Measurement of the current and spectral analysis on the continental shelf in the East China Sea

Direct measurements of current velocity and water temperature were undertaken at die mooring Sta. M (125°29.38′ N, 31°49.70′ E) on the continental shelf area in the East China Sea in June 1999 by R/V Xiangyanghong 14. The relationship between various time series of oceanic fluctuations is calculated by spectral analysis. The major results are as follows: (1) an average (u,v) of (6.9,-3.0 cm/s) at the 30 m depth is obtained during the 9-day observation, and that at the 45 m depth is (3.7,-1.1 cm/s), i. e., the mean flows are southeastward at both the 30 and 45 m depths;the currents become stronger gradually during the observation period; this may be mainly attributed to the transition of the tidal currents from neap to spring; (2)semidiurnal fluctuation is the most dominant in the current fluctuations, and rotates mainly clockwise; in the next place, there is also diurnal fluctuation;the local inertial period is close to the period of diurnal fluctuation, and an inertial motion is clockwise; thus, local inertial motion combines with diurnal fluctuation, and makes the spectral peaks in clockwise components much higher than those in counterclockwise ones; except for the fluctuations of above main periods, there is also the peak at 3 d period for counterclockwise components in the upper and lower layers; (3) the calculation of cross spectra between two time series of current velocities at the 30 and 45 m depths shows that both the current fluctuations at the 30 and 45 m depths are much alike, i. e., they are synchro; this shows that the flow field here is rather vertically homogeneous; (4)power spectra of temperature time series at both the 30 and 45 m depths show that the semidiurnal peak is the most predominant, and the second highest peak is the diurnal period; besides spectral peaks at above periods, there are also obvious spectral peaks at 6.8 h and 2 d; (5)plots of temperature time series at 16, 30, 35 45 and 50 m depths show that the temporal variations of temperature at these depths are synchro, which are like those in the velocity field; temperature records also show a gradual rise in temperature, which are also like those in the velocity field.