Numerical Study of the
K
uroshio Current in the Tokara Strait
Bin Wang
1,*
, Tianran Liu
2
, Naoki Hirose
2
and Toru Yamashiro
3
1
College of Oceanography, Hohai University, China;
2
Research Institute for Applied Mechanics, Kyushu University, Japan;
3
Department of Ocean Civil Engineering, Kagoshima University, Japan.
Keywords: Kuroshio current, Tokara Strait, high-resolution ocean modeling, Baroclinic tidal effect
Abstract: A high-resolution regional circulation model was configured over the southern coast of Japan including the
Amami Oshima to simulate the Kuroshio Current through the Tokara Strait (TS). The results are in good
comparison with the currentmeter data from the Acoustic Doppler Current Profiler (ADCP). Comparison
experiment with a higher horizontal resolution suggests the Kuroshio current through the four-islands chain
presents fine sub-mesoscale structures especially in the downstream. Also the sensitivity experiments
suggest the main axis of the Kuroshio in the model with tides moves to the south slightly (offshore) at the
eastern part of the TS. Furthermore, there is concentration of the stronger current (magnitude of current
speed > 60 cm/s) to the upper layer. The vertical shear of the current becomes larger and the Kuroshio
becomes more baroclinic due to the tidal effects.
1 INTRODUCTION
The Kuroshio Current (KC), the famous western
boundary current of subtropical gyre in the North
Pacific Ocean (NPO), transports large amounts of
heat, salt and nutrient from the tropical ocean(Qiu,
2001; Guo et al., 2012; Guo et al., 2013) [1-3] and
plays very important roles in water mass formation,
transformation and subduction (Nurser and Zhang,
2000). It flows northeastward along the continental
slope of the East China Sea (ECS), turns east
through Tokara Strait (TS), and proceeds eastward
along the southern coast of Japan until it separates
from the coast and enters the Pacific basin. Thus, the
TS is a key channel/passage connecting the ECS and
the NPO, which is regarded as a choke point in
considering climate change and cycles of water and
materials over the NPO (Nitani, 1972).
The investigations on the velocity, the volume
transport and the variations of the KC have been
addressed in numerous previous studies. Most of the
early studies have used the hydrographic
observations to estimate the temporal and spatial
variations of the KC, according to the geostrophic
calculation that assuming no motion in the deep
layers and neglecting the barotropic component of
the KC (Guo et al., 2012; Guo et al., 2013; Wei et al.,
2013). Later, on account of the availability of the
direct current measurements, the fine spatial
structures and the temporal variations of the KC
have been described in detail (Feng, 1999;
Yamashiro, 2008; Zhu et al., 2017).
The tide, as one of the most important physical
processes in the ESC, has been investigated for
several decades (Ogura, 1934), and it propagates to
the northwest in the TS against the KC. In
accordance with the direct current measurements
and the satellite altimetry analysis, besides the
barotropic (external) tide motions, the baroclinic
(internal) tides are also active in the ECS and its
adjacent water (Yamashiro, 2008; Zhu et al., 2017;
Niwa and T, 2004). It is widely accepted the
baroclinic tides can be an important sink of the
barotropic tide energy and play a significant role in
mixing in the deep layer (Munk and Wunsch, 1998;
Qiu et al., 2012).
Numerical models have suggested that the TS is
one of the most energetic straits in terms of
generation of internal tides around the Ryukyu-
Taiwan-Luzon island chain (Niwa and T, 2004;
Varlamov et al., 2015). The energy of the M
2
baroclinic tide converted from the barotropic tide is
Wang, B., Liu, T., Hirose, N. and Yamashiro, T.
Numerical Study of the Kuroshio Current in the Tokara Strait.
In Proceedings of the International Workshop on Environment and Geoscience (IWEG 2018), pages 395-399
ISBN: 978-989-758-342-1
Copyright © 2018 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved
395

subject to the local dissipation in TS (Niwa and T,
2004; Varlamov et al., 2015). The propagation speed
of the M
2
baroclinic tide is 3.5-4.5m/s around the
Ryukyu island chain, which is the same order of the
KC (0.75-1.5m/s). The advection effect of KC,
which is not represented in the early studies, is
considered to be important particularly around the
TK where the propagation direction of the baroclinic
tide is nearly parallel to the KC path (Niwa and T,
2004). The moored acoustic Doppler current profile
(ADCP) data set evidence that the internal tide in the
TS is greatly attributed to the third vertical mode
(Yamashiro, 2008; Zhu et al., 2017). It seems that
the internal tides interfere with each other and the
low frequency ocean circulation to create a
complicated wave pattern when it propagates
seaward. Recent development of the ocean modeling
allows the concurrent simulation of tides and lower-
frequency ocean circulation. Previous studies
suggested that there are active interactions between
the baroclinic tides and lower-frequency phenomena,
which result in the incoherent nature of the
baroclinic tide (Varlamov et al., 2015). The lower-
frequency ocean circulation affects the internal tides
has been investigated briefly. However, the effect of
tidal manipulation on the KC in this region,
including the horizontal advection and large
amplitude vertical displacement of isopycnal, still
has not been examined so far. Furthermore, the
previous simulated results indicated that the
baroclinic dissipation rate is more dependent on the
resolution of the bottom topography (Niwa and T,
2004), though only the rough estimate of the
dependence based on the empirical relationship
between the baroclinic energy conversion rate and
the horizontal resolution has been done. Thus, the
main purpose of this study is to clarify the effects of
tides on the KC in the TS at a high resolution.
This paper is organized as follows. Section 2
provides a description on the tide-resolving ocean
general circulation model for the Kuroshio region
with an extremely high resolution in TS, and reports
the ship-mounted ADCP data which are used to
validate the simulated results. Section 3 describes
and validates the simulation results. The final
section is devoted to summary of the present study.
2 MODEL CONFIGURATION
AND DATA
Niwa and Hibiya (Niwa and T, 2004) suggested the
resolution of the bottom topography might be a key
factor to predicate the baroclinic dissipation rate of
M
2
internal tide. Thus, the Z-coordinate ocean
model might have advantage to attain the goal of
present study. A high resolution regional circulation
model based on the Research Institute for Applied
Mechanics (Kyushu University) Ocean Model
(RIAMOM) is adapted to the southern coast of
Japan, named DREAMS_Energy (shorten as DR_E,
(Liu et al., 2018)), which is a 3D primitive equation
ocean model adopting the Arakawa B-grid and Z-
coordinate.
The model covers southwest of Japan with the
horizontal resolution of 1/60◦ longitude by 1/75◦
latitude and 33 layers in vertical (Figure 1a). The
detail configuration of model can be found in Liu et
al (Liu et al., 2018). To exam the horizontal
resolution effect on the Kuroshio Current, another
higher resolution model (1/180◦ൈ1/225◦) DR_T also
has been set up (Figure 1b). This model topography
is averaged from the JTOPO30 (~1km) and J-
EGG500 (500m). The initial and boundary
conditions are determined by the simulated results of
the DR_E. The other conditions of this model follow
DR_E. As compared experiment, another
experiment excluding tides, named as DR_T', has
been designed (Table 1). The analyzed period is
from 1 April 2012 to 30 September 2015. The
moving vessel ADCP data along the ferry line
between Kagoshima and Naha provided by the
Kagoshima University Faculty of Fisheries are used
Figure 1: (a) Bottom topography of the large
(DR_E) and (b) small (DR_T) models.
IWEG 2018 - International Workshop on Environment and Geoscience
396

to validate the simulated results. The observed
sections in the TS have been shown in Figure 2.
Table 1: A list of experiments.
Experiments
Horizontal
resolution
Forcing
DR_E
1/60◦ൈ1/75◦ (~1.5km)
With
tides
DE_T
1/180◦ൈ1/225◦
(~500m)
With
tides
DR_T'
1/180◦ൈ1/225◦
(~500m)
Without
tides
3 RESULTS
3.1 Current Validations
The simulated temperatures of DR_E are compared
with the observation data. And the basic features of
the temperature distributions are represented well by
the present model, which has not been shown
because of paper length.
Figure 2a and Figure 2b show the ADCP
observation and simulated Kuroshio Current pattern
of DR_E model on Oct. 21, 2014 at 50m in the TS,
respectively. On account of the hourly output of the
simulation results, which has the certain time
deviation with the ADCP observation, the
magnitude and directions of simulated current seem
to be a little different compared with the observation.
While, the spatial pattern of current has been
reproduced well. It suggests that the DR_E model
simulated the Kuroshio Current structure
realistically.
3.2 Horizontal Resolution Effect on
the Kuroshio Current
To investigate the horizontal resolution effect on the
Kuroshio Current, another higher resolution model
DR_T, which using approximate 500m mesh grids
size in horizontal direction. The simulated results of
DR_T will be addressed below. There are no
essential distinctions between DR_E and DR_T. The
strong current approaches the islands closer in
DR_T with stronger velocity (Figure 2c) compared
with that of DR_E (Figure 2b). Furthermore, the
sub-mesoscale eddies could easily be observed in
the higher resolution model results. Especially, after
the passage of the strong current through the narrow
channels between these islands, eddies could interact
to each other (Figure 3). In other words, the higher
resolution model could simulate more accurate
structure of the Kuroshio in the downstream of the
TS. It implies that the DR_E (1/60° × 1/75° mesh)
model already simulate the Kuroshio pattern
realistically.
(a)
(b)
Figure 3: Simulated magnitude of Kuroshio Current
during three years (a)DR_E model, (b) DR_T model.
(a)
(b)
(c)
Figure 2. (a) ADCP data on Oct. 21, 2014, (b)
simulated results of DR_E model and (c) DR_T
model.
Numerical Study of the Kuroshio Current in the Tokara Strait
397

3.3 Tidal Effects on The Kuroshio
Current
In order to investigate the tidal effects on the
Kuroshio Current, another experiment DR_T' is
designed, which excludes the tides from the open
boundaries and the other conditions are following
the DR_T. Compared to the model without tides
(DR_T'), the occurrence of the internal tides and
corresponding changes in the density fields are
found in the downstream of the TS in the results of
DR_T. As shown in Figure 4, the main axis of the
Kuroshio in the model with tides moves to the south
slightly (offshore) at the downstream of TS.
Furthermore, the stronger current (magnitude of
current speed > 60 cm/s) concentrates to the upper
layer. The vertical shear of the current becomes
larger and the Kuroshio becomes more baroclinic
after the consideration of the tide motions.
Nevertheless, the detailed mechanism for this
nonlinear interactions between the Kuroshio Current
and the internal waves must await further
investigation.
Figure 4: Magnitudes of the simulated Kuroshio Current
in summer along 130◦E for the DR_T model (color map)
and DR_T' model (contours).
4 SUMMARY
A high-resolution regional circulation model is
configured over the southern coast of Japan to
simulate the Kuroshio Current through the Tokara
Strait. The simulated results are in good agreement
with hydrographic and currentmeter observations.
The compared experiment suggested that the
simulated Kuroshio Current patterns are sensitive to
the horizontal resolution especially in the
downstream of the four-islands chain. Furthermore,
compared to the model without tides, the occurrence
of the internal tides and corresponding changes in
the density fields are found in the downstream of the
TS. The main axis of the Kuroshio in the model with
tides moves to the south slightly at the eastern part
of the Tokara Strait. Furthermore, there is
concentration of the stronger current (magnitude of
current speed > 60 cm/s) to the upper layer. The
vertical shear of the current becomes larger and the
Kuroshio becomes more baroclinic due to the tidal
effects. Nevertheless, the detailed mechanism for
this nonlinear interactions between the Kuroshio
Current and the internal waves must await further
investigation.
ACKNOWLEDGMENTS
This work was supported by National Natural
Science Foundation of China (Project 41706023),
the Natural Science Foundation of Jiangsu Province
(Grants No 20170871), the Fundamental Research
Funds for the Central Universities (2016B11514)
and the Open Fund of the Key Laboratory of Ocean
Circulation and Waves, Chinese Academy of
Sciences. The authors also would like to thank the
editor and the anonymous reviewers for their work
on this paper.
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