Analysis of the characteristics of offshore currents in the Changjiang (Yangtze River) estuarine waters based on buoy observations

A buoy of 10 m in diameter was used to record the current speed and direction in a vertical profile in the offshore area of the Changjiang (Yangtze River) Estuary (with an average water depth of 46.0 m) for one year. The results include: (1) the currents rotate clockwise and the current direction is consistent in a vertical profile without clear seasonal variations. (2) The horizontal current speeds are generally high, with a maximum of 128.5 cm/s occurring in summer and 105.5 cm/s appearing in winter commonly close to the surface. The average current speeds in the vertical profile fall in the same range (the differences are less than 8.0 cm/s), with the maximum of 47.0 cm/s occurring in summer and 40.8 cm/s in winter. The average current speed during spring tides is twice that during neap tides (26.5 cm/s). (3) Significant differences of speeds are observed in the vertical profile. The maximum current speed occurs at either surface (spring and winter) or sub-surface (summer and autumn), with the minimum current speed appearing at the bottom. The maximum average current speed of all layers is 57.9 cm/s, which occurs in the 18-m layer during summer. (4) The average speed of the residual currents ranges from 7.5 cm/s to 11.3 cm/s, with the strongest occurring in spring and weakest in winter. The residual currents of all layers are eastward during spring and winter, whereas northeastward or northward during summer and autumn. (5) The currents in the offshore of Changjiang Estuary are impacted collectively by diluted Changjiang River discharge, the Taiwan Warm Current, monsoon and tides.