On the Mixing of Waters at a Northern Offshore Area of Taiwan

Previous hydrographic surveys show that there exists a permanent upwelling center of Kuroshio subsurface water above the shelf break at the northeast offshore area of Taiwan. The water type of this cold dome is quite different from its Kuroshio origin. This indicates that the upwelling water is being mixed with shelf water and that the mixing is mainly in a vertical direction. The energy source of the mixing is partially supported by the internal tidal motion. A typical example was observed during December 3-4, 1989 from the CTD and ADCP surveys at an anchored station, During the survey period, the Kuroshio intruded onto the shelf and both the hydrographic and current profile data indicated a strong internal tide. The gradient Richardson number is smaller than 1 below 80 m and is about 4-10 in the thermocline, which is in the 40-80 m depth range. This indicates that the fl.ow is generally unstable and thus the turbulent mixing is enhanced.

The Kuroshio and the Gulf Stream are both intensified western boundary currents with similar dynamical structures.However, the extent of their in teractions with marginal seas is not the same.In the Middle Atlantic Bight, Csnady and Shaw (1983) have shown that the steep continental slope effectively• prevents the deep ocean circulation from entering the shelf.In the wide shelf of the East China Sea (ECS), the presence of Kuroshio originated water has long been recognized.A main process for the intrusion of Kuroshio subsurface water is through the upwelling at the shelf edge, and the strongest signal is probably from the persistent upwelling center located on the shelf break northeast of Taiwan (Chern and Wang, 1989).
TAO Vol.1,No.3In the South Atlantic Bight (SAB), there also exists a similar upwelling associated with the formation of Gulf Stream frontal eddies, but these eddies are transient events, with a residence time of about two weeks, while the upwelled water retains the same Cr, S) values when of their Gulf Stream origin (Lee et al., 1981).For the cold dome northeast of Taiwan, its locations always remain at similar places and the upwelled water has quite different (T, S) values as compared with the Kuroshio subsurface water (Chern et al., 1990).
This indicates that the upwelled water is being mixed with shelf waters.In this study, we present a typical example of Kuroshio intrusion onto the shelf with strong mixing induced by the internal tidal motions in the winter time.

HYDROGRAPHIC OBSERVATION
In the past three years, we have made six hydrographic surveys at offshore areas of north Taiwan.Although the data may depend on the weather condi tions at each survey time, all results show that there is a cold dome above the shelf break and the presence of Kuroshio subsurface originated water on the lower layer of the shelf (Chern andWang, 1989, 1990).In Fig. 1, the Kuroshio water is marked by high salinity (> 34.5) and the shelf water by relatively low salinity ( < 34.3), and these two waters are well separated in the surface layer.The T-S diagram also shows that the shelf waters are directly mixed with the subsurface Kuroshio water with a (T, S) value at about (16 °C, 34.6), which occurs at the 200-300 m depth range (estimated from the vertical T / S profiles of the seaward K uroshio water), and the mixing occurs mainly in a vertical directiqn.In the surface layer, this mixing may be driven by the wind stress but for waters below the thermocline, the mixing can only be caused by shear instability with the vertical shears supplied from inertial or internal wave motions.

OBSERVATIONS AT THE ANCHORED STATION
In order to understand the main process that provides the needed velocity shear for the vertical mixing between different waters, we kept the RV Ocean \\; ;���' � , .
• ' \-\_'. .:� • :-�:; .=• • :. : " \ � , • : • .i � / •� • 'lo.: ".' interval of 5 minutes.From the lowest internal wave mode theory, we know that the vertical gra dient of the horizontal velocity is large on the thermocline itself and the shear magnitude is proportional to the strength of stratification (Phillips, 1977).The strength is 1 cm s-1 m-1, agrees well with the thermocline va riation in the temperature profiles (Fig. 3).At depths beyond 80 m, there still exists quite a large velocity shear with a magnitude of about 1-2 cm s-1 m-1.By using the current meter measurements, moored at 15, 60 and 110 m depth from a nearby station, Chern (1986) showed that the difference of cur rents measured at the 60 m and the 110 m depth level agrees well with the prediction from a simple Ekman type bottom boundary layer model, and that the measured current shear remains highly coherent with the tidal variation at Keelung harbor.However, for the difference of currents at the 15 m and the 60 m depth, its magnitude is much larger than the model prediction and has lower coherence with the tide at Keelung.Therefore, we may infer that the current shear near the thermocline, at the 40-80 m depth range, is caused by the internal tide and the shear at a lower level is caused by the bottom friction.
We also computed the vertical gradient Richardson number, Ri, from each CTD cast and the hourly averaged AD CP data for the same time.At the northern offshore area of Tai wan, the water depth is about 120 m.
The�present survey shows that there exists an internal tide with an amplitude as large as 40 m on the thermocline with the sigina-t varied about 1-2 across it.
The velocity shear of the internal tide is the energy source of the vertical mixing . in this region.In such an active mixing region, there must be a continuous inflow of cold water in order to maintain the observed strong stratification and the internal wave motions.The persistent upwelling of Kuroshio subsurface water above the shelf break provides this cold water source.
It is interesting to note that the shelf-edge upwelling also occurs in the SAB (Bane et al., 1981).In contrast to the persistent and active mixed upwelling center northeast of Ta iwan, the upwelling in the SAB is generally associated with transient spin-off eddies and will travel with the stream.Those upwelled waters retain their original properties.

Fig. 1
shows a typical composite T-S plot of the survey over the shelf and slope region, based on data from the September 1987 cruise.Temperature and salinity data were obtained at each station with NBI CTD sampled at 32 Hz.Original data were smoothed and resampled at 1 m depth intervals.Each (T, S) point in Fig. 1 corresponds to one sample.
Fig. 1.Composite T-S plot of all hydrographic station data taken during 14 to 19 September 1987.

Fig. 2 Fig. 2 .
Fig. 2 shows the stick diagram of currents at the 20 m, 50 m and 80 m depth level and the surface temperature during the survey period.The currents are mainly northwestward with a temperature of about 24 °C.Since the daily averaged velocity is northwest at all depths and the salinity is about 34.3-34.6, Fig. 3b, this indicates the intrusion of Kuroshio originated water onto the the shelf at this site.The current variations are dominated by the semi-diurnal tidal constituent and even from the relatively weak variation of surface temperature, Figs. 3a, 3b shmys the depth var iations .for constant temperature and salinity levels.The existence of an internal tide of semi-diurnal constituent is obvious and its amplitude is about 40 m.The temperature variation also shows that except for the period between 10 a.m.-15 p.m.1 the .rising of low temperature water beneath the thermocline is accompanied by an increase of surface tem perature of over 24 °C.This 180° out of phase between the temperature below the thermocline and at the surface indicates the internal tide is at the lowest mode at the interface.
ADCP measurement should reveal this result.Fig.4shows the time variation of the velocity shear magnitude, which is computed from the hourly averaged velocity profiles.The data of the first two bin cells, which are 8 m deep, were discarded to avoid the influence of surface waves.In general, the yelocity shear in the mixed layer is weak and increases above 1 cm s-1 m-1 beyond the 40 m depth.The time variation of the depth, where the shear .

Fig. 4 .
Fig. 4. Depth variations of current shear strength (in cm s-1 m-1) calulated from the hourly averaged ADCP velocity profiles during the survey time as in Fig. 2.

Fig. 5 Fig. 5 .
Fig. 5. Depth variations of constant gradient Richardson number calculated from the CTD data .and the hourly averaged ADCP velocity profiles.during the survey time as in Fig. 2.
Kuroshio passes the north tip of Taiwan, it encounters the sharply curved steep continental slope of the ECS.An upwelling center of the Kuroshio sub surface water is formed above the shelf break at that place.The (T, S) points TAO Vol.1, No.3 of the CTD survey on the shelf and slope region north of Taiwan are roughly located in straight lines between the shelf water and the subsurface Kuroshio water.This indicates the direct vertical mixing between these waters.