Current measurements and spectral analyses in the Luzon Strait during spring of 2002

On the basis of the current measurements at 200,500 and 800 m from moored current meters with the time series data from March 17 to April 15 at the mooring station(20°49'57"N,120°48'12"E) in Luzon Strait during spring of 2002 cruise,the spectral analy ses and calculation have been made.The major observed results are as follows:(1) the average velocity in the observing days is(47.4cm/s,346°) at the 200 m levels.The maximum observed current velocity V_max and maximum daily current velocity V_d,max are(103.8 cm/s,10°) and(71.6 cm/s,339°) at 200 m level,respectively.The averag evelocity in the observing days is(20.3 cm/s,350°) at the 500 m level.V_max and V_d,max are(74.1 cm/s,17°) and(39.1 cm/s, 317°) at 500 m level,respectively.The above results mean that the Kuroshio intrudes north west ward at the 200 and 500 m levels of moored station through the Luzon Strait in SCS.(2) The average velocity in the observing days is(1.2 cm/s,35b) at the 800 m level.V_max and V_d,max are(10.8 cm/s,76°) and(4.7 cm/s,46°) at 800 m level,respectively.i.e.,their directions are north east ward.This means that the flow condition at the 800 m levels very differs from that at the 200 and 500 m levels,and their velocities decease with depth and their directions rotate clockwise with the change of depth.(3) The currents are stronger in April than these in March.(4) There exists the prominent fluctuations of tidal current.There exists also variation of tidal current with the change of depth.At 200,500 and 800 m levels,the semidiurnal peak is higher than the diurnal peak and the clockwise component is dominant in their fluctuations except for the height of diurnal peak is higher than that of semidiurnal peak for the clockwise component and the count erclockwise component is dominant in their fluctuations for semidiurnal peak at the 500 m level.(5) There is the prominent fluctuation of period of more than 15 d or about 14 d at the 200,500 and 800 m levels.For example,when f>0(the counterclockwise component),there are the prominent fluctuations of about 19 d periods at the 200 and 500 m levels,and there is the prominent fluctuations of about 14 d periods for clockwise component at the 800 m levels,respectively.(6) At 200,500 and 800 m levels,there are also the following prominent peaks:1) the fluctuation in the period range of about 4~6 d,which occurs in the weather process;2) the fluctuation in the period range of about 2~3 d;3) the inertial period of about 34.5 h,whose fluctuation is clockwise,and soon.(7) From the cross spectral estimates between two time series,it is shown that 1) there are significant coherence peaks for the periods of more than 15 d and 2~3 d and the diurnal and semidiurnal peaks and so on between the two time series of currents at 200 and 500 m dept hs.Bot h the current fluctuations at 200 and 500 m dept hs are synchro nous for the period of more than 15 d.2) t here ar e significant coher ence peaks for the periods of about 4~6 d,which occurs in the weather process,and 2~3 d and the diurnal and semidiurnal peaks and so on between the two time series of currents at 500 and 800 m depths.However,the phase difference of current fluctuations between two time series at 500 and 800 m levels have the time lags and forward phenomena for the above periods.