On The Low-frequency West-Guangdong Coast Current And Its Dynamical Mechanism

Statistical analyses are carried out of the data, which come from the ADP current meter and wind sensor mounted on the single-point mooring pontoon near the Zhu Island, West-Guangdong coast during the period of November 2000 and October 2001, and come from the multiple-station one-day synchronous current observations. Statistics shows that the astronomical tidal current is of an irregular semi-diurnal one; the leading direction of it is ESE-WNW; and the maximum possible current speed is less than 50.0cm/s. About the Low-frequency flow, the westward flow prevails around the year. The occurrence frequency and strength of the westward current are greater than those of the eastward current, no matter when in winter or in summer. The extreme current speed in each month of the year is 72.7cm/s to 141.4cm/s for the surface layer; 55.8 cm/s to 135.6cm/s for the middle layer; and 42.6 cm/s to 113.2cm/s for the bottom layer. The directions corresponding to the extreme current speeds for different layers and different months concentrate in SW to WNW, with the highest frequency in WSW. The extreme current speed for each month is controlled mainly by the speed of low-frequency flow.The West-Guangdong coast current is driven by buoyancy and monsoons. The runoff causes the low-density water piling along the coast; and the Coriolis effect makes the water flow to the west. In winter the northeast monsoon blows along with the direction of the geostrophic current, which causes strong westward West-Guangdong coast current with relatively steady cunent direction; for which the wind plays a leading role. In summer the runoff of the Zhujiang River is strong, and the geostrophic cunent caused by the buoyancy is strong. In this season the occurrence frequency of southeast wind is the highest along the West-Guangdong coast. Under the effect of southeast wind the westward coastal current is strengthened; but under the effect of strong southwest wind the westward coastal current will be overturned temporally. The maximum low-frequency current is caused by tropical cyclone. On one hand the strong wind force drives the cunent, on the other hand the tropical cyclone will cause strong precipitation, and hence the runoff as well as the ground water come to and pile in the coastal area, which will cause a strong westward geostrophic current. Existed observations show that off the West-Guangdong coast, east to the Leizhou Peninsula, there exists a cyclonic eddy. The circulation of this eddy strengthens that westward low-frequency coastal flow to certain extent, but its area, strength and formation mechanism need further study. The vertical distribution of the low-frequency coastal flow is related to disturbing factors. During the tropical cyclone period the wind is strong, and the vertical attenuation of the current speed is weak; and under the situation of small wind speed, low-frequency flow occurs mainly in the surface layer, and it weakens quickly in the middle and bottom layers.