On the Characteristics of Bottom Tidal Currents in Northeast Taiwan

A self-recording current metering subsurface mooring was deployd on the slope where the water depth is 270m, about 14 nautical miles SSE from the Peng-Chia-Yu islet northeast of Taiwan. Data shows that bottom tidal motions are very strong, and their periodicities are diurnal, semidiurnal and fourth diurnal. Semidiurnal tidal motions have the most energy, and act as the major component in the observed current field. Tidal current progressive vector diagram shows that the principal axis of current ellipse points NW-SE-Due to interaction with the sea floor, onshore tidal motions impinging on a sloping continental shelf will acquire a vertical motion which way contribute to the local upwelling.


INTRODUCTION
The Kuroshio coming from the Phillippine Sea flows mainly along the eastern coast of Taiwan toward the north.As the Kuroshio approaches the sharply curved continental shelf edge, it moves northeastward.Figure 1, after Sun (1987), shows the yearly mean (upper left panel) and seasonal averages (lower right panel) of the surface axial path of the Kuroshio in the vicinity of Taiwan based on 25 years  of GEK data.On the average, the Kuroshio axis moves shoreward in fall and winter, and seaward in spring and summer, in agreement with observations in 1966-1967 (Nitani, 1972) and in 1974-1975(Chu, 1976).Wang (1987) and Song (1987) show the near-surface salinity fields over the East China Sea in winter (December 1984-January 1985) and summer (June-July 1984).There is a r shape salinity front in winter and a n shape salinity front in summer.Chao (1991) pointed out that the primary influence of the Kuroshio on the circulation of the East China Sea comes from the region northeast of Taiwan, where the sudden expansion of shelf width causes the isobaths of the continental slope to become more zonal, almost perpendicular to the axis of the Kuroshio to the east of Taiwan.The bulk of the northward flowing left-bounded Kuroshio at depths is expected to follow isobaths northeastward to conserve potential vorticity.However, surface and subsurface Kuroshio water may overshoot and spill over the continental shelf.Conceivably, the intrusion of Kuroshio water onto the shelf generates upwelling and  anticyclonic circulation, and r shape of the Kuroshio water in the East China Sea in winter can be largely attributed to the shoreward surface Ekman drift associated with the northerly wind.Chuang and Wu (1991) showed that in winter 1990, in the outer shelf region, across a canyon northeast of Taiwan, existed a steady subsurface flow in the southward, along-slope direction, opposite to the nearby northward flowing Kuroshio current.Tides run transverse to the mean fl ow.Subtidal current fl uctuations were concentrated in the synoptic (2-4 days) band.
There exists a quasi-permanent upwelling center of Kuroshio subsurface water above the shelf break at the northeast offshore area of Taiwan, based on the hydrographic surveys and NOAA-HRPT SST image pictures (Yin, 1973;Uda and Kishi, 1974;Fan, 1980;Chem and Wang, 1989;Chem et al., 1990;Wong et al., 1991;Hsu, 199la,b).Yin (1973) pointed out that the cold water mass near N.N.E. of Taiwan is a mixing of intermediate water mass, Kuroshio water mass and coastal water mass, and the upwelling of the cold water is induced by wind, current and bottom topography.Since then, the causes of this cold dome were suggested as having many different factors, for example, 1) topographic upwelling (Yin, 1973;. Fan, 1980;Chem and Wang, 1989;Wong et al., 1991;Rou and Chen, 1991), 2) thermal' wind effect (Chem and Wang, 1990a), 3) oceanic front (Liu and Pai, 1987;Liu et al., 1988), 4) internal tide (Chem and Wang, 1990b).

Nakamura et al. (1990) discussed the characteristics of oceanic fronts in the East China
Sea from the viewpoint of oceanic currents.They found that variations in time scale of a few hours around Kuroshio Branch were connected with tidal current in the south cif the East China Sea.In this paper, we present the evidence of bottom tidal motion, internal tide and upwelled subsurface Kuroshio water.We discuss interaction of the semidiurnal bottom tidal current with sea floor and its relationship with cold dome at the shelf margin in the vicinity of the Peng-Chia-Yu islet northeast of Taiwan.Our results are consistent with finding of Nakamura et al. (1990).

OBSERVATIONS
Self-recording current meters (AANDERAA RCM-7s and/or RCM-4s) were deployed at station W (Figure 2) on the continental slope, 14 nautical miles SSE from the Peng-Chia-Yu islet, by using a subsurface mooring buoy system during the winter of 1990 and the spring of 1991.The location of station Wi s at 25°25.4 1 N, 122° 7.5 1 E with bottom depth around 270m.In this area, a quasi-permanent cold dome on the shelf had been well-documented.
Sampling interval for the RCMs was set at 10 minutes before deploying.After r e trieving the RCMs, data was carefully edited, and then standardized to one hour sampling records by further vector averaging.

RESULTS
The hourly records of temperature, north (V) and east (U) components of velocity were used to analyze power spectra and tidal current progressive vector diagram (PVD).Figure 3 shows the energy power spectra in which bottom tidal motions are evident.Their periodicities are diurnal, semidiumal and fourth diurnal.The semidiurnal tidal motions have a very strong energy peak, and more energy than others.Semidiurnal tidal motions are also evident in the Besides the regular variations mentioned above, there are small random variations in velocity and temperature records (Figures 6 and 7).These random variations may be due to high frequency internal waves.For the description of subtidal low frequency variation in the observed current field, we refer readers to the paper of Chuang and Wu (1991).Chong-Kung Wu

DISCUSSION
Figure 8 shows T-S diagram from CTD data at the mooring station W during leg 1 (Nov.3) and leg 2 (Apr.5).The water type of the whole water column is mostly the upwelled subsurface Kuroshio water (see Chem et al, 1990).Wong et al. (1991) studied the hydrographic data across the frontal region between the East China Sea and the Okinawa Trough northeast of Ta iwan, and concluded that the topographically-induced upwelling water was the modified Kuroshio water from about 300m deep.Its composition seems to remain rather constant with time with a temperature and salinity of around 13°C and 34. 4 psu respectively.Nakamura et al. (1990) used the hydrographic surrey in the south East China Sea and showed that on the bottom layer, the shelf warm water of more than 16°C is replaced by less than 16°C water about every 6 hours, corresponding to semidiumal tidal period.They concluded that this less than 16°C water is due to the advection of intermediate water of the Kuroshio to the bottom layer on the shelf.The range of temperature fluctuations in Figures 6 and 7 is between 16.2°C-12.4°Cand 16. 1°C-13.1°Crespectively.This indicates the cold water accompanied with onshore tidal motions (NW) at the mooring station W, is associated with the advection of intermediate water of the Kuroshio.Tang (1991) showed the presence of internal tide with amplitude as large as 30m on the thermocline from CTD data at the anchor station 5320, about 9 nautical miles NW from the       Nakamura et al. (1990) found that the semidiumal current is prevailing and the direction of principal axis of current ellipse (NW-SE) is almost perpendicular to the Kuroshio axis (NE).
The semidiumal fluctuation is also present in the temperature field, indicating the existence of internal tide.When tidal current is strong , the intrusion of the intermediate water of the Kuroshio system is accompanied with tidal excursion through the bottom layer of the continental shelf.As a result, the geographical location of oceanic front and horizontal gradient of water temperature changed.Our study is consistent with Nakamura's finding, namely, when prevailing onshore semidiurnal current is NW, the Kuroshio intermediate water moves onto shelf in the bottom layer on the continental shelf.

CONCLUSION
We have presented the evidence of bottom tidal motion, internal tide and upwelled subsurface Kuroshio water.Due to interaction with the sea floor, onshore tidal motions impinging on a sloping continental shelf will acquire a vertical motion (Pond and Pickard, 1983) which may contribute to the local upwelling.
"41UN'l'ER Fjg. 1.The Kuroshio paths defined by the axes of surface curre nt speed along the continental margin of the East China Sea based on 25 years of GEK data (after Sun, 1987).The yearly mean and seasonal means are shown on the upper left and lower right panels, respectively.

Fig. 2 .
Fig. 2. Map of the area northeast of Ta iwan, depth in meters.Location of station W denoted by solid square where water depth is around 270m.

Fig. 3 .
Fig. 3. Energy power spectra of north (V), east (E) component of velocity and temperature (T), where V component has been upper lifted three orders.Sticks D, M2 and M4 denote the frequencies of diurnal, semidiumal and fourth diurnal respectively.!denotes 90% confidence interval based on 10 degrees of freedom.

Fig. 4 .
Fig. 4. Hourly tidal current progres sive vector diagram stars from Nov. 4 to Nov. 5 1990 at station W. Instrument/bottom depth is 230m/263m.Princi pal axis of current ellipse points NW-SE direction, and its prin cipal axis is about 4 nautical miles long.

Fig. 5 .
Fig. 5. Hourly tidal curr ent progres sive vector diagram stars from Apr. 16 to Apr. 18 1991 at station W. Instrument/bottom depth is 245m/270m.Princi pal axis of current ellipse points NW-SE direction, and its prin cipal axis is about 5 nautical miles long.

Fig. 6 .
Fig. 6.Ten minutes sampling in terval records of tempera ture, current direction and current speed at station W during leg 1.

Fig. 7 .
Fig. 7.Ten minutes sampling in terval records of tempera ture, current direction and current speed at station W during leg 2.
mooring station W during the cruise #280 of the RN OCEAN RESEARCHER 1. Figures 9 and 10 show internal tide with an amplitude as large as 90m (see 15°C and 34.6 psu contour), from CTD data at the anchor station at 25°251N, 122°15'E about 7.5 nautical miles east from the mooring station W during the cruise #271 of the RN OCEAN RESEARCHER 1. Therefore internal tides are very common phenomenon in the vicinity of the Peng-Chia-Yu islet northeast of Taiwan .This is consistent with Maeda's finding (1979) that internal tide is dominant around the area of south East China Sea.