Deep-Sea Research Part I 132 (2018) 80–93

Contents lists available at ScienceDirect

Deep-Sea Research Part I
journal homepage: www.elsevier.com/locate/dsri

Impacts of two super typhoons on the Kuroshio and marginal seas on the
Paciﬁc coast of JapanT⁎

Hiroaki Tadaa,1, Yusuke Uchiyamaa,c, , Eiji MasunagabaDepartment of Civil Engineering, Kobe University, Kobe, Japan
Center for Water Environment Studies, Ibaraki University, Hitachi, Japanc
Coastal and Estuarine Environmental Department, Port and Airport Research Institute, Yokosuka, Japanb

A R T I C L E I N F O

A B S T R A C T

Keywords:
TyphoonKuroshio
Near-inertial oscillation
Eddy shedding
Topography
Regional Oceanic Modeling System

High-resolution downscaling ocean modeling was conducted to investigate the impacts of two super typhoons on
the Kuroshio in the fall of 2014 oﬀ the Kyushu and Shikoku Islands, Japan. The model result was compared withﬁeld observations and satellite altimetry. The synoptic and mesoscale oceanic structures around the Kuroshio
exhibit a good reproducibility. The typhoons generated near-inertial oscillations (NIOs) and near-inertial internal waves (NIIWs) around the Kuroshio path, particularly on the right side of the typhoon tracks. The NIOs
developed in the mixed layer to alter the direction of the Kuroshio by ~30°. The associated velocity oﬀ the
Shikoku and Kyushu Islands was signiﬁcantly decelerated by ~0.2 ms−1. The velocity almost vanished oﬀ
Kyushu Island and thus induced an unstable ﬂuctuating path shortly after both typhoons passed over that region.
The NIIWs were also excited at the thermocline, resulting in the oscillation of the Kuroshio path occurred in theentire water column. In contrast, oﬀ Shikoku Island, the typhoons shifted the Kuroshio path northward to enhance the interactions with the topographies. This shift caused considerable eddy shedding from the capes that
resulted in mesoscale counterclockwise circulations as cyclonic quasi-standing eddies with a shedding period of~3 days in the north of the Kuroshio path. The magnitude, direction, and meridional location of the path of theKuroshio prominently ﬂuctuated with the propagation of these eddies, manifested oﬀ Shikoku Island.
Furthermore, these eddies induced sporadic northward intrusions of the Kuroshio warm water through the Kii
Channel into the Seto Inland Sea (SIS), where a weak but persisting southward outﬂow prevails under normal
conditions. Therefore, the process could collectively be called the “typhoon-Kuroshio-eddy interaction”, which
conceptually diﬀers from the “typhoon-eddy-Kuroshio interaction” in the previous studies, where the Kuroshio
was modulated by eddy collision. The wind stress curl and intrusions associated with the typhoons jointly
provoked the inversion of the counterclockwise SIS residual circulation. The resultant spatially averaged volume
ﬂux was ~8 times as high as that under normal conditions.

1. Introduction
The Kuroshio is the western boundary current of the North Paciﬁc
Subtropical Gyre, which transports massive amounts of heat and salinity from lower latitudes to middle and high latitudes. The Kuroshio
generally ﬂows on the continental shelf break around Japan, while its
path is not always persistent but rather transient. For instance, theKuroshio typically takes either of the three paths oﬀ the Cape
Shionomisaki (Fig. 1): 1) nearshore straight (i.e., non-large-meander),2) oﬀshore straight, or 3) large-meander paths (e.g., Kawabe, 1985,
1995; Akitomo et al., 1991; Miyazawa et al., 2008, 2009). At the Tokara
Strait, the Kuroshio initiates its meandering either by a large inﬂow⁎

1(e.g., Nakamura et al., 2006), a small path meander (e.g., Kawabe, 1995;
Miyazawa et al., 2008), low potential vorticity (PV) water (e.g., Qiu andMiao, 2000), or high PV water (e.g., Akitomo and Kurogi, 2001).
The Kuroshio and its path are potentially inﬂuenced by typhoons.
Typhoons magnify the vertical mixing and Ekman transport due to thestrong surface wind stress, decrease the sea surface temperature (SST),
and alter the upper-ocean circulations (e.g., Yang and Sheng, 2008).
Inertial oscillations associated with typhoons are also known to be intense with velocity magnitudes as large as 1 ms−1 (e.g., Price, 1981;
Church et al., 1989), which is comparable to the mean velocity of theKuroshio. Based on the surface currents observed by radars, Morimoto
et al. (2009) reported that typhoons occasionally alter the current

Corresponding author at: Department of Civil Engineering, Kobe University, Kobe, Japan.
E-mail addresses: a8365@n-koei.co.jp (H. Tada), uchiyama@harbor.kobe-u.ac.jp (Y. Uchiyama), eiji.masunaga.oﬃce@vc.ibaraki.ac.jp (E. Masunaga).
Present address: Nippon Koei Co. Ltd., Tokyo, Japan.

https://doi.org/10.1016/j.dsr.2017.12.007
Received 17 March 2017; Received in revised form 22 December 2017; Accepted 24 December 2017
Available online 29 December 20170967-0637/ © 2017 Elsevier Ltd. All rights reserved.

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Fig. 1. The study area with bathymetry in color (m).
(a) Entire JCOPE2 domain with the embedded child
ROMS model domain (black box). (b) Blow-up of the
ROMS domain indicated by the black box in (a)corresponding to the outer black box in (b). (c) Blowup around the Seto Inland Sea (SIS) shown by the
inner black box in (b). The black dashed box in (c)indicates the approximate extent of the SIS. In (b),the best tracks of Phanfone (yellow line) andVongfong (red line) are overlaid; the typhoon days in
October 2014 in UTC are indicated. The black boldline in (b) represents the JMA ASUKA Line transect
used for Fig. 3. In (c), the red bold lines with the redcapital letters A-L are the transects used for Fig. 11;
with the lowercase letters a-i are the segments used
for Table 2. (For interpretation of the references tocolor in this ﬁgure legend, the reader is referred to
the web version of this article.)

Channel based on the SST composite maps (Section 5.1). We aim toclarify the oceanic responses to the two typhoons and pay particularattention to their impact on the Kuroshio and the SIS circulation. We
conducted retrospective, high-resolution downscaling ocean modelingusing the Regional Oceanic Modeling System (ROMS; Shchepetkin and
McWilliams, 2005, 2008) embedded in the Japan Coastal Ocean Predictability Experiment version 2 (JCOPE2) assimilative oceanic reanalysis (Miyazawa et al., 2009) forced by the high-resolution Grid
Point Value-Mesoscale Model (GPV-MSM) assimilative atmosphericproduct (e.g., Isoguchi et al., 2010). The ROMS model domain extends
over a fairly wide area, particularly southward, to encompass typhoons
from their early stage at lower latitudes. The ROMS horizontal grid
spacing is 2 km, which marginally resolves the complicated topography
and islands in the SIS.
The remainder of this paper is organized as follows. The model
conﬁguration is described in Section 2, followed by a comparison of themodel results with ﬁeld observations and satellite altimetry for validation in Section 3. We estimate the development of near-inertial oscillations (NIOs) and near-inertial internal waves (NIIWs) due to thetyphoon, mainly in the open ocean in Section 4. Subsequently, inﬂuences of the typhoons on the Kuroshio path and the adjacent coastal
waters including the SIS are discussed in Section 5. In Section 6, weanalyze the possible mechanisms of the typhoons altering the Kuroshio
path and the resultant topographic eddy shedding that recursively feeds
back to modify the Kuroshio at the continental shelf margins. Then the
conclusions are given in Section 7.

speed, direction, and axis of the Kuroshio oﬀ Taiwan due to the geostrophic currents enhanced by the sea surface height (SSH) gradientassociated with the typhoon passages. Sun et al. (2009) reported thatthe typhoon-induced localized upwelling intensiﬁes the preexisting
cyclonic eddies, giving rise to a large meander of the Kuroshio oﬀ
Shikoku Island. Such alterations of the Kuroshio path aﬀect not only thecoastal environment and ecosystem (e.g., Nakata et al., 2000; Yanget al., 2010; Sun et al., 2012) by changing the water temperature andSSH (e.g., Kawabe, 1985) but also the heat balance in the lower atmospheric boundary layer through the intensive eddy-current and airsea interactions along the Kuroshio path (e.g., Qiu, 2003; Taguchi et al.,2005; Tanimoto et al., 2009; Tokinaga et al., 2009), which consequently leads to feedback on the intensity and fate of the typhoons.
The transient Kuroshio substantially inﬂuences an extensive portionof the coastal waters on the Paciﬁc side of Japan, including the Seto
Inland Sea (SIS). The SIS is the largest semi-enclosed estuary in thenation, zonally stretching over ~450 km, with widths of 15–55 km,
bounded by the Honshu, Shikoku, and Kyushu islands (Fig. 1). The
Bungo Channel to the west and the Kii Channel to the east connect the
SIS to the Paciﬁc. The transient Kuroshio occasionally causes sporadic
intrusions of the Kuroshio into the Bungo Channel (Isobe et al., 2010),
which is often referred to as “kyucho” (Takeoka and Yoshimura, 1988).
The strong currents and abrupt temperature increase associated withkyucho result in serious damage of ﬁshery nets and ﬁsh cultivations(e.g., Takeoka and Yoshimura, 1988; Matsuyama et al., 1997). In addition, typhoons pass over or around the SIS several times a year,
mostly every fall. Hence, possible dynamical impacts of typhoons on the
Kuroshio and coastal water around the SIS due to the shift of the Kuroshio path are anticipated; however, they are poorly understood.
In the present study, we focus on two consecutive super typhoons
that passed over and close to the SIS in the fall of 2014. The ﬁrst one is
Typhoon Phanfone with an intensity of category 4 and the lowest sea
level pressure (SLP) of 935 hPa; the second one is Typhoon Vongfong,the most powerful typhoon in 2014, with category 5 and the lowest SLPof 900 hPa. Both are known to have altered the Kuroshio path oﬀ the Kii

2. The model
The hydrodynamic model we use is the ROMS, a 3D hydrostatic
Primitive equation ocean circulation model based on the Boussinesq
approximation. To accurately account for the eﬀects of the typhoons,the inverse barometer associated with the SLP anomaly is introduced in
the pressure gradient term. The GPV-MSM assimilative atmosphericproduct of the Japan Meteorological Agency (JMA) with a horizontal
81

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model validations (e.g., Uchiyama et al., 2017a, 2017b). The near-surface velocity magnitude and the Kuroshio path visualized by the timeaveraged geostrophic current estimated from the AVISO-SSH exhibit areasonable agreement with those of the JCOPE2 and ROMS (Fig. 2a, left
column).
The SSH variance (Fig. 2b, middle column) is regarded as a proxythat measures the intensity of the temporal variability of synoptic andmesoscale signals mostly due to eddies and Kuroshio meanders. The
ROMS-derived SSH variance reproduces several important features withmagnitudes and spatial distributions equivalent to the AVISO data. For
instance, an energetic area commonly arises north of 29°N oﬀ thesoutheast coast of Kyushu Island between 131°E and 133°E, where thewestward-traveling Rossby waves and mesoscale eddies collide with the
topographic ridge, giving rise to a large meander of the Kuroshio. The
SSH variance is then enhanced pronouncedly in the wide area around
32°N between 137°E and 141°E oﬀ the Enshu-nada Sea, where the large
meanders of the Kuroshio often occur. On the other hand, the SSH
variance oﬀ Shikoku Island between 133°E and 136°E is relatively
small, demonstrating that the Kuroshio path oﬀ Shikoku Island is rather
stable.
Subsequently, the annual mean SST distribution is compared with
the Merged satellite and in situ data Global Daily SST (MGDSST) of
JMA, as shown in Fig. 2c (right column). The MGDSST is the gridded
composite SST product at 1/4° including the AVHRR sensor of NOAA
and MetOp satellites, AQUA/AMSR-E microwave satellite, and in situ
SST data (Sakurai et al., 2005). Overall, the ROMS SST agrees reasonably well with that of the MGDSST and JCOPE2 with respect to the SST
distribution including the location of the Kuroshio represented bywarmer biases. However, the ROMS SST is slightly underestimated byabout 1–2 °C compared with that of the MGDSST, particularly in thenorth of ~30°N on the Paciﬁc side of Japan. It is worth noting that theparent JCOPE2 model already contains this underestimation on which
the reproducibility of the ROMS model substantially depends (Section
2).
The modeled stratiﬁcation is compared with in situ observations in
the vertical cross section along the Aﬃliated Surveys of the Kuroshio oﬀ
Cape Ashizuri (ASUKA) Line transect (e.g., Imawaki et al., 2001), shownas black line in Fig. 1b. Ship-based hydrographic measurements along
the ASUKA Line have been collected by a total of 155 cruises sinceNovember 1992 to evaluate the seasonal climatology of the density
ﬁeld (Fig. 3). Again, the ROMS model successfully reproduces thestratiﬁcation for fall in terms of the depths and slopes of the isotherms
and isohalines, with meridional northward steepening of the slopes. A
minor discrepancy is observed for the subsurface salinity centered at
800-m depth, with an overestimation of ~0.1 PSU, whereas it arises
from the parent JCOPE2 model similarly to the SST bias (Fig. 2c). Because the density in this region is mainly determined by the temperature rather than by salinity, the inﬂuence of the salinity deﬁciency on
the general oceanic structure is presumed to be subtle.
The above validations are conducted rather in longer-time scales
than near-inertial periods (Section 4), due primarily to the lack ofsuﬃcient data. However, we have validated high frequency reproducibility of the similar modeling in Buijsman et al. (2012), Kumar et al.,(2015, 2016), Marchesiello et al. (2015) and Kamidaira et al. (2018),
albeit at diﬀerent sites and periods of time. In addition, the ROMS
model has been applied by other modelers to the Gulf Stream region
under hurricane conditions (e.g., Xue et al., 2015; Mooney et al., 2016),
as well as to the Kuroshio region under a typhoon condition (e.g., Glennet al., 2016) to conﬁrm a good agreement with observation. Therefore,
the present ROMS model is presumed to be capable of reproducing theoverall 3D oceanic structure reasonably well under a typhoon conditionin the study area.

resolution of ~5 km is used for the determination of the surface momentum, heat, freshwater, and radiation ﬂuxes based on the Coupled
Ocean/Atmosphere Mesoscale Prediction System (COAMPS; Hodur,1997) bulk formula. Note that the shortwave solar radiation is replacedwith the composite of the in situ data provided by JMA for better reproducibility. Tides, wind waves, and swells are omitted for simplicity.
The vertical eddy mixing of the momentum, heat, and scalar (salinity)in the surface and bottom boundary layers is evaluated with the Kproﬁle Parameterization (KPP) turbulence model (Large et al., 1994;
Durski et al., 2004).
The ROMS model domain horizontally extends 1840 km × 1440 km
with a lateral grid spacing of 2 km to encompass a suﬃciently wide areathat includes as much tracks of the two typhoons as possible (Fig. 1),
and is represented by vertically stretched 32 s-layers, where grid height
reﬁnement occurs near the surface to range O (0.1–2 m) for the topmost grid cells. The daily averaged JCOPE2 reanalysis (Miyazawa et al.,2009) with a lateral grid spacing of ~10 km is used as the initial andopen boundary conditions. The ROMS model is embedded in the
JCOPE2 to convey the basin-scale information into the child ROMS
domain with the one-way oﬄine nesting approach, as described in
Mason et al. (2010) and Uchiyama et al. (2017a, 2017b). The model
topography is obtained from the SRTM30_Plus (Rodriguez et al., 2005;
Becker et al., 2009), which covers the global ocean at 30 geographic arc
seconds or approximately 1 km supplemented by the J-EGG500 at aresolution of 0.5 km for the coastal regions. The other boundary condition, in particular the sea surface salinity (SSS) to correct the longterm bias in the freshwater ﬂux, is imposed using the monthly SSS
climatology of the NOAA-COADS (Woodruﬀ et al., 1987) dataset. The
major river discharges into the domain are considered based on themonthly climatology dataset provided by the Japan River Association.
Four-dimensional nudging (also known as robust diagnostic; e.g.,Marchesiello et al., 2003) with a weak nudging inverse time scale of 1/
20 day−1 is applied towards the 10-day averaged JCOPE2 3D temperature and salinity ﬁelds to enhance the reproducibility of the Kuroshio path. The ROMS model runs for 24 months including the initial
spin-up for 18 month. For the following analyses, we mainly utilize theresult for the three-month period from September 1, 2014. The numerical conﬁgurations are summarized in Table 1.
3. Validation
We compare the model results with the satellite altimetry, SST data,and in situ observations conducted by JMA to conﬁrm the validity andreproducibility of the present ROMS model. The mean structure andtemporal variability of the near-surface currents including the Kuroshio
and its path are obtained from the gridded composite of multiple satellite altimetry data compiled by AVISO (e.g., Traon et al., 1998). The
delayed-time SSH anomaly data is available daily, with a horizontal
spacing of 1/4°. Although this data set has recently been reprocessed as
the DUACS/AVISO DT14 (Pujol et al., 2016), we rely on the previous
version because it has extensively been used in the previous studies for
Table 1
Computational conﬁgurations for the present ROMS model.
Computational period
Grid cells
Horizontal grid resolution
Baroclinic time step
Sea surface ﬂuxes
Short-wave solar radiation
Major river discharges
Boundary and initial conditions
T-S nudging
Topography
Vertical eddy mixing

1/1/2013–12/31/2014
920 × 720 × 32 vertical s-layers
2.0 km120 s
JMA GPV-MSM (2 hourly) + COADS(monthly clim.)
JMA in situ observation
Japan River Association (monthly
climatology)
JCOPE2 (daily averaged)
JCOPE2 (10-day averaged)
SRTM30 (SIO) supplemented by J-EGG500
KPP model (Large et al., 1994)

4. Near-inertial motions driven by typhoons
We deﬁne ﬂuctuations in the model results for the 21–30 h period82

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Fig. 2. Plan view plots of the annually averaged (a) surface velocity vectors and their magnitude in color, (b) sea-surface height (SSH) variance, and (c) sea-surface temperature (SST) for2014. The top panels in (a) and (b) are based on the AVISO data, while the SST in (c) are based on the MGDSST. The middle panels are based on the JCOPE2, while the bottom panels
show the ROMS model result.

concentrated EKE is barely transported below the thermocline andmostly dissipates in the mixed layer above a depth of 100 m, where the
NIIWs develop. It is worth mentioning that the NIIWs in Fig. 4 mightslightly be underestimated in the higher latitude with larger f with our
choice of the frequency ﬁlter, which would be a bit narrow for encompassing modulations of the NIIWs as speciﬁed by the dispersion
relation.
To assess the generation, horizontal propagation, and dissipation of
the NIIWs, we calculate the baroclinic energy ﬂux (e.g., Nash et al.,2005) around the inertial period. The depth-integrated near-inertial
baroclinic energy ﬂux, F, can be estimated from a correlation between
the perturbation of the baroclinic velocity and pressure as follows:band as the near-inertial component because the inertial period in themodel domain ranges between 20.3–30.8 h. A Butterworth frequency
band pass ﬁlter is applied to extract the near-inertial band in themeridional and zonal components of the horizontal current velocities,
temperature, and salinity. Figure 4 indicates the instantaneous eddy
kinetic energy, EKE = (u′2 + v′2 )/2 , and temperature anomalies, T ′, atthe depths of 5, 30, and 100 m. Note that u and v are the zonal andmeridional velocities, T is temperature, and the prime denotes the
bandpass-ﬁltered near-inertial ﬂuctuating component. When Phanfone
comes close to Shikoku Island, EKE is enhanced near the surface down
to 30 m over a wide area, mostly on the right side of the track (Fig. 4a).
Two days later, the area with near-surface high EKE extends northeastward and increases in the coastal sea and in the open ocean(Fig. 4b). The EKE barely extends and reaches a depth of 100 m, evenafter Phanfone passed. In contrast, the temperature anomaly is more
apparent at 100-m depth, where deepening of the thermocline associated with the enhanced vertical mixing occurs along the typhoon
track (not shown here, but partially evident in Fig. 8). It is conceivablethat NIIWs with an amplitude of approximately 0.4 °C are generated
below the typhoon track. These results suggest that the surface-η′ p′dz ,F = ⎜⎛
ubc
−h
⎝

η

∫

∫−h vbc′ p′dz ⎞,(1)

p′ (z ) = p0 + g∫z ρ′ (zˆ, T ′, S′) dzˆ

(2)

⎟

⎠

η

where h is the depth; η is the sea surface height (SSH); (ubc , vbc ) are thebaroclinic component of the horizontal velocities deﬁned as residuals′ , vbc′ ) are theafter subtracting the depth-averaged velocity; (ubc83

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Fig. 3. Seasonally averaged (a) temperature and (b)salinity for fall from JMA in situ observations (top),
JCOPE2 (middle), and the present ROMS model(bottom) in the vertical cross section along the
ASUKA Line transect (Fig. 1b).

a result, it is anticipated that the Kuroshio path is altered by interactions with the energetic NIIWs associated with typhoons. A further investigation is needed for determining the mechanism in future works.

perturbation of (ubc , vbc ); p′ is the pressure perturbation; p0 is the SLP
assumed to be zero; g is the gravity; ρ is the density of the sea water, S is
the salinity; z is the vertical axis with the mean sea level as origin,upward-positive; ẑ is a dummy variable for the integral calculus; andthe prime denotes the bandpass-ﬁltered near-inertial component. Notethat the pressure work is about three orders of magnitude larger thankinetic energy ﬂux, and thus we omit the later in the present analysis.
The magnitude of both components of F averaged over the black
rectangular oﬀ Shikoku Island shown in Fig. 5b-c considerably increases for ~10 days after Vongfong passes and dissipates gradually formore than one month (Fig. 5a). The comparison of the spatial distribution of F and its divergence (div F) averaged for a non-typhoon
period (i.e., September 2014; Fig. 5b and d) and the period including
the two typhoons (i.e., October 2014; Fig. 5c and e) reveals that the
typhoons intensify F by ~1 kW m−1 in extensive areas, particularly on
the right side of the tracks, compared with the non-typhoon period. This
1 kWm−1 is dynamically substantial because it reaches approximately
half of the baroclinic energy ﬂux of energetic internal tides evaluatedfor Monterey Bay, California (Fig. 6 in Kang and Fringer, 2012). The
vectors of F oﬀ Shikoku Island show that the baroclinic ﬂux develops
mainly southward, as if it moves away from the Kuroshio and turns
clockwise (e.g., in the area of the black box). Conversely, the baroclinic
ﬂux F moves to the Kuroshio in the south of Kyushu Island at ~132°E
and 29°N while it is intensiﬁed by the two typhoons. This area corresponds to a bend of the Kuroshio path at which the ﬂow direction
considerably changes (Fig. 2a). The divergence of F (div F; Fig. 5e) is
consistently distributed with a strong convergence (i.e., negative div F)
near these two areas (denoted by the black and green boxes in Fig. 5e)
where the NIIWs are supposed to dissipate locally. For the non-typhoon
period, F along the Kuroshio is divergent in the north (e.g., K1), while it
is convergent in the south (e.g., K2) (Fig. 5d). This meridional div F
pattern is enhanced for the typhoon period with an evident appearanceof the positive div F to generate the southward propagating NIIWs in
the south of the Kuroshio at ~135°E and 32°N (northwest area in theblack box). In turn, the positive div F is not apparent in the south of
Kyushu Island, where a negative div F is predominant. Therefore, the
NIIWs lose their energy around the Kuroshio oﬀ Kyushu Island, whereas
they extract baroclinic energy from the Kuroshio oﬀ Shikoku Island. As

5. Inﬂuences on the Kuroshio and coastal waters5.1. Modiﬁcation of the Kuroshio by the typhoons
The observation supports that a typhoon can alter the path andstructure of the Kuroshio oﬀ Shikoku Island through interactions of thetyphoon with the associated eddies (i.e., the typhoon-eddy-Kuroshio
interaction; Sun et al., 2009). The ﬁshery department of the Mie Prefecture, Japan, releases the daily averaged Kuroshio path oﬀ the Kii
Channel inferred from satellite and in situ surface temperature andcurrent data including those reported by ﬁshery boats and research
vessels, as shown in Fig. 6 for the period of the two typhoons. The
Kuroshio path largely meanders northward to be oriented towards theKii Channel after the passage of both typhoons. Fig. 7 shows themonthly averaged surface velocity vectors and their magnitude for the
non-typhoon (September 2014) and typhoon (October 2014) periods
based on the model. The faster ﬂow shown by reddish colors represents
the Kuroshio path, which decelerates by ~0.2 ms−1 due to the twoconsecutive typhoons on the south and east coasts oﬀ Kyushu Island
encompassing the oﬀshore area of the Bungo Channel around 132°E and
32°N. The deceleration is also evident in the time series of thestreamwise northward velocity averaged over the EK Line (Fig. 8a).
Immediately after Phanfone passes the EK Line on October 5, thestreamwise velocity begins to oscillate pronouncedly in and below themixed layer at a period of ~24 h (i.e., near-inertial period). The mixed
layer has deepened since October 3 to 75–80 m with ﬂuctuations at thenear-inertial period. The velocity averaged in the vertical, from thesurface to the depth of 80 m (Fig. 8c), also demonstrates near-inertial
periodic ﬂuctuations after the passage of Phanfone, with an apparent
deceleration bias of approximately −0.4 ms−1. This deceleration is nota near-surface process, but occurs rather barotropically down to deeper
than 400 m at least, with modest deepening of the Kurhoshio core from~80 m to ~100 m depth (not shown). In contrast, the Kuroshio is lesstransient oﬀ Tosa Bay (TB Line, Fig. 8b and d), suggesting that the84

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Fig. 4. Snapshots of the (top) eddy kinetic energy (EKE), and (bottom) temperature anomalies in color at the depths of 5, 30 and 100 m, from left to right, (a) on October 5 when Phanfone
approached Shikoku Island, and (b) two days later. Both the black and white contours are the sea-level atmospheric pressures (SLPs) from the GPV-MSM. The broken line is the best trackof Phanfone.

is pronounced in the northern part of the channel associated withcounterclockwise circulations shown by bluish colors. The surface relative vorticity normalized by the Coriolis parameter (ranging from5.66 × 10−5 to 8.58 × 10−5 s−1) ﬂuctuates with the velocity; positive(negative) vorticity is predominant in the north (south) of the Kuroshio(Fig. 9d). The typhoons also signiﬁcantly reduce the temperature of theKuroshio at lower frequency, more evidently than at the inertial frequency (Fig. 9e), in response to the slowly evolving nearshore recurring
circulations. In addition, warm water of the Kuroshio occasionallypours into the channel after the typhoons, as visualized by the observed
path shown in Fig. 6. These intrusion events are quantiﬁed by evaluating the northward heat ﬂux q across the cross section along the MS
Line (Fig. 7b).

Kuroshio oﬀ the eastern Kyushu Island is readily aﬀected by the typhoons. This is presumably due to the Kuroshio passing by the topography around Cape Ashizuri, which sharpens the Kuroshio front mechanically due to the northward shift of the Kuroshio by the typhoons(Fig. 6). The shift results in the intensiﬁed pressure gradient force due
jointly to the increased SSH and density gradients that promotes
geostrophic zonal acceleration of the Kuroshio and thus stabilizes its
path.
5.2. Intrusion of the Kuroshio into the Seto Inland Sea (SIS)
We subsequently analyze the intrusions of the Kuroshio due to thetyphoons oﬀ the Kii Channel along the KC Line (Fig. 9). The marching
typhoons increase the SSH such that the Kuroshio path shifts northward
to approach the channel (Fig. 9a). The NIOs are initiated prominently inthe oﬀshore region after the passages, as visualized in the velocities(Fig. 9b-c), whereas lower frequency variability at periods of 2–3 daysq=

∫A ρCp un T dA,(3)

where q is the heat ﬂux deﬁned as northward positive, Cp is the speciﬁcheat capacity, un is the northward velocity normal to the MS Line, and A
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Fig. 5. (a) Time series of the depth-integrated baroclinic energy ﬂux F from September 1 to November
30, 2014, averaged over the subdomain shown in (b)and (c). The red line is the zonal, eastward (u)component of F (Fx) and the blue line is the meridional, northward (v) component (Fy). The timeaveraged F vectors are shown in for (b) the non-typhoon period (Sepember 1–30, 2014, orange shadein (a)), and (c) the period that includes the two typhoons (October 1–31, 2014, red shade in (a)) with
their magnitude in color. (d)–(e) are the same as(b)–(c), but for the divergence of F (div F). The boldbroken lines in (a) correspond to the times when
Phanfone (left) and Vongfong (right) were closest to
the analysis area. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)signiﬁcant temporal variations. Noticeable positive incoming eddy heatﬂux appears only after the typhoons pass. The variance-preserving
power spectrum of q during the typhoon period from September 28 to
October 22 has three distinctive peaks at around periods of ~0.5, 1, and2 days (Fig. 10). The peak at 2 days corresponds to low frequency
variability of the counterclockwise circulation in the Kii Channel, northof the MS Line. The intermediate peak at 1 day is the inertial frequency,
representing the NIOs. The other peak is in the semi-diurnal period,
coinciding with the ﬂuctuations that occur after Vongfong passes

is the area of the cross section. We further introduce a decomposition q= < q > + q′′, where q′′ is the eddy (high-frequency) component
and < q > is the mean (low-frequency) component with a cut-oﬀ
period of 30 days. The double prime denotes the ﬂuctuating eddy
component for periods shorter than 30 days.
The mean heat ﬂux < q > is negative, which reﬂects long-termsouthward ﬂux to the Paciﬁc out of the SIS, consistent with the persistent clockwise circulation formed in the SIS. However, the eddy heatﬂux q” predominates over the mean ﬂux and is generally negative with

Fig. 6. The Kuroshio paths estimated by the ﬁsherydepartment of the Mie Prefecture before and after (a)Phanfone and (b) Vongfong passed through this area.
The area bounded by blue lines indicates theKuroshio path (a) on October 4 and (b) October 12
before Phanfone and Vonfong respectively. The redlines indicate the Kuroshio path (a) on October 6 and(b) October 14 after the typhoons. The black lines
reﬂect the best tracks of Phanfone (solid) andVongfong (broken) (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)86

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Fig. 7. Monthly-averaged surface velocity vectors for(a) Sepember 2014 and (b) October 2014 and theirmagnitude in color. In (b), the black lines with thetwo-letter acronyms indicate the transects, the redline is the cross section for the heat ﬂux analysis(Figs. 9f and 10); the blue line is the characteristic
length scale for the eddy shedding frequency estimation (Section 6); and the black box represents the
subdomain for the computation of the enstrophy,
PeKe and KmKe (Figs. 15 and 16). The acronyms are
EK (East coast of Kyushu Island), CA (Cape Ashizuri),
TB (Tosa Bay), KC (Kii Channel), and MS (Cape
Muroto to Cape Shionomisaki). (For interpretation of
the references to color in this ﬁgure legend, thereader is referred to the web version of this article.)path due to the typhoons (Figs. 6 and 9a) inﬂuences the intermittent
directional inversion of the mean clockwise circulation in the SIS,
which provokes the intrusion at the Kii Channel and discharge out of at
the Bungo Channel. The negative ﬂuxes in the SIS are not only aﬀectedby the typhoon-induced inverse circulations but also by counterclockwise wind stress associated with the approaching Phanfone (notshown) and Vongfong (Fig. 12), particularly at the transects far fromthe two channels. Note that Vongfong induces more negative ﬂuxes
because its track is closer to the SIS (Fig. 1b) and its wind stress is more
intense than that of Phanfone. The averaged volume ﬂux in the SIS is~0.01 Sv (1 × 104 m3 s−1) in the clockwise direction under normal,
non-typhoon conditions, whereas the typhoons alter it to approximately−0.08 Sv in the opposite direction. Because 0.08 Sv is ~130 times asmuch as the sum of the mean discharges from all major rivers pouring
into the SIS, the typhoons are anticipated to inﬂuence the estuarine
environment.
To quantitatively measure the inﬂuence of the typhoons on the SIS
environment, we evaluate a water exchange time scale Te expressed as:(Fig. 9f). Perhaps this high-frequency variability reﬂects higher harmonics of the NIOs because the model does not account for tides, which
is usually the prevailing source of the semi-diurnal variability. Note that
previous studies (e.g., Isobe et al., 2010) have reported that intrusionsof the Kuroshio warm water occur at the Bungo Channel and Kii
Channel due to the ﬂuctuations of the Kuroshio path.

5.3. Impacts on the SIS circulation
To quantify the inﬂuences of the two typhoons on the circulation in
the SIS, time series of the volume ﬂuxes across the 12 transects (Fig. 1c)are plotted in Fig. 11. Overall, weak, positive, and clockwise transport
is predominant throughout the SIS, except for the periods of the two
typhoons. Both the typhoons abruptly make the sign of the ﬂuxes negative, leading to intensiﬁed counterclockwise circulations. Extensive
negative ﬂuxes are observed in particular near the Kii (L) and Bungo (A)channels, followed by moderate positive ﬂuxes as aftershocks. Thissuggests that the meridional (northward) transition of the Kuroshio

Fig. 8. (a) and (b) Hovmöller diagrams of the streamwise northward velocity as a function of time and depth averaged over the EK Line (left column; Fig. 7b) and the TB Line (rightcolumn) with the mixed layer depth estimated from the KPP model (black line); (c) and (d) depth-averaged velocity of (a) and (b) down to 80-m depth. The bold broken lines in (c) and (d)represent the times when Phanfone (left lines) and Vongfong (right lines) were closest to the transects.

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Fig. 9. Hovmöller diagrams of (a) SSH, (b) transverse southwardand (c) streamwise eastward velocities at 50-m depth, (d) normalized relative vorticity at the surface, and (e) surface potential
temperature along the KC Line (Fig. 7b). The ordinate is thenorthward distance xk measured from the oﬀshore end of the KC
Line, while the abscissa is the date in 2014. (f) Time series of theheat ﬂuxes q (blue line) at the MS Line, where the red line is its
high-frequency component q’’ and the green line is the mean heatﬂux < q > . The black solid lines in (a)–(e) are the time-averaged
Kuroshio axis from September 20 to October 22, 2014, evaluatedaccording to the JMA deﬁnition, where the maximum current
occurs at 50-m depth, while the broken lines are the location of
the MS Line crossing the KC Line. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the
web version of this article.)

Fig. 10. Variance-preserving frequency spectrum of heat ﬂux q across the MS Line locatedat the entrance of the Kii Channel. The broken lines indicate the spectral peak periods
around 2, 1 (near-inertial period), and 0.5 days (semi-diurnal period), from right to left.

Te = V / ∑ Fk

,

k

(4)

where V is the volume of the segment (a – i; see Fig. 1c) and Fk is theoutgoing volume ﬂux (m3s−1) from the segment through the surrounding transect k. The maximum Fk is used for the typhoon periods,
whereas the monthly averaged Fk for Sepember 2014 is applied for the
non-typhoon period. As summarized in Table 2, both typhoons signiﬁcantly reduce the exchange time scale Te of all the segments compared with those for the non-typhoon period. Even for the segments far
from the two channels, Te reduces to ~30% of that under the normal
conditions. In other words, the water exchange in the SIS is considerably enhanced by the typhoons (~3–20 times), resulting in thepotential impact on the estuarine environment.

Fig. 11. Time series of volume ﬂuxes in the SIS across the 12 transects A–L plotted inFig. 1c. The clockwise transport ﬂowing from L (west, at the Bungo Channel) to A (east, at
the Kii Channel) is deﬁned as positive. One scale in the ordinate is 0.05 Sv. The shaded
areas represent approximate periods during Phanfone (orange) and during Vongfong(red). (For interpretation of the references to color in this ﬁgure legend, the reader isreferred to the web version of this article.)

6. Typhoon-Kuroshio-eddy interaction

its maximum velocity, along the TB Line oﬀ Tosa Bay, the upstream
region of the Kii Channel (Fig. 13). Although the low frequency
variability is apparent, a distinct peak appears around the near-inertial
frequency in the clockwise-rotating components due to the NIOs on theoﬀshore side of the Kuroshio path, as indicated by the dotted ellipse
near P1. On the coastal side, it peaks at periods of ~3 days with

6.1. Topographic eddy shedding
To assess the typhoon-induced oscillations of the Kuroshio path that
occurred oﬀ Shikoku Island, we conduct rotary spectral analysis on thehorizontal velocities at 50-m depth, where the Kuroshio normally has
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Fig. 12. Wind stress vectors and their curl in color,
averaged for October 12–13 when Vongfong approached the SIS. The right panel is the blow-up of
the entire model domain on the left.

on Strouhal number. For instance, the universal Strouhal number St
deﬁned in Bearman (1967) has occasionally been used for oscillating
motions in geophysical ﬂuids, such as Kármán vortex streets behindislands:

Table 2
Water exchange time scale Te in days represented by Eq. (4) for the nine segments of the
SIS (a – i, Fig. 1c) during Phanfone and Vongfong and for the non-typhoon, normal period(September 2014).
Segment

Non-typhoon

Phanfone

Vongfong

a
b
c
d
e
f
g
h
i

552
767
1260
124
122
271
418
1556
1696

219
897
303
69
60
199
249
272
274

105
554
184
41
39
93
73
84
88

St = fH / U ≅0.181,(5)where f is an oscillation frequency, U is the characteristic velocity, andH is the characteristic length scale. Eq. (5) is known to be valid for awide range of Reynolds numbers. A similar result in a form of St(~0.18) is derived from a linear stability analysis of shear turbulence(e.g., Schar and Smith, 1993). If we assume that U = 1.0 ms−1, basedon the mean Kuroshio, is the velocity, and H = 50 km, based on thelateral length of Cape Ashizuri (blue line in Fig. 7b), is the eﬀective
thickness of the shear layer, then we obtain f = 0.311 cpd (i.e., a periodof 3.22 days), consistent with the spectral peak frequency near P2 in
Fig. 13. Therefore, topographically generated mesoscale eddies shedfrom Cape Ashizuri are the plausible main cause of the low frequency
velocity variability.
The estimated eddy-shedding period of ~3 days is further ensured
quantitatively using a surface enstrophy time series on the downstreamside of Cape Ashizuri (Fig. 15a). The enstrophy of the positive andnegative relative vorticity components at the surface is evaluated separately to determine the prevailing rotation in Tosa Bay. Although thenegative component is prominent before Phanfone passes on October 5,
the positive component signiﬁcantly intensiﬁes with temporal ﬂuctuations at a period of several days after the passage. The variance-preserving frequency spectra for the positive and negative enstrophy
components peak at ~3 days, particularly for the positive vorticity
component (Fig. 16). Therefore, the typhoons considerably promote
counterclockwise, cyclonic eddies shed from Cape Ashizuri, leading to~3-day period velocity ﬂuctuations in Tosa Bay. We should note that,
whereas Fig. 15a shows smaller enstrophy ﬂuctuations after Vongfong(i.e., after Oct. 13) than Phanfone, it is mostly due to our choice of thecounterclockwise rotation, denoted by P2.
To examine the possible mechanism responsible for the ~3 day peak
period, sequential plots of the ﬂuctuating component of the normalized
relative vorticity at the surface are depicted in Fig. 14, where the slow
components for periods longer than 30 days are subtracted as the
background vorticity mainly due to the Kuroshio. On October 6, justafter Phanfone passed, a pair of positive and negative vorticity shed atCape Ashizuri, subsequently rolling up into circular coherent eddies.
These eddies then partly accumulate in Tosa Bay but mostly propagate
eastward to the Kii Channel. The eddy-shedding period is ~3 days long,corresponding to the low frequency velocity spectral peak at Tosa Bay(P2 in Fig. 13). As a side remark, although the period of the studies is
diﬀerent, such a ~3 day eddy-shedding period was detected in the Kii
Channel through an ADCP measurement (Uchiyama et al., 2016).
6.2. Eddy shedding frequency
Such an eddy shedding frequency might roughly be conﬁrmed based

Fig. 13. Velocity rotary spectrum for September 28 to October 22
along the TB Line (Fig. 1c) at z = −50 m, showing the clockwise(right) and counterclockwise (left) components. The ordinate is
the shoreward distance measured from the southern end of the TB
Line, while the abscissa is the frequency in cycles per day (cpd).
The white broken lines are the corresponding inertial frequency.
The gray area corresponds to undetectable lower frequencies.

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Fig. 14. Temporal evolution of the normalized vorticity anomaly at the surface for October 5–12, 2014 oﬀ Shikoku Island. The vorticity anomaly is evaluated from the eddy component oflateral velocity for the periods shorter than 30 days to exclude the background vorticity in the slowly evolving transient Kuroshio. The black broken circles are shown to track the negative
vorticity anomaly, whereas the black circles reﬂect the positive vorticity anomaly.

Fig. 15. Time series of (a) enstrophy of the normalized relative vorticities at the surface
for positive (cyclonic, red) and negative (anti-cyclonic, blue) components; (b) barotropic
conversion rate, KmKe (Eq. (6)); and (c) baroclinic conversion rate, PeKe (Eq. (7)). Allvariables are averaged over the rectangular area on the east of Cape Ashizuri shown by
the black box in Fig. 7b. The KmKe and PeKe values are integrated vertically down to 200m depth. (For interpretation of the references to color in this ﬁgure legend, the reader isreferred to the web version of this article.)

Fig. 16. Variance-preserving frequency spectra of enstrophy from September 28 to
October 22 for the positive (red) and negative (blue) components shown in Fig. 15a. The
broken lines are the periods of 1, 2, and 3 day, from right to left. (For interpretation of thereferences to color in this ﬁgure legend, the reader is referred to the web version of thisarticle.)analysis subdomain (black box in Fig. 7b). As seen in the Fig. 9d, eddies
associated with Vongfong ﬂuctuate as intensively as those with Phanfone along the KC line.

energy, KmKe (viz., barotropic conversion rate), and the conversion ofeddy potential energy to eddy kinetic energy, PeKe (viz., baroclinic
conversion rate).

∂u
∂u
∂u
∂v
∂v
∂v
Km K e = −⎛ u"2+ u"v"+ u"w"+ u"v"+ v "2+ v "w " ⎞ ,
∂
x
∂
y
∂
z
∂
x
∂
y
∂z ⎠
⎝
(6)

6.3. Eddy generation mechanism⎜

The energy conversion rates in the eddy kinetic energy (Ke) conservation equation quantify the relative importance of instability andeddy-mean interaction mechanisms (e.g., Marchesiello et al., 2003;
Dong et al., 2006; Klein et al., 2008; Uchiyama et al., 2017a). To better
understand the mechanism of the eddy generation and resultant intensiﬁcation of enstrophy, we focus on the two leading terms that account for the conversion of the mean kinetic energy to eddy kinetic

Pe K e = −

g
ρ"w",
ρ0

⎟

(7)

where (x, y, z) are the horizontal and vertical coordinates; w is thevertical velocity; ρ0 (1027.5 kg m−3) is the Boussinesq reference density; the overbar is an ensemble-averaging operator; and the double
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prime denotes the ﬂuctuating eddy component for periods shorter than
30 days. A positive KmKe implies the extraction of Ke from the mean
kinetic energy to generate eddies by shear instability. Likewise, if PeKeis positive, baroclinic instability is expected to produce Ke.
Fig. 15b-c illustrate the time series of KmKe and PeKe integrated
vertically down to a 200-m depth and averaged over the same subdomain as the enstrophy in Fig. 15a. Intensiﬁcation of the positive KmKe
occurs after the typhoons passed coincidently with the increase of enstrophy. They are slightly out of phase, whereas the increase of thepositive KmKe corresponds to the increase of the temporal change inenstrophy. The positive KmKe lasts over a relatively long period after
Phanfone passes, similarly to the enstrophy intensiﬁcation. In contrast,
PeKe ﬂuctuates around zero with periodic largely positive and negativevalues alternatively appearing with less notable synchronicity with the
enstrophy variations. In summary, the enstrophy ampliﬁcation on theeastern side of Cape Ashizuri is mainly promoted by the shear instability due to topographic eddy shedding with a characteristic frequency of ~3 days approximated by the universal Strouhal number,while the baroclinic instability associated with the sharpening of theKuroshio SST front (not shown) plays a secondary role.
6.4. Typhoon-Kuroshio-eddy interaction
Fig. 17 shows the time series of the location, streamwise velocity,and prevailing direction of the Kuroshio axis at the three transects oﬀ
Cape Ashizuri (CA Line; Fig. 7b), Tosa Bay (TB Line), and the Kii
Channel (KC Line) to examine how the typhoons modify the Kuroshio
and its path. Here, the Kuroshio axis is determined as the streamline at
50-m depth that connects the lateral locations of the Kuroshio core,
where the maximum velocity occurs, according to the deﬁnition of
JMA. At Cape Ashizuri, the axis is shifted northward (shaded in red) by~10 km after the passages of the two typhoons (Fig. 17a). As shown in
Fig. 15, a prominent increase of KmKe and resultant enstrophy ampliﬁcation take place concurrently with these northward shifts. Upon thearrival of Phanfone on October 5, the velocity starts to oscillate and to
decelerate at ~0.2 ms−1. The direction is also modiﬁed to rotate theaxis clockwise to 30° with the typhoon passages at 6:00 on October 7.
Thus, the typhoons push the Kuroshio path northward to close to thecoastal topography, leading to the deceleration and clockwise turn of
the axis. Likewise, at Tosa Bay, the axis moves northward at the passages, whereas it reverts back southward shortly after (Fig. 17b). In
contrast, a direction change occurs in the counterclockwise directionthat turns the axis to the north, indicating the intrusion of the Kuroshio
into the bay.
Northward axis meanders of up to 15 km appear in the Kii Channel(KC Line) as a result of the typhoon-induced shift (Fig. 17c). The velocity deceleration is also evident, particularly with Vongfong approaching on October 13; the velocity gradually recovers in ~5 days.
Similarly to the TB Line, topographically generated eddies shed from
Cape Ashizuri propagate eastward to the Kii Channel (Fig. 14) to provoke the oscillation of the Kuroshio at the CA line. It is worth notingthat the axis direction oscillates with an amplitude of ~10° without the
typhoons, whereas it increases to ~25° when the two typhoons passwith distinctive periodicity at the period of ~3 days. On the contrary,
the oscillating periods at the CA and TB lines are dominated by theperiod of ~1 day, the near-inertial period. These results suggest that the
combination of NIOs and eddies shedding from the two capes jointly
promotes the ﬂuctuations of the Kuroshio axis.
We then investigate the spatiotemporal ﬂuctuations of the Kuroshio
path due to the typhoons at depths down to 500 m in the frequency
space. Analogously to the deﬁnition of the Kuroshio axis, the locationsof the Kuroshio path at an arbitrary depth is determined as the horizontal coordinate at which the maximum velocity occurs at that depth
along the transect. The two transects, one is oﬀ the east coast of Kyushu
Island (EK Line) and the other is oﬀ Tosa Bay (TB Line), are examinedusing the frequency spectra of the locations of the Kuroshio path as a

Fig. 17. Time series of the shoreward distance from the southern end of the transect tothe Kuroshio axis (top), the maximum velocity of the Kuroshio at 50-m depth (middle),
and the corresponding counterclockwise angle of the ﬂow direction measured from theeast (bottom) at (a) CA (Cape Ashizuri) Line, (b) TB (Tosa Bay) Line, and (c) KC (Kii
Channel) Line. The red (blue) shaded areas exhibit greater (smaller) values than the time
averages. (For interpretation of the references to color in this ﬁgure legend, the reader isreferred to the web version of this article.)function of depths down to 500 m deep (Fig. 18). At the EK Line, thespectrum is signiﬁcantly intensiﬁed in the upper ocean from the surface
down to ~150 m with a considerable downward decrease (Fig. 18a).
Because a peak appears at the inertial frequency (broken lines inFig. 18), the NIOs are the predominant factors inducing the ﬂuctuationsof the Kuroshio path. Another peak at very low frequencies (periods oflonger than 5 days) is apparent, whereas it is also conﬁned in the upper
ocean. In turn, the peak around the inertial frequency is much smallerat the TB Line, except for depth deeper than 400 m, above which the
Kuroshio main body exists (Fig. 18b). The spectral energy at the TB Lineis much smaller than that at the EK Line, which is consistent with themore stable path oﬀ Shikoku Island compared with that oﬀ Kyushu
Island (Section 5). The most notable peak appears at around the periodof 3 days at all depths at the TB Line as a consequence of the topographic eddy-shedding.
In summary, the “typhoon-Kuroshio-eddy interaction”, according to
the sequence of their occurrence, took place oﬀ Shikoku Island. This
process diﬀers conceptually and phenomenologically from the “typhoon-eddy-Kuroshio interaction” reported by Sun et al. (2009), wherecollisions of eddies intensiﬁed by a typhoon initiated a meander of the91

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Fig. 18. Power spectral density (m2cpd−1) of thealong-transect locations of the Kuroshio core analyzed for October 1–22 as a function of depth (a) oﬀ
the eastern Kyushu Island (EK Line; Fig. 7b) and (b)oﬀ Tosa Bay (TB Line; Fig. 7b). The Kuroshio core isdetermined at each depth where the maximum
streamwise Kuroshio velocity takes place. The
broken lines indicate the inertial frequency along thetransects. The solid line in (b) shows the approximate
eddy-shedding period of 3 days. Notice that diﬀerent
color scales are used.

the velocity rotary spectrum. We thus call this process as the “typhoonKuroshio-eddy interaction”, which diﬀers from the typhoon eﬀects on
the Kuroshio studied in Sun et al. (2009). The enstrophy of the associated counterclockwise eddies with positive relative vorticity is apparently excited after the typhoon passes with a ﬂuctuating period of~3 days. The energy conversion analysis reveals that the positive barotropic conversion KmKe synchronously arises with the positive enstrophy, whereas the baroclinic conversion PeKe is of secondary importance. Hence, the shear instability associated with the topographic
eddy generation is the key mechanism responsible for the sub-inertial
oscillations.
The near-inertial dynamics and sub-inertial eddy shedding concurrently aﬀect the Kuroshio path and alter its axis at ﬂuctuating periods of ~1 day and ~3 days. The typhoons lead to a signiﬁcant change
in the Kuroshio axis, which mostly turns into counterclockwise direction to promote northward ﬂow near the coast. Indeed, the pronounced
intrusions into the Seto Inland Sea (SIS) occur at the Kii Channel after
the typhoons pass. The SIS has a persistent clockwise (eastward) circulation of ~0.01 Sv under the non-typhoon conditions, whereas theintrusions abruptly promote counterclockwise (westward) circulationof ~0.08 Sv, resulting in inversed circulations that remarkably intensify
the water exchange rate in many of the segments of the SIS.

Kuroshio. In contrast, the present interaction mechanism is initiated by
the northward shift of the Kuroshio axis due to the two consecutive
typhoons, which results in topographic eddy shedding. Both the interactions are anticipated to be crucial in modifying the Kuroshio, andthus have potential impacts on nearshore dynamics and associatedcoastal environment.

7. Conclusions
We investigate the impact of two consecutive super typhoons that
occurred in the fall of 2014 on the Kuroshio and adjacent coastal waters
in the northwestern Paciﬁc using high-resolution, retrospective ROMS
modeling. The modeling framework is conﬁgured based on a one-way
oﬄine nesting technique that enables the downscaling of the assimilative JCOPE2 oceanic reanalysis with a realistic atmospheric forcing
based on the assimilative GPV-MSM reanalysis products. A model-datacomparison is conducted to conﬁrm that the present model reasonably
well reproduces the Kuroshio and 3D oceanic structure on the Paciﬁcside of Japan.
The two typhoons prominently generate near-inertial motions,particularly on the right side of the typhoon tracks. The near-inertial
internal waves (NIIWs) are induced in the thermocline at ~100-m
depth. These NIIWs are extensively energized by ~1 kW m−1 near the
Kuroshio oﬀ the Shikoku and Kyushu islands, which is considered to bequite signiﬁcant because it is equivalent to half of the energy ﬂux ofinternal tides. Furthermore, the NIIWs interact with the Kuroshio andcause its path to ﬂuctuate oﬀ the east coast of Kyushu Island (EK Line,
Fig. 7b) and oﬀ Tosa Bay (TB Line, Fig. 7b), where the NIIWs are mostenergetic. However, the ﬂuctuations of the path are substantially suppressed at the TB Line because of the northward shift of the path due tothe typhoons that decelerate the ﬂow rate and stabilize the path. The
near-inertial oscillations (NIOs) are also intensiﬁed in the mixed layer,
which promote periodic changes in the velocity and direction of theKuroshio axis. The two super typhoons cause the considerable deceleration of the streamwise velocity of the Kuroshio at 0.2–0.3 ms−1 onaverage. This deceleration occurs as a part of the NIOs, and thus thevelocity and direction of the Kuroshio axis are instantaneously andlargely altered at the period of ~1 day.
In addition to the generation of such high-frequency, near-inertial
variability, the Kuroshio oﬀ Shikoku Island evidently shifts northwardwith the typhoons, leading to the quasi-persistent deceleration andchange in the mean direction of the Kuroshio axis that lasts for several
days after the passages. This northward shift enhances the interactions
between the Kuroshio and topography and promotes eddy shedding
from Cape Ashizuri. The shedding period is estimated to be ~3 days
based on the universal Strouhal number (Bearman, 1967), which isconsistent with the peak period in the counterclockwise component of

Acknowledgements
This research was supported by the JSPS Grant-in-Aid for ScientiﬁcResearch (Grant Numbers: 15H00977, 15H04049, and 15KK0207) andthe JST-CREST program (Grant Number: JPMJCR12A6).
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