wind, salinity, and water levels, leads us to conclude the following:

There is an obvious strong–current zone at a depth of 20 m to 30 m in the coastal current sea area in the Yangjiang sea area, the residual current velocity being 38.2 cm/s–100.3 cm/s according to the current observational data in the strong–current zone. The conservation law of potential vorticity in a rotating fluid can explain the motion of currents along the isobath. In a barotropic geostrophic current, Taylor–Proudman theory demonstrates that velocity cannot change with depth, so the current should flow along the isobath, and the low-frequency current along the shelf direction is one order or more than that of the transverse shelf current [14]. Therefore, the strong coastal current zone in Yangjiang shallow sea is caused by topography and nonlinear effects.

The coastal waters of Yangjiang, Guangdong Province, are typical of wind-driven currents. The near-shore currents are westward during the northeast monsoon and easterly during the southwest monsoon. The effect of wind can affect the whole shallow-water layer. Wind stress is the main factor that determines the current direction. The shallower the water depth in shallow water, the faster the response. The response time is between minutes and days. However, during the observation period, the maximum value of the surface current appeared in the south–west direction in spring and summer, rather than in the winter with the strongest northeast monsoon.

In this study, it was found that, in spring and summer, in the Yangjiang River area, there are sudden strong currents in the coastal current waters, at a depth of 20–30 m. The maximum flow velocity in the sea area at Station 1 in May 2019 was 164.7 cm/s, and the maximum flow velocity in the sea area observed in August 2019 was 161.8 cm/s. Before the occurrence of strong–current phenomena, such as on 7 March, 23 March, 1 April, 1 May, 5 May, and 1 August, the east component of the wind speed had a negative value after a period of time. When a strong–current phenomenon occurred, the east component of the wind speed had a positive value. Strong–current phenomena occurred only when the southwest monsoon in spring and summer had relaxed and the wind direction of the coastal current sea area had changed to a strong northeasterly wind. In addition, the sudden strong–current phenomena in the Yangjiang coastal waters were closely related to the increase in water level in the coastal waters west of the Pearl River Estuary in Guangdong. Precipitation and continental runoff make the sea surface form a certain slope. In spring and summer, the runoff of the Pearl River into the sea reaches a peak. Under the action of the northeast wind, the water level on the coast of western Guangdong rises, forming a barotropic pressure–gradient force from the shore to the outer sea, generating a southwestward geostrophic current in the same direction as the wind-driven current. Under the joint action of the wind-driven current and geostrophic current, a sudden strong southwestward coastal current is generated.