Baroclinic Tidal Energy and Vertical Mixing in the Continental Margin and Peng-hu Channel off
Southwestern Taiwan
Huan-Jie Shao* and Ruo-Shan Tseng*
*Department of Marine Biotechnology and Resources,
National Sun Yat-sen University
Kaohsiung, Taiwan 804
(e-mail:rstseng@mail.nsysu.edu.tw)
Abstract: The Peng-hu Channel (PHC) is a funnel-shaped deep passage separating the Taiwan
Island and the Peng-hu Archipelago. With approximately 40 km wide and 100 m deep in the
northern part, the PHC becomes wider and deeper as it extends further southward along the
channel where it is connected to the continental shelves of the northern South China Sea (SCS).
Previous studies have indicated that the PHC, with a strong northward flow due to the pressure
gradient between the two ends, plays an important role on the material and volume transport
between the South China Sea and East China Sea (ECS).
In this study, three cruises were carried out during 2011 and 2012 to measure the currents,
temperature and microstructure turbulence at several stations in the PHC and the adjacent
continental margin of the northern SCS. More than eighty profiles of CTD-turbulence moduleLADCP data were collected from these cruises. Strong internal tides with an overturning scale
of about 100 m were found to exist below 200 m depth in the continental slope. Further
downstream of PHC at the continental shelf, internal tides of moderate intensity can still be
observed. However, in the narrowest section of PHC, no internal tidal signals were detected,
and strong barotropic tidal current with a maximum speed of approximately 2 m/s was
observed. Our results suggest that the baroclinic tidal energy is dissipated completely in the
continental slope and shelf. Microstructure turbulence observations reveal that dissipation rate
of turbulent kinetic energy in the 50-m thick bottom boundary layer of the PHC is much higher
than those in the open ocean. It is inferred that intense turbulent mixing in the PHC brings the
SCS waters and Kuroshio waters from the continental shelves and transports further northward
to the ECS.
Keywords: Internal tidal, Microstructure, Peng-hu Channel, Vertical mixing
1. INTRODUCTION
Internal tides (or baroclinic tides), which are generated by the interaction of the barotropic tidal currents with
variable bottom topography, play an important role in dissipating tidal energy and energizing the mixing to drive the
thermohaline circulation. Several key parameters governing their generation include the bottom slope, the height of
bottom topography and tidal excursion (Klymak et al. 2011). One of the primary generating sites of the internal tides
in the world ocean is the Luzon Strait, which with its two distinct ocean ridges (the Luzon Island Arc and the
Hengchun Ridge) can generate very strong baroclinic tides with an amplitude over 100 m. As the internal tide
radiates from the Luzon Passage and propagates westward into the deep ocean of the SCS, it gradually transforms
into nonlinear solitary wave packets and impacts the continental slope. The breaking and scattering of this strong
internal tide when it impacts the continental slope were investigated by Klymak et al. (2011). Direct measurement of
turbulence was made by St. Laurent (2008) along the continental slope, shelfbreak and shelf near the Dongsha
Plateau, and his results indicate that the most intense turbulence levels occur in the shelfbreak region, with the
integrated dissipation levels exceed 50 mW/m^2, which is an order of magnitude above the open-ocean value.
Internal tide in the coastal region is complicated and largely unpredictable, mainly due to some factors such as the
variability of coastal morphology, stratification and circulation. Some prominent topographic features often found in
continental margin are the submarine canyon and tidal channel. Canyons maybe both effective conduits for
funneling open ocean baroclinic energy onto the inner shelf and sites for internal tides generation through
topographic scattering of barotropic tides (Kunze et al. 2002). A recent numerical study (Chiou et al. 2011) was
made to investigate the sources of baroclinic tidal energy in the Gaoping Submarine Canyon (GPSC) off
southwestern Taiwan. Their results indicate that baroclinic tidal energy in the GPSC is mainly generated on
the Hengchun Ridge and on the steep topography of the Taiwan Banks; the local generation of baroclinic
energy only accounts for 4.4% of the total value. On the other hand, previous studies on the baroclinic
and barotropic tidal energy and associated mixing in tidal channels were rather scarce.
In this study we focus on the Peng-hu Channel, which is a funnel-shaped and N-S oriented deep passage separating
Taiwan and Peng-hu Archipelago. Connecting the continental margin of the northern SCS with its opening, the PHC
is not far from and has some similarities to the GPSC. The baroclinic tidal energy and associated mixing in the PHC
is investigated in this study by using direct turbulence and current measurements. The observations, results and
discussion will be described in the following sections.
2. OBSERVATIONS
Three cruises of field measurements were conducted by R/V OR3 on the Penghu Channel during 2011 and 2012. A
total of six stations were surveyed in this study. Among the six stations, four stations (P1, P2, P3 and P4) are located
at the narrowest section of the northern PHC with depth of 110 m and the other two stations (S1 and S2) are at the
mouth of the PHC connecting the continental margin with depth of 150 m and 600 m, respectively (Fig. 1).
Hydrographical casts by using of CTD/LADCP/Turbulence module were conducted repeatedly at some stations in
each cruise with the interval varies between 1 and 4 hours (Table 1). The measurement length of each cruise lasts
between 25 and 40 hours. More than one hundred profiles of data have been collected. Two self-contained, lowered
ADCPs (Teledyne-RDI 300 KHz) and a turbulence package (Microrider made by Rockland Scientific Inc.) were
attached with the CTD frame to measure the currents and turbulence intensity. Powered by the CTD, the turbulence
package consists of an airfoil shear probe, a thermistor and a micro-conductivity probe operated with a sampling rate
of 512 Hz. Current profiles with 8-m vertical resolution were obtained from the LADCP measurements. Shear data
as measured by the shear probe was found to be contaminated by the vibrations of the CTD to a certain extent, and is
thus not used in this study. The dissipation rate of temperature variance, chi, was derived from the temperature
gradient data as measured by the thermistor.
Table 1 Number of CTD casts and sampling interval
Time
Station
CTD casts
2011/9
P1
11 casts, 2hr
P2
6 casts, 4hr
2012/3
P3
12 casts, 2hr
P4
11 casts, 2hr
15 casts, 1hr
2012/9
S1
16 casts, 2.5hr
S2
17 casts, 2.5hr
Fig. 1: Map showing study region in the Peng-hu
Channel and on the continental margin.
3. RESULTS AND DISCUSSION
The first two cruises, completed on September 2011 and March 2012, surveyed the narrowest region of the PHC.
The time series of LADCP velocity profiles is decomposed into the barotropic and baroclinic components, with the
former representing the depth-averaged current and the latter derived from subtracting the barotropic current from
the total current. Plotted in Fig. 2 are the profiles of baroclinic current (top panel), barotropic current (middle panel)
and the integrated levels of dissipated power (bottom panel), computed by depth-integrating the dissipation rate
times the density, at four stations in the PHC. Consistent with findings from a number of earlier field measurements,
Fig. 2 indicates that flows in the PHC are dominated by the semidiurnal tidal currents with a much larger northward
speed during flood and a smaller southward speed during ebb. It is also obvious that the PHC flows are primarily
barotropic, with the maximum observed current speed reaching 1.9 m/s and it occurs at the surface layer. The
baroclinic tidal current varies from 0.2 to -0.4 m/s and the energy only accounts a small proportion of the total
energy. The integrated dissipation levels apparently vary with the tidal phase, with higher values occurring at the
flood stage. Dissipation level in the 50-m thick bottom boundary layer is higher than that in the upper 50 m layer.
Turbulence is enhanced at P2 and P4, which are located at the sill of the PHC (Fig. 2(b) and 2(d)). Overall speaking,
the integrated dissipation level in the PHC is of the order of 10-50 mW/m^2, which is comparable with those
measured by St. Laurent (2008) near the Dongsha Plateau.
Fig. 2: Time series of the profiles of the northward baroclinic flow (top panel), barotropic flow (middle panel) and
integrated energy dissipation level (bottom panel) at stations (a) P1, (b) P2, (c) P3 and (d) P4 in the PHC. The brown
and blue bars in the bottom panel represent dissipation levels integrated over column below and above 50-m depth.
The third cruise of this study, conducted on September 2012, focused on the continental margin just south of PHC.
Figs. 3 and 4 show the profiles of chi (top panel), baroclinic current (middle panel) and integrated dissipation level
(bottom panel) at S1 and S2. The baroclinic tidal currents at the continental shelf (S1) and the continental slope (S2)
both have speed ranging from 0.2 to -0.3 m/s. On the other hand, the barotropic tidal current (not shown here) in
these two stations has about the same magnitude as the baroclinic part. The internal tide is predominantly
semidiurnal at S1 and S2, although the latter shows more markedly. On the S1, the integrated dissipation level for
the 70-m thick bottom layer is much higher than that of the upper layer, and the higher value occurs at a time when
there is a strong vertical shear between the upper and bottom layers. The integrated dissipation level for the bottom
layer below 200-m depth varies consistently with the semidiurnal period, and the high values occur at a time when
the baroclinic tidal current is northward.
To summarize, our measurements of turbulence and current reveal the noticeable internal tidal variations at the
continental slope and the shelf. As the internal tide propagates into the PHC, the barotropic current is greatly
enhanced due to the topographic constraint of the channel. As a result, the baroclinic tidal energy and its associated
turbulent mixing are not clear or obscured by the strong tidal currents. More studies are needed to investigate the
sources and energy transformation of the baroclinic tides.
Fig. 3: Time series of the profiles of the chi (top panel), northward baroclinic flow (middle panel) and integrated
energy dissipation level (bottom panel) at station S1. The bars are the same as Fig. 2 but for 70-m depth.
Fig. 4: Time series of the profiles of the chi (top panel), northward baroclinic flow (middle panel) and integrated
energy dissipation level (bottom panel) at station S2. The bars are the same as Fig. 2 but for 200-m depth.
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