Geophysical Research Letters
RESEARCH LETTER
10.1002/2017GL076916
Key Points:• Internal tidal dynamics areinvestigated with a high-resolution
numerical model• Tidally induced internal waves explain
92% of the total internal wave ﬂux
over the Izu-Ogasawara Ridge, off theJapan mainland• Approximately 30% of the total kinetic
energy is excited by tidal forcing over
the ridge

Supporting Information:• Supporting Information S1
Correspondence to:
E. Masunaga,eiji.masunaga.ofﬁce@vc.ibaraki.ac.jp

Citation:
Masunaga, E., Uchiyama, Y., Suzue, Y., &Yamazaki, H. (2018). Dynamics ofinternal tides over a shallow ridge
investigated with a high-resolution
downscaling regional ocean model.
Geophysical Research Letters, 45,
3550–3558. https://doi.org/10.1002/
2017GL076916
Received 21 DEC 2017
Accepted 27 MAR 2018
Accepted article online 31 MAR 2018
Published online 16 APR 2018

Dynamics of Internal Tides Over a Shallow Ridge InvestigatedWith a High-Resolution Downscaling Regional Ocean Model
Eiji Masunaga1, Yusuke Uchiyama2, Yota Suzue3, and Hidekatsu Yamazaki4

1

Center for Water Environment Studies, Ibaraki University, Hitachi, Japan, 2Department of Civil Engineering, Kobe
University, Kobe, Japan, 3CTI Engineering Co., Ltd., Nagoya, Japan, 4Department of Ocean Science, Tokyo University ofMarine Sciences and Technology, Tokyo, Japan

Abstract This study investigates the dynamics of tidally induced internal waves over a shallow ridge, the
Izu-Ogasawara Ridge off the Japanese mainland, using a downscaled high-resolution regional ocean
numerical model. Both the Kuroshio and tides contribute to the ﬁeld of currents in the study area. The model
results show strong internal tidal energy ﬂuxes over the ridge, exceeding 3.5 kW m1, which are higher thanthe ﬂuxes along the Japanese mainland. The ﬂux in the upstream side of the Kuroshio is enhanced by aninteraction of internal waves and currents. The tidal forcing induces 92% of the total internal wave energy
ﬂux, exhibiting the considerable dominance of tides in internal waves. The tidal forcing enhances the kinetic
energy, particularly in the northern area of the ridge where the Kuroshio Current is not a direct inﬂuence.
The tidal forcing contributes to roughly 30% of the total kinetic energy in the study area.

Plain Language Summary Internal waves are ubiquitous phenomena in oceans, and they play asigniﬁcant role in global ocean circulations and climates. However, details of internal waves have not been
understood well due to limitations of ﬁeld surveys. This study investigates the dynamics of tidally induced
internal waves (internal tides) over a shallow ridge using a high-resolution numerical simulator with ahorizontal resolution of 1 km. The model results show that tidal forcing dominates in internal wave energy
over the ridge, and an interaction of internal waves and currents enhances internal waves in the upstreamside of the Kuroshio. Internal tides explain 92% of the total internal wave ﬂux over the ridge. Tides
generate roughly 30% of the total kinetic energy in the study area.

1. Introduction
Tidally induced internal waves, namely, internal tides, are important phenomena in oceanic physicalprocesses. Internal tides enhance their nonlinearity as they propagate into shallow regions, resulting inpronounced mass transports (e.g., Masunaga, Arthur, et al., 2017) and turbulent mixing (e.g., MacKinnon &Gregg, 2003). Recent studies have revealed the dynamics and mechanisms of nonlinear internal tides inshallow regions, using both observational and numerical approaches. The observational studies have foundseveral processes associated with nonlinear or breaking internal tides in shallow slopes, resulting in strong
sediment resuspension and turbulent mixing due to strongly enhanced shear ﬂows (e.g., Cheriton et al.,2014; Masunaga et al., 2016). Although signals of nonlinear internal tides have occasionally been observedby mooring systems, it is generally difﬁcult to understand all the processes involved with internal tides,
including their generation, propagation, and dissipation, from spatially sparse observations. By contrast,
numerical simulations are better suited to investigate these processes. Recent modeling studies havesucceeded in examining shelf-scale generation, propagation, and dissipation processes of internal tides(e.g., Kang & Fringer, 2012; Masunaga, Fringer, et al., 2017; Varlamov et al., 2015).

©2018. American Geophysical Union.
All Rights Reserved.

MASUNAGA ET AL.

In this study, we conducted numerical simulations to investigate internal tidal dynamics over the
Izu-Ogasawara Ridge that extends southward from the Japanese mainland (Figure 1a). This shallow ridge islargely inﬂuenced by both the western boundary current (Kuroshio) and tides. The Kuroshio Current passes
over the ridge from west to east at a maximum speed of about 2 m s1 (e.g., Uchiyama, Kanki, et al., 2017).
Barotropic (BT) tidal waves are known to propagate from east to west over the ridge, which results in strong
tidal ﬂows. In addition, this area is known as an important area for maintaining marine ecosystems in the
Northwest Paciﬁc Ocean, because of strong mass transport and mixing over the shallow ridge (e.g.,
Hasegawa et al., 2004). Masunaga, Fringer, et al. (2017) reported that tidal ﬂows induce highly nonlinear internal tides in this area. It has also been reported that the intensity of internal tides over the ridge is much higher

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Geophysical Research Letters10.1002/2017GL076916

Figure 1. (a) Numerical domains and topography for the (left) ROMS-L1 and (right) ROMS-L2 models. The temporally averaged L2 results for 21 July 21 to 4 August
2013 from Run #1 for (b) the subsampled surface velocity vectors with their magnitudes in color and (c) sea-surface temperatures (color). The solid black andthin gray contours in (b) and (c) show the time-averaged sea surface height at intervals of 0.1 m with isobaths spaced at intervals of 500 m. The black dotted box
shown in (a) indicates the L3 domain used for the sensitive test described in the supporting information.

than it is along the Japanese mainland (Masunaga, Fringer, et al., 2017; Varlamov et al., 2015). Varlamov
et al. (2015) conducted numerical modeling of M2 internal tides with a horizontal resolution of 1/36°(~3 km), which may not be sufﬁciently ﬁne to reveal details of the internal tide dynamics, particularly incoastal areas. Thus, Masunaga, Fringer, et al. (2017) instead used a high-resolution, triangle unstructured
grid numerical model (SUNTANS) with greatly idealized forcings. High-resolution models with realistic
oceanic conditions are necessary for understanding the details and effects of internal tides over the ridge.
This study investigates the internal tidal dynamics over the Izu-Ogasawara Ridge, using a high-resolution
downscaled Regional Oceanic Modeling System (ROMS) with a staggered horizontal grid coordinate underrealistic oceanic and atmospheric forcings.

2. Model Conﬁgurations
Numerical simulations were performed with a double-nested conﬁguration, based on a research version of
ROMS (e.g., Shchepetkin & McWilliams, 2005). The outer domain (ROMS-L1) is embedded in the dailyaveraged Japan Coastal Ocean Predictability Experiment 2 (JCOPE2) oceanic reanalysis product (Miyazawa
et al., 2009). The horizontal grid spacing of 3 km and the vertically stretched 32 layers in the s-coordinate
are used for the L1 model. The L1 model is forced by realistic surface wind stresses, surface ﬂuxes, and riverdischarges (see Uchiyama, Suzue, & Yamazaki, 2017, for more details). The high-resolution L2 model with ahorizontal grid spacing of 1 km is nested in the L1 model using a one-way ofﬂine nesting technique (e.g.,
Mason et al., 2010). The L2 model consists of 800 × 674 horizontal cells with 40 vertical s-coordinate layers.
The open boundaries of the L2 model are forced by sea surface height and three-dimensional horizontal

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Geophysical Research Letters10.1002/2017GL076916

currents and scalars (temperature and salinity) from the parent L1 model output. In addition, BT tidal forcing
estimated from the TPXO 7.0 tide model (Egbert & Erofeeva, 2002) is imposed along the perimeter of the L2model. Baroclinic (BC) tides inclusive of internal tides are thus spontaneously computed in the L2 model withrealistic large-scale inﬂuences, such as those from the Kuroshio Current. The amplitudes and phases of thetidal forcing were corrected using Newton’s method to improve the reproducibility of tidal signals (see details
in the Appendix of Masunaga, Fringer, et al., 2017). The ocean surface is forced by hourly wind stress (from theJapan Meteorological Agency Grid Point Value Mesoscale Model) and monthly climatological surface heat,
freshwater ﬂuxes, and radiation ﬂuxes (from the National Oceanic and Atmospheric Administration
Comprehensive Ocean-Atmosphere Data Set, NOAA-COADS; Woodruff et al., 1987). Monthly averaged
climatological freshwater discharges are added as point sources of freshwater and mass from 31 river mouths
located along the coast in the L2 domain. The K-Proﬁle Parameterization mixed-layer turbulence model is
employed for computing vertical momentum eddy viscosity and scalar eddy diffusivity (Large et al., 1994).
Horizontal eddy momentum viscosity and scalar eddy diffusivity are set at 0 m2 s1, except for in the small
background hyper viscosity and diffusivity associated with an upstream-biased third-order advection scheme
in ROMS. To maintain numerical stability, the BC time step size is set to 30 s for the L2 model.
To investigate the effects of the tides, Kuroshio Current, and winds separately, three model runs were conducted in this study: Run #1 is the baseline case in which all the forcing (three-dimensional ﬂows and tides
along the open boundaries, winds, and heat and freshwater ﬂuxes at surface) is considered in the model
to be as realistic as possible, Run #2 is the same as Run #1 but without tides, and Run #3 is the same as
Run #1 but without tides and winds. The L2 model was spun-up from 29 December 2011 to 30 July 2013
without tides. Then, the tidal forcing was imposed starting on 1 July 2013, and the tidal computation was
carried out for 2 months. The model output from 21 July until 10 August is used in the following analyses,
and thus, the ﬁrst 3 weeks are excluded as the model spin-up with tides takes place. A sensitivity test of thehorizontal grid resolution (grid spacing of 1 km and 300 m) was also conducted to investigate effects of themodel resolution on numerical results. The sensitivity test showed that differences of the kinetic energy (KE)and internal wave ﬂux in these two model cases are only a few percent (see supporting information).
The oceanic model used in this study, ROMS, has extensively been validated for the Kuroshio regions in previous studies (e.g., Kamidaira et al., 2017; Uchiyama, Kanki, et al., 2017; Uchiyama, Suzue, & Yamazaki, 2017) todemonstrate that the model can reproduce the Kuroshio Current and mesoscale and submesoscale eddies
with high accuracy. The computed tidal elevations in the L2 model are compared with the observed dataat the Okada tidal gauge station located on Izu-Oshima Island (Figure 1a). The root-mean-square error (E)between the modeled and observed tides can be estimated by the following formula (Carter, 2010):
rﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

1 2
E¼
a þ a2M  aO aM cosðGO  GM Þ
2 O

(1)

where a is the tidal amplitude, G is the phase, and the subscripts O and M denote observations and predictions by the model, respectively. The estimated E for the four major tidal constituents—S2, M2, K1, and O1—is0.013, 0.004, 0.020, and 0.016 m, respectively. The relative error, E/aO, is 7.8%, 1.2%, 9.0%, and 10.0% for S2, M2,
K1, and O1, respectively (the mean error of 7.0%). BT surface tidal motions can be validate by using observed
data obtained from tidal gauge stations or global tidal models; however, it is difﬁcult to obtain large-scale BCinternal tidal motions due to the limitation of ﬁeld surveys. Furthermore, the area around the Izu-Ogasawara
ridge is known as a great ﬁshing ground; mooring surveys in this area are limited by ﬁshery communities.
Although the modeled data are not validated with BC internal motions, we assume that the model
performance is sufﬁcient for investigating internal tidal dynamics in the study area.

3. Internal Waves Over the Ridge
The model results show that the interaction of BT tides and the shallow ridge generates strong tidal ﬂows,
leading to internal tidal generations. Figures 2a–2d show time series of sea surface height, temperature,
and BC ﬂow components at the point shown by the red asterisk in Figure 1a. Large isothermal displacements,
accompanied by strong BC ﬂows, reaching 0.8 m s1, appear during the spring tides around 24 July and
7 August. The isothermal displacements exceed 50 m where the depth is ~120 m, which is approximately

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Geophysical Research Letters10.1002/2017GL076916

Figure 2. Results from Run #1. (a) Tidal elevation, (b) temperature, and the (c) eastward (zonal) and (d) northward(meridional) components of baroclinic velocity at the location indicated by the red asterisk shown in Figure 1a (right).
(e) Time-averaged and vertically integrated internal wave ﬂux, (f) the divergence of internal wave ﬂux, (g) the barotropic tobaroclinic conversion rate, and (h) the baroclinic dissipation rate. The gray contours in (e)–(h) are isobaths spaced atintervals of 500 m.

25 times larger than surface elevations. To investigate the energetics of internal waves, the BC internal wave
energy ﬂux is computed. The dominant BC ﬂux is expressed as follows (Kang & Fringer, 2012):
F E ¼ p0 u0 BC ;(2)

where p is the pressure, uBC is the BC velocity, and the prime denotes the perturbation component within
the internal wave frequency. The perturbation p0 and u0 BC are deﬁned for frequencies higher than
3.86 × 106 s1 (i.e., a period of less than 72 hr) to include tidal signals and their harmonics. Figure 2eshows the time-averaged and depth-integrated BC ﬂux, < F E >, where the over bar denotes a depthintegral operator, and the brackets indicate a time-averaging operator; Figure 2f shows the divergenceof the BC ﬂux, ∇H · < F E >, over 2 weeks (~one spring-neap cycle, 21 July 21 to 4 August). Strongly
enhanced internal wave ﬂuxes are distributed along the ridge. The intensity of the ﬂux exceeds
3.5 kW m1, which is consistent with the results found in Masunaga, Fringer, et al. (2017).

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Geophysical Research Letters10.1002/2017GL076916

Figure 3. (a) The maximum internal Froude number from Run #1, the (b) semidiurnal and (c) diurnal internal wave ﬂux from Run #1, (d) the internal wave ﬂux fromRun #2 (w/o tides), and (e) the internal wave ﬂux from Run #3 (w/o tides and winds). The gray contours are isobaths spaced at intervals of 500 m.

The high westward BC ﬂux appears in the upstream (i.e., western) side of the Kuroshio Current, with its direction opposing that of the Kuroshio Current (Figure 2e, ~137.5°E, 33°N). For surface gravity waves, it is wellknown that the wave amplitude increases when the wave direction is opposite to the direction of background currents (e.g., Mei, 1992). Recent idealized numerical experiments by Lamb and Dunphy (2018)
showed that an interaction of internal waves and background currents enhances the upstream internal wave
ﬂux from a ridge. The high BC ﬂux in the upstream side of the Kuroshio Current is supposed to be generated
by an interaction of internal tides and the Kuroshio.
The time-averaged and depth-integrated BT to BC (BT-BC) conversion rate (C) and BC dissipation rate (ε) are
given by (Kang & Fringer, 2012)< C > ¼ < ρ0 gW 0 >;(3)

< ε > ¼ ∇H · < F E >  < C >;(4)

where ρ0 is the perturbation density and W is the BT vertical velocity (W =  ∇H · [(d + z)UH], where d is thetotal depth and UH is the depth-averaged lateral BT velocity). The magnitude of both the BT-BC conversion
rate (< C >) and dissipation rate (< ε >) is high over the ridge. They show a patchy structure where ampliﬁed< C > and < ε > manifest next to each other, corresponding to negative and positive values of the divergence of the internal tides. This implies that the internal waves mostly dissipate where they are generated.
Therefore, the generated internal waves contribute to mixing processes near their local generation sites.
On the other hand, although the intensity is not high, the internal wave energy radiates away from the ridge,
for example, as the westward ﬂux on the upstream side of the Kuroshio Current. Such offshore-ward internal
wave propagations are believed to contribute to deep ocean mixing processes.
The shape of the isotherm displacements is vertically asymmetric, which suggests the appearance of nonlinear
internal waves that include high-frequency components (Figure 2b, especially during the spring tides, around
24 July and 7 August). Such nonlinear internal waves occur when they propagate along shallow regions, andthe internal Froude number (Fri = UH/ci, where UH is the horizontal BT ﬂow speed and ci is the internal wave
speed) exceeds the critical value of 1 (e.g., Hibiya, 1986; Masunaga, Fringer, et al., 2017; Venayagamoorthy &Fringer, 2006). The maximum Fri is plotted in Figure 3a, showing the critical value over the ridge where thetotal depth d is less than 500 m. Strong internal waves under the high Froude number condition are
anticipated to lead to highly nonlinear wave motions over the ridge, such as those shown in Figure 2b.
In order to investigate the frequency responses of internal waves, the BC ﬂux was computed from band-passﬁltered data for the frequencies of semidiurnal (~M2 and S2) and diurnal (~K1 and O1) bands (Figures 3b and3c). The semidiurnal BC ﬂux shows strong internal waves over the ridge, and its spatial structure explains that
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Geophysical Research Letters10.1002/2017GL076916

of the total internal wave ﬂux shown in Figure 2e. The magnitude of the semidiurnal (diurnal) BC ﬂux contributes to ~73% (~5%) of the total internal wave ﬂux integrated over the ridge. Because the study area islocated in midlatitudes, the diurnal waves can propagate only as Kelvin waves trapped along shallow topography. Masunaga, Fringer, et al. (2017) reported that the resonance of the internal Kelvin waves (i.e., diurnalinternal tides) occurs around an island located on the Izu-Ogasawara Ridge if the wavelength matches theisland’s scale, which results in a largely enhanced, diurnal, internal tidal energy. However, the intensity ofthe diurnal internal tides is much lower than that of the semidiurnal internal tides in this result. The
stratiﬁcation (i.e., buoyancy frequency) in our simulation period may not satisfy the resonance condition ofisland-trapped internal tides.
Internal waves are generated not only by tides but also by strong geostrophic ﬂows (inertial currents, e.g., theKuroshio Current) and winds. Recent studies have suggested that near-inertial internal waves play importantroles in the vicinity of the Kuroshio Current (e.g., Nagai et al., 2015). In addition, winds are known to besources of internal waves (e.g., Kitade & Matsuyama, 2000). In order to evaluate the effects of tides, theKuroshio Current, and winds on internal wave dynamics, we compare the BC ﬂuxes from the three model runs
(Figures 2e and 3d and 3e). The BC ﬂuxes for Runs #2 and #3 are much lower than the ﬂux in the run with tidal
forcing (Run #1). For the case without tidal forcing (Run #2), although the internal wave energy ﬂux reaches~1.5 kW m1 in the Kuroshio Current path around the latitudes of 32–33.5°N, the ﬂux over the ridge is much
lower than it is for the case with tidal forcing (Run #1). The spatially averaged internal wave ﬂux over the ridge
for Run #2 (without tidal forcing) is 8% of that for Run #1, in which ~92% of the internal wave ﬂux is inducedby the tidal forcing. Therefore, the intensity of the internal tides is much higher than the intensity of the internal waves due to the Kuroshio Current and winds (near the inertial internal waves and wind-induced internal
waves). The 92% of the total internal wave ﬂux caused by tides is 14% higher than the sum of semidiurnal(~M2) and diurnal (~K1) internal wave ﬂuxes (78%). This difference implies that nonlinear or higher harmonicinternal tides contribute to 14% of the total internal wave ﬂux.
The spatially averaged internal wave ﬂux for the case without tides and wind (Run #3) is slightly higher (6%)
than for the case without tides (Run #2). This suggests that winds do not enhance the internal waves, butrather act to dissipate their energy. The vertical eddy viscosity and diffusivity estimated by the K-ProﬁleParameterization model are considerably reduced by design if wind stress is omitted, because eddy viscosityand diffusivity are computed based on wind stress and buoyancy ﬂux at the ocean surface (e.g., Large et al.,1994). This condition results in small vertical eddy viscosity and diffusivity that cause less damping of theinternal waves for Run#3.

4. Kinetic Energy
The time-averaged KE density (< ρu2 =ð2dÞ > , where ρ is the water density) is plotted in Figure 4(Figures 4a and 4b show the results from Runs #1 and #2, respectively). The Kuroshio Current causes highKE density along its path. The highest KE density of ~103 J m3 appears where the Kuroshio Current
passes over the shallow ridge at approximately 33°N and 140°E, with a maximum ﬂow speed of approximately 2 m s1. The KE density is also high around the islands located on the northern ridge at around
34–35°N and near the mouths of the Tokyo Bay and Ise Bay for the tidal forcing case (Run #1). On theother hand, the KE density in these areas is lower in the case without tidal forcing than it is for the tidal
forcing case. The differences in the KE are represented in Figure 4c as the ratios of the KE density between
the case with the tidal forcing (Run #1, KETide) and the case without the tidal forcing (Run #2, KEw/o Tide).
Figure 4c demonstrates that the tidal forcing enhances the KE by a factor of 10 over the northern ridge.
Thus, tidal forcing dominates the KE over the northern ridge where the Kuroshio Current does not havedirect inﬂuences. Nevertheless, the difference between the KE of Runs #1 and #2 is rather small along
the Kuroshio Current path, suggesting that the Kuroshio Current has a more dominant effect on theincreased KE in this area than do the tides.
The KE is divided into the BT and BC components as follows:

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KETotal ¼ KEBT þ KEBC þ KEBTBC

(5)


1 
KEBT ¼ ρ U2 þ V 2
2

(6)

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Figure 4. The kinetic energy (KE) density for (a) Run #1 and (b) Run #2 (w/o tides), (c) the ratio of the depth-integrated KE
between Runs #1 and #2, and (d) the volume-integrated KEBT (white bars) and KEBC (yellow bars) over the ridge area
denoted by the black dotted box. (e) Same as (d), but in the area shallower than 500 m depicted by the thick gray contours18in (c). The numbers labeled on the bars in (d) and (e) are the integrated KEBT and KEBC for each case (units of ×10 J).
Note that KEBT and KEBC are deﬁned by equations (6) and (7).


1  2
2
KEBC ¼ ρ u0 þ v 0 þ w 2
2
KEBTBC ¼ ρðUu0 þ Vv 0 Þ

(7)
(8)

where U and V are depth-averaged BT lateral velocities, u0 and v0 are horizontal BC velocities, and KEBTBC
represents the cross term eliminated by the depth integration (Kang & Fringer, 2012).
Figure 4d shows the time-averaged and volume-integrated KEBT and KEBC over the ridge, represented by theblack dotted box in Figures 4a–4c for the three cases. Figure 4e is the same as Figure 4d, but only for areas
shallower than 500 m, depicted by the thick gray contours in Figure 4c. The more types of forcing the model
includes, the more total KE increases. In Figure 4d, KETotal for Run #3 is about 67% of that for Run #1, suggesting that 2/3 of KETotal in the entire ridge area is generated neither by tides nor by wind, but by the Kuroshio
Current. KEBC for Run #3 explains 36% of the KETotal, presumably due to the baroclinicity associated with thevertical shear of the Kuroshio Current (e.g., Uchiyama, Kanki, et al., 2017) rather than the internal waves, as
shown in Figure 3e. From the difference between Runs #3 and #2, the wind forcing is found to explain
approximately 13% of the KETotal for Run #1, which mainly contributes to KEBT. Similarly, the tidal forcing signiﬁcantly enhances both KEBT and KEBC, which respectively explain 12% and 8% of the KETotal for Run #1. The
increase in KEBC by the tidal forcing can apparently be attributed to energetic internal tides.
In the shallow areas (Figure 4e), the relative contributions from the winds and tides to KETotal are greater thanthose in the whole ridge area shown in Figure 4d. Winds and tides (BT + BC) explain 26% and 29% of KETotal
for Run #1, respectively, whereas the contributions of BT and BC (internal) tides are 19% and 10%, respectively. These results demonstrate that both the local wind stress and BT tides, which are ampliﬁed aroundthe ridge topography, more signiﬁcantly contribute to the KETotal in the shallow area. The contribution of
the internal tides is signiﬁcant by similar amplitude in both depth ranges. The contribution is 10% for theshallow area, as shown in Figure 4e, whereas it is not considerably reduced in the deep ocean (at 8%) forthe entire ridge area in Figure 4d.

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5. Summary
High-resolution numerical experiments were conducted with a double-nested JCOPE2-ROMS downscaling
system to investigate the internal wave dynamics over a shallow ridge inﬂuenced by tides and theKuroshio Current. We chose to investigate the Izu-Ogasawara Ridge that extends southward from theJapanese main island and crosses the Kuroshio Current path. The model realistically showed details of
the internal wave structure over the ridge and revealed that 78% of the total internal wave (BC) ﬂux over
the ridge is dominated by semidiurnal (~S2 and M2) internal tides. Meanwhile, the contributions fromdiurnal (~K1 and O1) tides, winds, and geostrophic ﬂows (e.g., the Kuroshio Current) to the internal waves
are much less than those from the semidiurnal internal tides. Internal tides explain 92% of the total internal
wave ﬂuxes. The internal Froude number exceeds the critical value, which results in highly nonlinear internal
wave motions on the shallow ridge. The internal tidal ﬂux in the upstream side of the Kuroshio is enhanced byan interaction of internal tides and currents. Approximately 30% and 10% of the total KE are generated by thetides (BT + BC) and internal tides (BC), respectively. Especially over the shallow northern ridge, the tidal
forcing enhances the KE by a factor of 10.
The tidal forcing is sometimes neglected in developing global/regional oceanic models to simplify model
conﬁgurations; however, this study indicates that tidal forcing contributes to roughly 30% of the total KE overa shallow ridge. In addition, internal tides dominate approximately half of the tidally induced KE, and theyalso contribute to deep ocean mixing processes. This study suggests that tides and internal tides playsigniﬁcant roles in oceanic physical processes, and they must be considered to better understand, forexample, energy budgets and mixing processes in the ocean.
Acknowledgments
This study was supported by JST-CREST
grant JPRMJCR12A6. E. Masunaga
acknowledges the JSPS Grant-in-Aid for
Scientiﬁc Research (15K06227).
Y. Uchiyama also acknowledges JSPS
Grant-in-Aid for Scientiﬁc Research(15KK0207 and 15H04049). The ROMS
model used in this study is distributedby its developers and is available athttps://www.myroms.org/. The JCOPE2
reanalysis data are available for scientiﬁc research purposes from Japan
Agency for Marine-Earth Science andTechnology (JAMSTEC) and are available at http://www.jamstec.go.jp/jcope/
htdocs/e/distribution/index.html. The
JMA GPV-MSM data set is provided by
Japan Meteorological Agency (JMA) andis available at http://database.rish.
kyoto-u.ac.jp/arch/jmadata/data/gpv/
original/. The NOAA-COADS climatological data set is distributed by NOAA at
https://www.esrl.noaa.gov/psd/data/
gridded/data.coads.1deg.html. TPXO
tidal data set is available at http://
volkov.oce.orst.edu/tides/.

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