Green Island located in the typhoon active eastern Taiwan coastal water is the potential Kuroshio power plant site. In this study, a high resolution (250–2250 m) shallow-water equations (SWEs) model is used to investigate the effect of typhoon on the hydrodynamics of Kuroshio and Green Island wake. Two wind induced flows, typhoon Soulik and Holland's wind field model, are studied. Simulation results of the typhoon Soulik indicate that salient characteristics of Kuroshio and downstream island wake seems less affected by the typhoon Soulik because typhoon Soulik is 250 km away Green Island and the wind speed near Green Island is small. Moreover, Kuroshio currents increase when flow is in the same direction as the counterclockwise rotation of typhoon, and vice versa. This finding is in favorable agreements with the TOROS observed data.
The SWEs model, forced by the Kuroshio and Holland's wind field model, successfully reproduces the downstream recirculation and meandering vortex street. Numerical results unveil that the slow moving typhoon has a more significant impact on the Kuroshio and downstream Green Island wake than the fast moving typhoon does. Due to the counterclockwise rotation of typhoon, Kuroshio currents increase (decrease) in the right (left) of the moving typhoon's track. This rightward bias phenomenon is evident, especially when typhoon moves in the same direction as the Kuroshio mainstream.
According to Zdravkovich (2003) flow past a bluff obstacle with the Reynolds
number
Vortex streets occur frequently in the atmosphere and oceans (Chelton et al., 2011; Nunalee and Basu, 2014). The phenomenon of vortex shedding behind bluff bodies has long been of interest to the fluid dynamics community and has been intensively studied by many researchers (Zdravkovich, 2003; Roshko, 2012; Tritton, 1959; Williamson, 1996). The downstream island wakes produce the upwelling and enhance the nitrate concentration in the upper ocean. This kind of phenomenon is often captured by satellite imagery (Hubert and Krueger, 1962; Li et al., 2000; Thomson et al., 1977; Young and Zawislak, 2006; Zheng et al., 2008), field measurement (Barkley, 1972), and by numerical modeling (Dong et al., 2007; Heywood et al., 1996; Ruscher and Deardorff, 1982; Wolanski et al., 1984).
The Kuroshio, a western boundary current of the sub-tropic North Pacific
Ocean, originates from the North Equatorial Current and flows northward to
the eastern coast of Taiwan. The passage of the Kuroshio mainstream
parallels to the eastern shoreline of Taiwan. Green Island is located at
(121
Typhoons, also known as tropical low-pressure cyclones, occur in the tropical
and subtropical seas, and are a frequent cause of serious disasters in
coastal countries and regions. Taiwan is located in the Northwest Pacific –
one of the most typhoon active areas of the world. An annual average of four
to five typhoons hit the island according to the records of the Central
Weather Bureau (CWB) of Taiwan (
Typhoons have a significant impact on the physical and chemical
characteristics of the water of the Kuroshio. The storms disturb the
Kuroshio's path and reduce its flow rate, while strengthens upwelling and
lowering the surface temperature. The impact of typhoons on the upper water
layers is quite dramatic in the Kuroshio. Suda (1943) suggested that
typhoons are the main cause for the Kuroshio meandering near Japan. Sun et al. (2008) conducted a numerical analysis for the impact of typhoons on the
Kuroshio's path south of Japan, found that violent disturbances on the sea
surface caused by a typhoon produces a strong upwelling, further
strengthens the counterclockwise eddy and causes the Kuroshio to deviate
southward from original path by about 2
Taiwan has an excellent marine current energy resource as it is an island
country and experiences the abundant marine current flows. However, this
resource has yet to be explored. The Kuroshio is known for its strong and
fast flow. It could be a potential source of renewable energy as it flow
steadily all year round, and Green Island is the potential test site for
Kuroshio current energy of Taiwan. Chen (2013) proposed a conceptual design
for the Kuroshio power plant. Assessment of potential Kuroshio power test
site has been performed using the in situ measurements and modelling by Hsu et al. (2015). Safety, maintenaces and operations of the Kuroshio power plant may be
subject to impacts from earthquakes, typhoons, climate change, and other
natural factors. To prevent the potential damages caused by typhoon-driven
waves, meandering vortex shedding, and the cavitation occurred on the
surface of turbine blade, turbines and platforms should be submerged several
tens of meters below the water surface. Effect of monsoon on the Green
Island wake has been previously studied (Hsu et al., 2015; Liu et al.,
1992). The effect of typhoon
on the Kuroshio and Green Island wake is presented using a high resolution
(250–2250
Shallow-water equations (SWEs) for describing the shallow water flows and space-time least-squares finite-element method (STLSFEM) used to solve SWEs are briefly introduced in this section.
The two-dimensional vertical-averaged SWEs are a simple form for describing
the horizontal structure of the ocean dynamics (Buachart et al., 2014;
Johnson, 1997; Tan, 1992; Vreugdenhil, 2013). The
two-dimensional viscous SWEs in a non-conservative form read
Holland's wind field model (Holland et al., 2010) is employed to compute the typhoon wind field.
The equation of the gradient of wind speed is
The wind shear stresses on the water surface are usually expressed in terms of
the wind speed at 10
We use the two-dimensional SWEs to illustrate the Space–Time Least-Squares
Finite-Element Method (STLSFEM). The two-dimension SWEs read
Upon applying the least square method, we thus have
In this section, the effect of typhoon Soulik and wind field of Holland's model (Holland et al., 2010) on the Kuroshio and Green Island wake are presented.
A high resolution (250–2250
However, Kuroshio is a sub-surface flow where flow mainly occurs at the top
400
The flow field data of HYCOM (HYbrid Coordinate Ocean Model) on 7
November 2014 after interpolation to the model grids, shown in Fig. 2a, is
utilized to initiate the model. The choice of this particular flow field as
the initial condition is based on the hypothesis proposed by Zheng and
Zheng (2014) in which the downstream Green Island wake is prone to occur when
Kuroshio mainstream heads on the island. Figure 2b shows the boundaries of
the study domain.
Figure 3 shows a composite ERS-1 SAR image of downstream island vortex trains of Lanyu Island and Green Island taken with one week interval sequentially between 17:01 UT 25 September (lower) and 19:02 UT 2 October (top), 1996. Two downstream island vortex trains are clearly seen. One a little far from the coast, called the Lanyu Island (also called the Orchid Island) vortex train, occurs downstream of Lanyu Island (a little beyond the southern boundary of image), and consists of three vortexes, two cyclonic appearing as bright shading and one anticyclonic appearing as dark shading vortexes. Another near the eastern coast of Taiwan, called the Green Island vortex train, occurs downstream of Green Island, and consists of three pairs of cyclonic vortexes (bright center) and anticyclonic vortexes (dark center). Spatial and temporal scales of downstream Green Island wake due to passing of the Kuroshio has been numerically studied (Liang et al., 2013). The main difference between Liang et al. (2013) and the present study lies in the point that a constant inflow used in Liang et al. (2013) is replaced by a spatial varying inflow from HYCOM in the present study.
The SWEs model is applied to simulation the typhoon Soulik event. Typhoon
Soulik hit Taiwan area during 7 to 14 July 2013. Figure 4 depicts the
track of typhoon Soulik. Typhoon Soulik, developed from a tropic depression
on 7 July 2013 through a tropical storm, moderate typhoon, severe typhoon, and
then to decrease its strength and make landfall on Taiwan on 7 December 2013. The
typhoon Soulik data with the temporal interval every 6 h are from the
CWB of Taiwan, which are computed by the NFS (Non-hydrostatic Forecast
System). NFS uses the structured orthogonal meshes, however, SWEs model uses
the unstructured triangular meshes. Therefore, wind data needs to be
interpolated into the computational nodes of the SWEs model. The IDW
(inverse distance weighted interpolation; Shepard,
1962) is employed to interpolate
the data from NFS into nodal points of the SWEs model. In this study, the 5
Figure 5a depicts the global view of the study area, where the enclosed
region near the Green Island indicates the sub-domain that will be presented
in the succeeding plots of the downstream recirculation and island wakes.
Figure 5b depicts the local view of the sub-domain as well as location of
the cross section
Figure 6 depicts the typhoon wind field and flow streamlines as well as
vectors of wind (red) and flow (blue) along cross section
Figure 7a illustrates the location of 22.84
Figure 8 depicts flow streamlines around the Green Island of no wind forcing
in a period of vortex shedding. There are small recirculations in the lee of
Green Island. Its size is about the size of the island (
According to the statistical analysis of typhoons from 1911 to 2010 by the CWB
of Taiwan, 83.6 % of typhoons pass the eastern Taiwan coastal waters and
13.5 % pass the western Taiwan coastal waters. Therefore, two typical
tracks of typhoons, namely the SN typhoon (17 % of total) moving from
southwest to northeast and the EW typhoon (67 % of total) moving from east
to west, shown in Fig. 9, are chosen to investigate the effect of typhoon on
the Kuroshio and Green Island wake. Two typical values of the typhoon moving
speed, 2.5
In order to better quantify the net influence of typhoon on Green Island
wake, we subtract the flow field of SN typhoon case and EW typhoon case with
no wind forcing case. The net influence of typhoon on the flow field is
defined by
Comparing Fig. 10 with Fig. 11, we notice that the impact of the slow moving
typhoon (
Figure 12 plots the wind field as well as wind vectors (red) and flow (blue)
vectors along
Figures 13 and 14 plot the “net”
Figure 15 plots the wind field as well as wind vectors (red) and flow vectors
(blue) along
Figure 16 shows a comparison of time series of
Safety, maintenances, and operations of Kuroshio power
plant may be subject to impacts from earthquakes, typhoons, climate change, and other natural factors. Typhoons have a significant impact on the physical and chemical characteristics of
the water of the Kuroshio. A shallow
water model based on the shallow water equations (SWEs) and the space-time
least-squares finite-element method (STLSFEM) has been developed. The model
has been applied to study the hydrodynamics of Kuroshio and Green Island
wakes with a small computational domain (
A high resolution (250–2250
In the second simulation case, the SWEs model, forced by the Kuroshio and Holland's wind field model, successfully reproduces the downstream recirculation and meandering vortex streets. Kuroshio and the downstream Green Island wake is found significantly affected by the moving typhoon that passes the Green Island directly, especially for the shallow waters and the lee of the islands. Computed results clearly reveal the rightward bias phenomenon – due to counterclockwise rotation of typhoon, Kuroshio currents increase in the right of the moving typhoon's track and decrease in the left of the moving typhoon's track. Computed results also show that the slower the moving typhoon, the more significant impact of typhoon on the Kuroshio and downstream Green Island wake.
This study is supported by the NSC NEP-I project under the grants of NSC103-3113-P-019-001 and NSC103-3113-P-019-012.
Study domain and boundaries as well as contours of flow speed from HYCOM.
Track of typhoon Soulik from 7 to 14 July 2013, where
symbol
Flow streamlines of the Green Island wake of no wind forcing case.
Schematic diagram of the study domain and track of SN and EW typhoon.
Contours of
Contours of
Contours of
Contours of
Comparison of