PUBLICATIONSJournal of Geophysical Research: Oceans
RESEARCH ARTICLE
10.1002/2017JC012847
Key Points:
 The primary production along theKuroshio is examined using a
climatological, high-resolution,
coupled circulation-biogeochemicalmodel Eddy-driven downward nitratetransport suppresses the primary
production in the Kuroshio Region by
shear and baroclinic instabilities
 In turn, the downwelling is much lesspronounced in the Kuroshio
Extension, where nitrate upwelling
due to cyclonic eddies prevails

Correspondence to:
Y. Uchiyama,uchiyama@harbor.kobe-u.ac.jp

Citation:
Uchiyama, Y., Y. Suzue, andH. Yamazaki (2017), Eddy-driven
nutrient transport and associatedupper-ocean primary production along
the Kuroshio, J. Geophys. Res. Oceans,
122, 5046–5062, doi:10.1002/
2017JC012847.
Received 28 FEB 2017
Accepted 23 MAY 2017
Accepted article online 26 MAY 2017
Published online 22 JUN 2017

Eddy-driven nutrient transport and associated upper-ocean
primary production along the Kuroshio
Yusuke Uchiyama1,2,3, Yota Suzue1, and Hidekatsu Yamazaki3,4

1

Department of Civil Engineering, Kobe University, Kobe, Japan, 2Coastal and Estuarine Environment Division, Port andAirport Research Institute, Yokosuka, Japan, 3Core Research for Evolutionary Science and Technology, Japan Science andTechnology Agency, Tokyo, Japan, 4Department of Ocean Sciences, Tokyo University of Marine Science and Technology,
Tokyo, Japan

Abstract The Kuroshio is one of the most energetic western boundary currents accompanied byvigorous eddy activity both on mesoscale and submesoscale, which affects biogeochemical processes in
the upper ocean. We examine the primary production around the Kuroshio off Japan using a climatologicalocean modeling based on the Regional Oceanic Modeling System (ROMS) coupled with a nitrogen-based
nutrient, phytoplankton and zooplankton, and detritus (NPZD) biogeochemical model in a submesoscale
eddy-permitting conﬁguration. The model indicates signiﬁcant differences of the biogeochemical responses
to eddy activities in the Kuroshio Region (KR) and Kuroshio Extension Region (KE). In the KR, persisting
cyclonic eddies developed between the Kuroshio and coastline are responsible for upwelling-induced eutrophication. However, the eddy-induced vertical nutrient ﬂux counteracts and promotes pronounced southwardand downward diapycnal nutrient transport from the mixed-layer down beneath the main body of the Kuroshio, which suppresses the near-surface productivity. In contrast, the KE has a 23.5% higher productivity thanthe KR, even at comparable eddy intensity. Upward nutrient transport prevails near the surface due to predominant cyclonic eddies, particularly to the north of the KE, where the downward transport barely occurs,
except at depths deeper than 400 m and to a much smaller degree than in the KR. The eddy energy conversion analysis reveals that the combination of shear instability around the mainstream of the Kuroshio withprominent baroclinic instability near the Kuroshio front is essential for the generation of eddies in the KR, leading to the increase of the eddy-induced vertical nitrate transport around the Kuroshio.

1. Introduction

C 2017. American Geophysical Union.
V

All Rights Reserved.

UCHIYAMA ET AL.

The marine ecosystem acts to mediate global warming by absorbing and ﬁxing carbon in the ocean. Nellemann et al. [2009] reported that marine organisms account for more than 50% of all biological carbon in
the world. The estimated carbon ﬁxation due to primary production in the coastal seas is approximately
250 g m22 yr21, which is approximately twice as much as that in the open ocean [Martin et al., 1987]. At thecoastal margins, active primary production mainly occurs due to the nutrient supply to the upper ocean
through terrestrial and riverine discharge and coastal upwelling, whereas the productivity in the open
ocean is generally much smaller than that in coastal regions. Nevertheless, the open ocean occupies
approximately 90% of the ocean, substantially inﬂuencing the global carbon cycle and its inventory. Eddies
and other mesoscale processes, such as fronts and ﬁlaments, are ubiquitous in the upper ocean, potentiallyaffecting the biological production. Because nutrients are transported vertically from the nutrient-rich subsurface to the euphotic zone by localized upwelling due to cyclonic mesoscale eddies and fronts, the highproductivity manifests mostly around them in the open ocean [e.g., Falkowski et al., 1991; Strass, 1992]. Satellite ocean color images revealed that the spatial variability in the surface chlorophyll is distributed not
only on the mesoscale [e.g., Gower et al., 1980] but also on the submesoscale [e.g., Denman and Abbott,1994]. Numerical studies have been conducted in idealized conﬁgurations to examine the inﬂuences of themesoscale and submesoscale variability on the biological production [e.g., Franks and Walstad, 1997; Spalland Richards, 2000; Levy et al., 2001; Levy, 2003; Pasquero, 2005; Bracco et al., 2009]. In addition, regional biological responses to the mesoscale and submesoscale variability have been observed in the northern Gulf
of Alaska [e.g., Crawford et al., 2007] and were modeled in synoptic regional conﬁgurations for the North
Atlantic [e.g., Oschlies and Garçon, 1998; Oschlies, 2002], yet explicitly focusing on the enhancement of

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Figure 1. (a) The ROMS-NPZD model domain (bold box) and bathymetry (color: m); and (b) blow-up region shown by the thin box in (a)with the surface velocity magnitude greater than 0.5 m s21 at intervals of 0.25 m s21 (gray contours). Also shown in Figure 1b as black thinrectangles are the approximate extents of the Kuroshio Region (KR) and the Kuroshio Extension Region (KE) used for the kinetic energy
spectra in Figure 4, the two transects at 1388E and 1458E (black lines) used for the cross-sectional plots in Figures 10, 12, 14, and 15, andfor Table 3. The red star marks the locations for the Hovm€oller plots in Figures 7 and 8.

primary production associated with eddy-induced upwelling. However, recent studies have suggested thateddies could conversely suppress the productivity. For instance, Gruber et al. [2011] showed that unusual
low productivity might be caused by eddy-induced downward nutrient transport in eastern boundary
upwelling systems including the U.S. West Coast, where an abundant supply of upward transport of subsurface nutrients is generally expected.
The Kuroshio is the energetic western boundary current of the North Paciﬁc Subtropical Gyre. It ﬂows northeastward along the continental shelf break on the Paciﬁc side of Japan and then detaches off the Boso Peninsula at 35.68N and 141.08E, followed by further eastward veering to enter the North Paciﬁc as a free jetreferred to as the Kuroshio Extension (Figure 1). The Kuroshio off Japan is accompanied by vigorous eddy
and frontal activity both on the mesoscale and submesoscale [cf., Waseda et al., 2003]. Observations haveshown that cyclonic eddies associated with the Kuroshio enhance the chlorophyll-a (chl-a) concentration to2.5 mg L21 and the associated carbon production rate of 40 g C m22 [e.g., Kimura et al., 1997; Nakata et al.,2000] in the Enshunada Sea, which we call the ‘‘Kuroshio Region (KR)’’ (Figure 1b, left box). Note that in thepresent study, we assume that chl-a is used as a proxy of the primary production [e.g., Behrenfeld andFalkowski, 1997; Tiera et al., 2005]. In contrast, in the Kuroshio Extension Region (KE; right box in Figure 1b),
both the upward and downward nutrient transport have been observed near the Kuroshio front [Clayton
et al., 2014], resulting from concurrent adiabatic isopycnal subduction and diabatic diapycnal turbulent mixing [Nagai et al., 2012]. Such a subduction was also detected for the Kuroshio front in the East China Sea as
downward transport of the less saline surface water [Matsuno et al., 2009] in which ageostrophic crossfrontal ﬂow due to baroclinic instability [e.g., Spall, 1995] crucially interplayed with the shallow shelf-break
topography. However, Sasai et al. [2007, 2010] demonstrated that cyclonic eddies promote the primary productivity in the KE based on the MOM3-based OFES model [Masumoto et al., 2004] in a mesoscale eddyresolving conﬁguration coupled with the nitrogen-based NPZD model of Oschlies [2001]. Therefore, whether
oceanic eddies in the upper ocean along the Kuroshio promote localized upwelling-induced eutrophic conditions or subduction/downwelling-induced oligotropic conditions are a quite fundamental but open
question.
In the present study, we conduct a high-resolution, mesoscale eddy-resolving, submesoscale eddypermitting simulation around Japan encompassing an extensive portion of the Kuroshio in a climatologicalsetting for ten years with a horizontal resolution of 3 km. The model is based on the Regional Oceanic
Modeling System (ROMS) [cf., Shchepetkin and McWilliams, 2005, 2008] coupled with a simpliﬁed version of
the nitrogen-based pelagic NPZD biogeochemical model of Fasham et al. [1990], Moisan and Hofmann[1996], and Di Lorenzo et al. [2004], embedded in and initialized by the Japan Coastal Ocean Prediction
Experiment 2 (JCOPE2) assimilative oceanic reanalysis [Miyazawa et al., 2009] with the atmospheric forcing
from the Grid Point Value of the Global Spectral Model (GPV-GSM) assimilative atmospheric product [e.g.,Roads, 2004]. We aim to clarify the biogeochemical responses to mesoscale and submesoscale oceanic variability through eddy-induced nutrient transport. Particular attention is paid to substantial differences in the

UCHIYAMA ET AL.

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primary production and its driving mechanism in the KR and KE regions. The remainder of this paper isorganized as follows. A description of the coupled climatological model conﬁguration is presented in section 2. Section 3 describes the model-data comparisons to validate the model with respect to the mean
dynamics, eddy activities, and surface biological production around the Kuroshio. Subsequently, analyses
and discussions on the eddy-induced nitrate transport (section 4) and its driving mechanism based on eddy
energy conversion analysis (section 5) are provided, followed by conclusions in section 6.

2. Methods
2.1. Hydrodynamic Model
The downscaling oceanic modeling for the Kuroshio region off Japan (Figure 1) is based on a researchversion of ROMS developed privately at University of California, Los Angeles (A. F. Shchepetkin, personalcommunication). The ROMS model is a terrain-following, hydrostatic regional ocean circulation model
expressed in horizontal curvilinear coordinate and vertically stretched surface-following and terrainfollowing s coordinate [Shchepetkin and McWilliams, 2005]. The K-Proﬁle Parameterization (KPP) mixedlayer turbulence model [Large et al., 1994] is employed for the vertical momentum and tracer mixing,
while the lateral mixing is not explicitly taken into account other than biharmonic hyperdiffusion inherent
in the third order upstream-biased lateral advection scheme. The monthly climatology of the JCOPE2
reanalysis [Miyazawa et al., 2009] is exploited to initialize the ROMS model, and to impose open boundary
conditions of sea surface height (SSH), temperature, salinity, and zonal and meridional horizontal current
velocities. The JCOPE2 is the operational oceanic reanalysis product for the northwestern Paciﬁc with ahorizontal resolution of 10 km incorporating 3-D-VAR data assimilation with a vast amount of satellite
and in situ observations such as the Argo proﬁling ﬂoats. We rely on the one-way ofﬂine nesting
approach described in Mason et al. [2010] to convey the basin-scale information in the parent JCOPE2
into the child ROMS model. The ROMS model is designed to encompass the KR and KE regions around
Japan, consisting of 784 3 864 horizontal grid cells with 3 km horizontal resolution and 32 verticallystretched terrain-following and surface-following s-layers. The model topography is obtained from theSRTM30_PLUS product [Rodriguez et al., 2005; Becker et al., 2009], which covers the global ocean at 30
geographic arc seconds, supplemented by the J-EGG500 (http://www.jodc.go.jp/data_set/jodc/jegg_
intro.html) at 500 m resolution for the coastal region around Japan. Surface wind stresses are imposedfrom the daily climatology of the GPV-GSM operational atmospheric product of the Japan Meteorological
Agency (JMA) at the resolution of TL959L60 (viz., at 20 km horizontally). Surface heat, freshwater, andradiation ﬂuxes are based on the NOAA-COADS monthly climatology [Woodruff et al., 1987]. To improve
the reproducibility of the Kuroshio, we introduce a four-dimensional nudging (also known as robust diagnostic) [e.g., Marchesiello et al., 2003] for temperature and salinity (TS) with a weak nudging inverse timescale of 1/20 d21 toward the 10 day averaged climatological JCOPE2 TS ﬁelds. The numerical conﬁguration is summarized in Table 1.
2.2. Biogeochemical Model
The biogeochemical model is based on a nitrogen-based NPZD pelagic model [Fasham et al., 1990; Moisan
and Hofmann, 1996; and Di Lorenzo et al., 2004], and the numerical conﬁguration follows that of Gruber
et al. [2006]. The present NPZD model is reconciled for examining the biological responses to the ambienthydrodynamics as simple as possible, and,thus, is reduced to consider the concentrations (mmol Nm23) of only ﬁve components: 1
Table 1. Numerical Conﬁgurations
nitrate NO2
3 , ammonium NH4 , phytoComputational period
10 years (climatological run)plankton ½Phyt , zooplankton ½Zoo, andGrid cells
784 3 864 (3 32 vertical s-layers)
detritus ½Det . The full set of conservation
Horizontal grid resolution
3 km
Baroclinic time step
150 sequations with the source and sink terms for
Surface wind stress
JMA GPV-GSM (daily climatology)
each of the ﬁve components is written as:
Surface ﬂux
NOAA COADS (monthly climatology)
SST to restore
Initial/boundary conditions
TS nudging
Topography

UCHIYAMA ET AL.

Pathﬁnder-AVHRR (monthly climatology)
JCOPE2 (monthly climatology)
JCOPE2 (10 day averaged)
SRTM30_Plus supplemented by J-EGG500

EDDY-DRIVEN NUTRIENT TRANSPORT

D  2
NO3 52Q1 ;
Dt

(1)

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D  1
NH4 52Q2 1l½Zoo½NH1   ½Zoo1l½Det½NH1   ½Det ;
4
4
Dt


D
½Phyt52 l½Phyt 1l½Phyt½Det  ½Phyt 2G1Q1 1Q2 ;DtD½Zoo52 l½Zoo 1l½Zoo½NH1  1l½Zoo½Det  ½Zoo1b  G;
4
Dt


D
½Det 52 l½Det 1l½Det½NH1   ½Det 1ð12bÞ  G1l½Phyt½Det  ½Phyt;
4
Dt

(2)
(3)
(4)
(5)

where D=Dt is the material derivative including the tendency, 3-D advection, vertical settling of½Phyt; ½Zoo; and ½Det , and vertical eddy diffusive ﬂuxes determined by the KPP model. Similarly to thehydrodynamic model ROMS, the horizontal subgrid scale diffusion is not explicitly introduced in the NPZD
model. The rate of nitrate uptake by phytoplankton (Q1 ), the rate of ammonium uptake by phytoplankton(Q2 ), and the rate of phytoplankton grazing by zooplankton (G) are given by: 21½NH1 
4
aJ  ½Phyt   NO2
3 e


;
Q1 5
2
KNO23 1 NO3
aJ  ½Phyt   NH1
 1 4 ;KNH14 1 NH4(7)gmax  ½Phyt   ½Zoo
:KPhyt 1½Phyt 

(8)

Q2 5

G5

(6)

where aJ is the light limited growth rate in d21 as a function of photosynthetically active radiation (PAR)and the maximum growth rate [Fasham et al., 1990, equations (4) and (5)]. Chlorophyll-a concentration isretrieved by equation (8) in Gruber et al. [2006]. The other model parameters are modestly tuned for the
study site as summarized in Table 2.
The initial and boundary conditions for nitrate and phytoplankton are obtained from the World Ocean Atlas
2005 (WOA05) [e.g., Garcia et al., 2006] and the MODIS/Aqua satellite data (https://oceancolor.gsfc.nasa.gov/
cms/data/aqua). The nitrate concentration is imposed as a combination of the seasonal (for depths down to
500 m) and annual (below 500 m depth) climatology. Because the resolution of the WOA05 is 18 3 18, whichis too coarse to correctly represent the coastal nitrate, the initial concentration near the coast shallower
than 500 m is linearly interpolated in the vertical toward the vertical nitrate proﬁles at 500 m depth. Vertical
proﬁles of phytoplankton concentration are inferred from the monthly climatology of the surface
chlorophyll-a (chl-a) from the MODIS/Aqua using the algorithm proposed by Morel and Berthon [1989] forthe initial and open boundary conditions. The zooplankton concentration is assumed to be proportional tochl-a [Gruber et al., 2006]. The remaining variables, ammonium and detritus, are set to zero in the initial andboundary conditions. Note that all the reanalysis, satellite, and in situ data used in the present study are
publically available. The coupled ROMS-NPZD model is run for ten years in a climatological conﬁguration.

Table 2. Parameters of the Biogeochemical Model

UCHIYAMA ET AL.

Parameter

Symbol

Value

Unit

Half-saturation for phytoplankton NO2
3 uptakeHalf-saturation for phytoplankton NH1
4 uptake
Phytoplankton ammonium inhibition parameter
Phytoplankton loss to sink rate
Phytoplankton mortality to detritus rate
Zooplankton maximum growth rate
Zooplankton assimilation efﬁciency
Zooplankton half-saturation for ingestion
Zooplankton loss to sinkZooplankton speciﬁc excretion rate
Zooplankton mortality to detritus
Detrital loss to sink rate
Detrital breakdown to NH1
4 rate

KNO23
KNH14
1
l½Phyt

1.5
0.5
1.5
0.01
0.072
0.75
0.75
1.0
0.025
0.1
0.025
0.02
0.03

mmol N m23
mmol N m23(mmol N)21
d21
d21
d21
n.d.
d21
d21
d21
d21
d21
d21

l½Phyt½Det
gmax
b
KPhyt
l½Zoo
l½Zoo½NH1 
4
l½Zoo½Det
l½Det
l½Det½NH1 

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The results for the 10th model year are utilized in the following analyses unless otherwise stated. Initialadjustments during the model spin-up occurred mostly in the ﬁrst model year, while approximately periodic, climatological results are obtained after the third model year.

3. Upper-Ocean Dynamics and Biogeochemistry
3.1. Upper-Ocean Dynamics
First of all, the model results are compared for validation with the delayed-time AVISO satellite altimetry
data (viz., gridded daily SSH composite at 1/48 resolution) [e.g., Traon et al., 1998], Merged satellite and insitu data Global Daily Sea Surface Temperature (MGDSST, at 1/48 resolution) [Sakurai et al., 2005], and the
JCOPE2 reanalysis (Figure 2). The annual mean climatology is obtained from 20 year averages (1994–2013)of each of the three data sets. The Kuroshio main body visualized by the time-averaged near-surface geostrophic current velocity estimated from the AVISO-SSH is approximately the same as that of the JCOPE2
and ROMS models (Figures 2a–2c). Although ROMS yields slightly faster velocities due to the grid-size
reﬁnement from 10 km (JCOPE2) to 3 km (ROMS), which allows to more realistically resolve the buoyancy
gradient and results in the geostrophic acceleration of the Kuroshio, the overall pattern resembles that of
the AVISO and JCOPE2. Likewise, the model-derived SST shows a satisfactory agreement with the MGDSST
and JCOPE2, particularly the sharp transition along the Kuroshio fronts (Figures 2d–2f). A modest deﬁciency
in the SST reproducibility is found in ROMS, which is most conceivable in the northern part of the Japan
Sea, whereas this warm bias is already inherent in the parent JCOPE2. Hence, we conclude that the present
JCOPE2-ROMS downscaling model is sufﬁciently capable of reproducing the mean oceanic hydrodynamics
along the Kuroshio off Japan.
3.2. Eddy Activity Near the Surface
Figure 3 shows the daily averaged relative vorticity near the surface normalized by the background rotationf (the Coriolis parameter) in winter (1 January) and summer (1 August) in the tenth model year. Horizontal
velocity shear associated with the Kuroshio generates axisymmetric pairs of positive (viz., counterclockwise
rotating, cyclonic) and negative (viz., clockwise rotating, anticyclonic) vortex streamers on the northern andsouthern sides of the Kuroshio in both seasons. Submesoscale eddies with typical diameters of O (1–10 km)
arise more evidently in winter than in summer because of the mixed-layer deepening and surface coolingthat precondition baroclinic and symmetric instabilities near the surface [e.g., Thomas and Taylor, 2010]. In
contrast, mesoscale eddies with diameters of O (100 km) are more prominent in summer than winter, consistent with Sasaki et al. [2014]. The mesoscale eddies are generally shed from the coastal topographies
such as capes and islands, and spontaneously generated near the Kuroshio front, occasionally resulting inmeandered paths [e.g., Endoh and Hibiya, 2001]. For the latter, the release of the available potential energy
around the Kuroshio front leads to baroclinic instability and associated eddies [e.g., Waseda et al., 2003].
Such eddy generation occurs typically at a period of several days. The mesoscale eddies manifested in the
KE are transported downstream along the Kuroshio at about 1–28 per day. In contrast, in the KR, quasiperiodic cyclonic-eddy shedding takes place from the upstream headland topography. These eddies eventually
coalesce due to inverse cascade to evolve into the cyclonic lateral recirculation under the relatively stable
Kuroshio path [e.g., Yamagata and Umatani, 1989; Nishimura, 1998]. Submesoscale eddies do not manifest
on the Kuroshio path, where the SST front is prominent (Figure 2f), but rather in the region 18 away fromthe front. The submesoscale dominance is also found in the surface kinetic energy wave number (k) spectra
as elevated power at higher wave numbers in the KR and the KE (see Figure 1b for the subdomain deﬁnitions), as shown in Figure 4. The spectral power for the KR and KE is greater at higher wave numbers in winter with gentler spectral slopes close to the k23 law expected for quasi-geostrophic turbulence [e.g.,Charney, 1971], rather than to the k22 law [cf., Capet et al., 2008; Sasaki et al., 2014]. The KR shows less prominent seasonal differences, whereas interseasonal differences in the spectral energy are more evident in the
KE for our choice of analysis subdomains (Figure 1b). These results demonstrate that both regions areaffected by vigorous eddy activity with more energetic submesoscale eddies in winter.
The surface eddy kinetic energy, EKE 5 u0 2 1v 0 2 =2, is then evaluated from the ROMS result (Figure 5a),where u and v are the zonal and meridional components of the horizontal velocity, the overbar is an
ensemble-averaging operator, and prime denotes high-pass ﬁltered ﬂuctuations relative to the seasonal
components for timescales longer than 90 days. High EKE is distributed along the Kuroshio, corresponding

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Figure 2. Plan view plots of the climatological surface velocity vectors and their magnitude in color (left column) and sea-surface
temperature (SST; right column). The top panels are from the (a) AVISO and (d) MGDSST data. The middle and bottom plots depict the
JCOPE2 reanalysis and the present ROMS model.

to the vorticity snapshot in Figure 3. The EKE is conﬁned near the Kuroshio path in the KR, while it is extensively distributed in the meridional direction in the KE. In the KR, the Kuroshio path is rather persistent,
although the KE jet leads to meandered paths with much greater spatiotemporal variability (not shown).
Figure 5b shows the annually averaged SSH and subsampled surface current velocity vectors in the
expanded subregion indicated by the magenta box in Figure 5a. The Kuroshio passes Cape Shionomisaki(CS) to take the eastward path and then veers northeastward at 138.28E off Cape Omaezaki (CO) and getsclose to the coast again before entering the KE. At the margin between the Kuroshio and coastline, twoquasi-persistent standing eddies are found: a mesoscale cyclonic eddy with lowered SSH at 100 km off CO

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Figure 3. Instantaneous (daily averaged) surface relative vorticity normalized by the Coriolis parameter (i.e., dimensionless) on (a) 1 January(winter) and (b) 1 August (summer) in the 10th model year. A pair of positive and negative vortex streamers is apparent around the Kuroshio
path with substantial changes in the eddy size. Submesoscale eddies with diameters of O (1–10) km are more evident in winter.

and an anticyclonic eddy with slightly elevated SSH in the east of CS. Because cyclonic eddies are accompanied by localized upwelling that promotes the nutrient supply from the nutrient-rich subsurface to thenear-surface euphotic zone, the mesoscale cyclonic eddy off CO is expected to maintain the biological production in the KR, as suggested by Kimura et al. [1997].
3.3. Upper-Ocean Biogeochemistry
The annually averaged surface chl-a climatologies from the ROMS-NPZD model and the MODIS/Aqua satellite
data are compared in Figure 6. Several basic features are consistently observed even with the simple nitrogenbased NPZD model, although the model result is not perfect. For instance, segmentation in the production
occurs on the northern and southern sides of the Kuroshio, with much higher chl-a to the north. Coastalenhancement of chl-a is commonly shown, whereas the model shows an underestimation because of the lackof the land-derived nutrient inputs. In turn, chl-a is slightly overestimated in the northeastern part of the model
domain, where the primary productivity is known to decrease due to iron limitation [e.g., Denman and Abbott,
1994; Denman, 2008; Moore et al., 2013], which is not considered in the present model. Nevertheless, the modeled surface chl-a concentrations of 1.2–1.8 mg L21 in the KR and 1.0–1.5 mg L21 in the KE are also quantitatively
comparable to the in situ measurements [e.g.,
Kimura et al., 1997; Clayton et al., 2014]. Therefore, we consider the simpliﬁed geophysicsbiogeochemistry relationship employed here to
be useful to diagnose biological responses to
eddying ﬂow ﬁelds in the upper ocean aroundthe Kuroshio, avoiding further complexity ofmore sophisticated biogeochemical models. We
leave these issues for future work.

UCHIYAMA ET AL.

Figure 4. Surface kinetic energy wave number spectra for winter(January–March) and summer (July–September) in the KR and KE
subdomains shown in Figure 1b. The spectral slopes become gentler inwinter than in summer due to energetic submesoscale eddies.

The model shows that the cyclonic eddy
developed in the coastal area off CO in the KR(Figure 5b) supplies subsurface nutrients tothe near-surface euphotic zone, leading toactive production in the coastal area north of
the Kuroshio path (Figure 6b). However, chl-a
is reduced along the Kuroshio path, where the
EKE is considerably high (Figure 5a). In contrast, the maximum chl-a appears mostly on
the mean Kuroshio path in the KE around thezonal band of the Kuroshio Extension jet at
358N and 141–1488E (Figures 2c and 5b),where the EKE is also high (Figure 5a). TheseEDDY-DRIVEN NUTRIENT TRANSPORT

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results suggest that the eddy activities
associated with the Kuroshio in bothregions inﬂuence the primary production differently.

Figure 5. Plan view plots of the annually averaged (a) surface eddy kinetic energy(EKE), and (b) sea surface height (SSH) in color with the surface current velocity vectors in the blow-up region shown by the magenta box in Figure 5a. The red circles
indicate Cape Omaezaki (CO) and Cape Shionomisaki (CS). The EKE is distributed
mostly along the Kuroshio path. A counter-clockwise, cyclonic mesoscale eddy isformed off CO.

To understand the meridional transition of the primary production acrossthe Kuroshio, the vertical structures ofnitrate and chl-a are examined. We
choose the stations at 1388E and 1458E
centered at the Kuroshio core, which isdeﬁned by the location of the maximum velocity magnitude at 50 m
depth, and the stations at 618 meridionally away from the core (Figure 1b).
€ller diaFigure 7 shows the Hovmo
grams of the modeled nitrate and chla at the three stations at 1388E (KR);the white lines represent the KPPderived mixed-layer depths [Largeet al., 1994]. Nitrate is distributed near
the surface on the north side (Figure7a) to maintain the high production in
the mixed-layer (Figure 7d). Upward
nitrate transport and the associatedhigh biological production are pronounced in colder seasons (December–March) when the mixed-layer
deepens. Going southward (i.e., equatorward), nitrate is lowered, barely
reaches the surface, and results in
reduced chl-a. In warmer seasons,however, high chl-a is observed withinand slightly below the mixed-layer,
leading to nitrate depletion (Figures
7d–7f). The spring bloom with

Figure 6. Surface chlorophyll-a (chl-a) concentrations from (a) the climatology of MODIS/Aqua satellite data, and (b) the annually averaged
ROMS-NPZD model. The productivity is high in the north of the Kuroshio, where the cold subarctic water dominates, and coastal margins,
whereas the southern chl-a is considerably reduced.

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Figure 7. Hovm€oller diagrams of the log10-based nitrate (left column) and chlorophyll-a (right column) concentrations in the (a and d)
northern, (b and e) Kuroshio core, and (c and f) southern locations along the 1388E line in the Kuroshio Region (KR) as shown by the redstars in Figure 1b. The white lines are the KPP-determined mixed-layer depths. Substantial meridional transitions are apparent both innitrate and chlorophyll-a; the northern location has a higher productivity and nutrient depletion in the upper ocean than the southernlocations.

increased chl-a occurs at the two southern stations in April and May (Figures 7e and 7f). Similar seasonalvariability, meridional differences in nitrate, and chl-a are observed in the KE at 1458E (Figure 8), while theupper limit of the high nitrate concentration is lowered more pronouncedly than that in the KR (Figure 7). It
is worth noting that the nitrogen budget analysis (not shown) indicates that nitrogen exists mostly in thephytoplankton and zooplankton compartments in both the KR and KE during the bloom. The increased zooplankton further enriches the detritus that settles down by 100 m, resulting in the increase of ammonium
below the mixed-layer. Furthermore, both nitrate and chl-a have high frequency ﬂuctuations at a period ofseveral days, suggesting that the transient Kuroshio and associated eddies are responsible for generating
the vertical nitrate ﬂuxes that maintain the upper-ocean production in a different way in each of the tworegions.

Figure 8. Same as Figure 7, but for the 1458E line in the Kuroshio Extension Region (KE; see Figure 1b). The near-surface chlorophyll-a at
the southern stations (e and f) is higher than that in the Kuroshio Region (KR) in Figure 7.

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Table 3. Seasonally and Annually Averaged Chlorophyll-a (chl-a) and Eddy-Induced Vertical Nitrate Flux (N0 w 0 ) Integrated Vertically
From the Surface Down to 50 and 200 m depths as labeled, While Averaged Spatially Over the Two Transects at 1388E (KR) and 1458E(KE), as Shown in Figure 1ba
Variable

RegionChl-a (mg L21 m)

KRKE

Nitrate ﬂux(mmol N m21 s21)

KE/KR
KR
KE

Depth (m)WinterSpringSummer

Fall

Annual

50
200
50
200
200
50
200
50
200

34.1
69.6
29.4
79.7
1.145
21.31
22.85
0.13
3.41

41.8
65.4
44.0
80.7
1.235
20.68
21.34
0.10
2.36

29.4
34.5
26.8
36.7
1.062
0.08
0.16
0.11
0.47

27.4
33.6
26.9
39.9
1.188
20.02
20.31
0.00
20.18

33.2
50.8
31.8
59.3
1.167
20.48
21.08
0.09
1.51

a
Note that we deﬁne the four seasons as December–February (winter), March–May (spring), June–August (summer), and September–
November (fall), respectively. KE/KR denotes the ratio of the chl-a in the Kuroshio Extension Region (KE) to that in the Kuroshio Region(KR).

Surface and subsurface primary production occurs more prominently in the KE, except for the northernlocation, where the nitrate concentrations below the mixed-layer and the near-surface chl-a are higher inthe KR due to the cyclone-induced upwelling (section 3.2). Table 3 summarizes the time-averaged primary
production as the vertically integrated chl-a concentration from the surface down to z 5 250 and 2200 m,
averaged laterally along the 1388E and 1458E transects (Figure 1b). Evidently, the KE is always more productive than the KR at 16.7% (annual) down to 200 m depth as indicated by the chl-a KE/KR ratio, with the seasonality ranging from 6.2% to 23.5%. Spring bloom is detected particularly in the photosynthetically active
near-surface layer down to a depth of 50 m in both regions, as exhibited in Figures 7 and 8. Note that thenear-surface chl-a differences between the KR and KE in Table 3 are statistically signiﬁcant with p < 0.002(i.e., at the 99.8% signiﬁcance level) according to the two-sample t test, except for summer with p 5 0.15.

4. Eddy-Induced Nitrate Transport
To assess the spatial distribution of nitrate inﬂuencing the upper-ocean production, the isoconcentration
depths (ICDs) of the annually averaged nitrate of 20 mmol N m23 from the ROMS-NPZD model are depicted
in Figure 9. The ICDs are closely related to the surface chl-a distribution (Figure 6b), with apparent inverse
proportionality. The shallower the ICDs are, the higher is the surface chl-a. On the Paciﬁc side of Japan, thedeeper ICDs coincide with depleted production, in particular in the south of the Kuroshio path, where subtropical warm water dominates. The ICDs
almost surface in the KR, where theupwelling due to the standing cyclonicmesoscale eddy is expected to promote
the upward nutrient transport. However,the southward ICD deepening occurs
with a steeper slope than in the KE, where
the ICDs in the north of the Kuroshio are
deeper than those in the KR. The deepest
ICDs emerge in the south of the Kuroshio(as deep as 600 m) in both regions, suggesting that the downward subsurface
nitrate transport takes place across theKuroshio to induce the southward downgradient of the ICDs. The ICDs of the Kuroshio core also vary seasonally (notshown); they are generally elevated in
Figure 9. Annually averaged depths of the nitrate isoconcentration (ICDs) ofwarmer seasons, as partly depicted in Fig23
20 mmol N m . Notice the shallowing in the north of the Kuroshio andures 7 and 8.
southward sharp deepening in the meridional direction across the Kuroshio.
The ICDs almost surface near the shore of the Kuroshio Region (KR; Figure 1b),
manifesting a more intensive southward downgradient than that in the
Kuroshio Extension Region (KE).

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To investigate the vertical nitrate transport in both regions, cross-sectional

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Figure 10. Cross-sectional plots of the annually averaged eddy kinetic energy (EKE; upper row) and nitrate concentration (lower row)along the 1388E transect (left column) in the Kuroshio Region (KR; Figure 1b) and the 1458E transect (right column) in the Kuroshio
Extension Region (KE) in color. Also plotted by contour lines are the eastward velocities in m s21 normal to the section (upper) and thepotential density in kg m23 (lower). The magenta lines are the annually averaged mixed-layer depth estimated by the KPP model.

distributions of the annually averaged EKE and nitrate concentrations along the two transects at 1388E and
1458E (Figure 1b) are shown in Figure 10. The eastward velocities normal to each transect and potential
density overlaid as contours. The KPP-determined mixed-layer depths are also plotted. The Kuroshio main
body is located near the surface at 338N (KR) and 35.58N (KE), obliquely extending southward down to adepth of 800 m, visualized by the velocity contours of 0.2 m s21. The EKE peaks slightly northward of thevelocity maximum (see also Figure 5). The EKE in the KE is less energetic, whereas it has a more extensive
meridional distribution than in the KR. The southward downgradient (or subduction) of the density andnitrate consequently takes place around the Kuroshio path, although the meridional transition is more
intensiﬁed in the KR (1388E) with elevated nitrate ICDs and isopycnals in the north of the Kuroshio. Notethat such shoreward elevations and consequent southward nitrate downgradient are not evident in the
poorly resolved WOA05 data (not shown). Because the present submesoscale eddy-permitting model indicates correlations in the spatial distributions between EKE and nitrate, these results suggest that the verticalnitrate transport is potentially affected by transient dynamics characterized by eddies associated with the
Kuroshio.
To examine the inﬂuence of eddies on the nitrate transport, we evaluate the eddy-induced nitrate ﬂux. The(vertical) nitrate ﬂux FV may be decomposed into the mean and eddy components as:
FV 5 Nw  |{z}Nw 1 |ﬄ{zﬄ}
N0 w 0 ;
mean

(9)

eddy



where w is the upward vertical velocity; N is the nitrate concentration ( NO23 ); and the prime denotes theﬂuctuating (eddy) component for the periods shorter than 90 days computed with the Butterworth frequency ﬁlter. Likewise, the eastward and northward horizontal ﬂuxes (FH ) are determined using u and v
instead of w. Figure 11 shows the annually averaged eddy-induced horizontal (meridional) and verticalnitrate ﬂuxes vertically integrated from the surface down to z 5 2200 m. The southward and upward transport is deﬁned as positive. The southward eddy ﬂux is pronounced along the Kuroshio path over the large
zonal extent from 1268E to 1438E, while highly positive and lowly negative patches appear in the KE further
to the east. The northward ﬂux prevails in the north of the Kuroshio along the coast with the meridionallystretched distribution between 358N and 438N. The vertical eddy ﬂux is also conﬁned near the Kuroshio
path but in a more complicated manner with manifesting sign changes as the Kuroshio ﬂows eastward.
Although negative, downward ﬂux is prominent in the KR, the KE is dominated by weak but upward ﬂux,
consistent with the elevated nitrate distribution in Figure 10. These spatial patterns of the eddy ﬂuxes
explain the surface chl-a distribution reasonably well (Figure 6b). In the coastal area north of the KR, there
are also weak upward ﬂuxes, which correspond to the standing cyclonic eddy-induced upwelling (Figure5b) and to the resultant high chl-a. In the north of the KE, where the cold subarctic water prevails, the intensive upward ﬂux associated with prevailing cyclonic eddy (Figure 3) is attributed to the high production. On

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Figure 11. Annually averaged (a) horizontal and (b) vertical components of the eddy-induced nitrate ﬂux (equation (9)), integrated verticallyfrom the surface down to 200 m depth. The southward and upward directions are deﬁned as positive. The eddy ﬂuxes are distributed mostly
near the Kuroshio path with prominent spatial differences in the Kuroshio Region (KR) and Kuroshio Extension Region (KE).

the contrary, chl-a in the south of the Kuroshio, where the warm subtropical water prevails, is reduced
remarkably owing to the southward and downward eddy ﬂuxes.
The subsurface eddy ﬂuxes for the two transects along 1388E (KR) and 1458E (KE) illustrate signiﬁcant structural differences (Figure 12). The southward and downward eddy ﬂuxes in the KR are distributed around 33.58N, where the Kuroshio main body exists (Figure 10a) within and below themixed-layer. Because this downward ﬂux does not occur along the isopycnals (Figure 10b), eddy
mixing and associated strong turbulence are anticipated to promote this diapycnal transport. By
contrast, the ﬂuxes in the KE emerge at depths deeper than 400 m. Thus, in the KR, a substantial
amount of nitrate is removed from the near-surface euphotic zone on the coastal side and is transported downward beneath the Kuroshio by eddies, leading to the intensiﬁed nitrate reduction. In
other words, the vigorous downward eddy transport competes against the cyclone-induced upwelling in the coastal part of the KR. In the KE, however, the downward eddy ﬂux takes place at depths
greater than 400 m, while the moderate upward eddy ﬂux dominates in the upper layer (Figure12c). This substantial difference in the vertical eddy nitrate ﬂuxes is also evident in Table 3. In general, the KR is dominated by negative, downward ﬂuxes except for summer, whereas the KE mainly
has positive, upward ﬂuxes. As a consequence, the eddy-induced downward nitrate transport is

Figure 12. Cross-sectional plots of the annually averaged, horizontal (top row) and vertical (bottom row) components of the eddy-induced
nitrate ﬂuxes (equation (9)) along the 1388E (left column) and 1458E (right column) transects. The southward and upward directions aredeﬁned as positive. The magenta lines are the associated KPP-evaluated mixed-layer depth. Notice that the near-surface intensiﬁcation
occurs in the KR (1388E) while the subsurface, weaker ﬂuxes are found in the KE (1458E).

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anticipated to restrict the primary production around the Kuroshio path more extensively in the KR
than in the KE.
Note that Nagai et al. [2012] reported based on a detailed measurement that shallow warm tongues anddeep cold tongues were formed at the Kuroshio front in the KE due to isopycnal subduction and diapycnal
turbulent mixing. The latter tongues are structurally quite similar to the deep downward eddy ﬂux found in
the KE in the present study. It is also worth noting that the vertical mean nitrate ﬂux (Nw) is insigniﬁcant as
it is approximately an order of magnitude smaller than the vertical eddy ﬂux (N0 w 0 ). In addition, because
the difference in the vertical eddy ﬂux results mostly from the difference in the vertical velocity variancesrather than the nitrate variances (not shown), mesoscale and submesoscale eddies associated with the transient Kuroshio have an inevitable importance for the intensiﬁed nitrate downward transport in the KR. As
shown in Table 3, these vertical eddy ﬂuxes vary seasonally to promote the nitrate seasonal variability in theupper ocean (Figures 7 and 8) caused by the seasonal variability of the eddy ﬁeld, where submesoscale mixing is signiﬁcantly enhanced in colder seasons, while mesoscale mixing is predominant in warmer seasons(Figures 3 and 4).

5. Eddy Generation Mechanisms
The eddy generation mechanism is examined using the energy conversion rates in an EKE conservation
equation useful for quantifying the relative importance of the instability and eddy-mean interactions [e.g.,Marchesiello et al., 2003; Dong et al., 2006; Klein et al., 2008]. We investigate the two important quantities,
the barotropic and baroclinic conversion rates, KmKe and PeKe, respectively, deﬁned as follows:


@u
@u
@u
@v
@v
@v
Km Ke 52 u0 u0
1u0 v 0
1u0 w 01v 0 u0
1v 0 v 0
1v 0 w 0
;
(10)
@x
@y
@z
@x
@y
@z
Pe Ke 52

g 0 0
qw ;
q0

(11)

where x, y, and z are the horizontal and vertical coordinates; q is the density of the sea water; q0 51027:5 kg
m23 is the reference density; and g is the gravitational acceleration. Note that KmKe represents the conversion of the mean kinetic energy to EKE by horizontal and vertical Reynolds stresses due to shear instability if
KmKe > 0. In contrast, PeKe represents the eddy available potential energy converted to EKE by baroclinic
instability if PeKe > 0. When KmKe and PeKe are negative, EKE is decreased and leads to ﬂow stabilization,
merging of eddies, or restratiﬁcation.
The annually averaged conversion rates are plotted as the horizontal distributions, integrated verticallyfrom the surface down to z 5 2200 m (Figure 13), and as the cross-sectional distributions along the 1388E(KR) and 1458E (KE) transects (Figure 14). Figure 15 shows the meridional proﬁles of the vertically integrated,
eddy-induced horizontal (southward) and vertical (upward) nitrate ﬂuxes (FH and FV), KmKe, and PeKe along
the two transects. In Figure 13, both conversion rates are mostly distributed near the Kuroshio path, where
EKE is high (Figure 5), with distinctive meridional sign changes in KmKe. A positive KmKe is manifested on thenorthern (southern) side of the Kuroshio path in the KR (KE). Inﬂection point instability associated with zonal
free jets should have a positive KmKe on both sides of the velocity maximum, as seen in the KmKe distribution
around the KR, while the topographic eddy shedding overrides in many places along the Kuroshio including
the KE. A positive PeKe is generally provoked along the path, except for a few places including the Izu Ridge(at 34.238N and 139.258E). Figure 13 shows that the positive KmKe developed in the south of the Kuroshio
originates from the Izu Ridge, where PeKe changes its sign to negative and subsequently extends to the KE
and further to the east. In turn, PeKe is positive in the KR and the west of the KE, while it almost vanishes in
the KE. Similarly, the headland topography around Cape Shionomisaki (viz., CS; Figure 5b) has a positive
KmKe on the northern side of the Kuroshio in the KR. These results show that the ridge and headland topographies have a considerable impact on the altering roles that eddies play and yield some residual effects on
their downstream areas.
Near the surface, the maximum velocity (Figure 10) appears at 33.18N (KR) and 35.88N (KE); however, Figures 14 and 15 indicate that the maximum positive KmKe occurs at 33.58N (KR) and 35.48N (KE). In the KR,
the near-surface EKE peak (Figure 10) collocates with the maximum KmKe, where PeKe also signiﬁcantly

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Figure 13. Annually averaged (a) barotropic conversion rate KmKe and (b) baroclinic conversion rate PeKe (equations (10) and (11)) integrated vertically from the surface down to 200 m depth. Both KmKe and PeKe are provoked around the Kuroshio path with pronounced
sign changes in the streamwise and meridional directions.

increases its magnitude. The KmKe and PeKe maxima are observed where the nitrate isoconcentration deepens southward due to the eddy-driven nitrate downward transport in the KR (Figures 9, 10b, and 12a). This
deepening is further ampliﬁed by the near-surface FV (Figure 15a), which changes its direction from upwardon the northern side of the Kuroshio to downward on the other side. The former is coincident with theintensiﬁed positive KmKe, while the latter occurs at the positive PeKe peak (Figure 15b). However, in the KE,
the peaks of the near-surface EKE and KmKe are not coincident and remarkably small PeKe values appear (Figures 14c–14d and 15d), while all of them are intensiﬁed near the mixed-layer. Both the KmKe and PeKe areattenuated when going deeper and marginally contribute to the subsurface eddy nitrate ﬂuxes centered at
500 m depth (Figure 12), where nitrate is abundant (Figure 10). In contrast, the maximum positive KmKe at
35.48N occurs near the surface in the KE, where nitrate is considerably depleted, resulting in quite small
eddy ﬂuxes (Figure 15c). In the extensive area in the north of the KE between 388N and the Kuroshio front
at 35.68N, a weak but positive PeKe is provoked (Figure 15d) to promote baroclinic instability that leads tothe cyclonic eddy-induced nutrient upwelling (Figure 15c).
In summary, the large, positive KmKe induced from CS and the positive PeKe associated with the sharp front
in the KR (Figure 2f) jointly enhance shear instability and baroclinic frontal instability in the upper oceanthat promote eddying ﬂows on the north side of the Kuroshio, leading to an extensive diapycnal southward

Figure 14. Cross-sectional plots of the annually averaged, barotropic conversion rate KmKe (top row) and baroclinic conversion rate PeKe(bottom row) along the 1388E (left column) and 1458E (right column) transects. The magenta lines are the associated KPP-evaluatedmixed-layer depth. Shear and baroclinic instabilities jointly promote near-surface eddy-driven downward transport at around the areawith higher eddy kinetic energy (EKE; Figure 10a) in the KR (1388E), although both of them are insigniﬁcant in the KE (1458E).

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Figure 15. Meridional proﬁles of the eddy-induced nitrate ﬂuxes (red: horizontal ﬂux FH; blue: vertical ﬂux, FV), barotropic conversion rate
KmKe (yellow), and baroclinic conversion rate PeKe (green) along the 1388E (left column) and 1458E (right column) transects. All variables arevertically integrated from the surface down to 200 m depth. Notice different scales are used for the y axes in each plot. Along 1388E (KR),the downward eddy ﬂux is induced by baroclinic instability (PeKe  0) at the Kuroshio front, while the upwelling occurs mostly throughshear instability (KmKe  0) in the north of the downward ﬂux. However, the near-surface downward ﬂux is not evident along 1458E (KE)with a weak upward ﬂux in the north of the Kuroshio.

and downward nutrient transport. In the KE, particularly east of 1438E, the negative KmKe provoked remotelyfrom the Izu Ridge in conjunction with the attenuated PeKe in the upper ocean hinder the downward reduction of the nutrient-rich near-surface water, while subsurface southward and downward nitrate eddy transport occurs in the south of the Kuroshio path.

6. Conclusions
We examined the inﬂuences of mesoscale and submesoscale eddies associated with the transient Kuroshio
on the primary productivity in the Kuroshio Region (KR) and Kuroshio Extension Region (KE) off Japan usinga climatological oceanic modeling based on ROMS coupled with a simple pelagic NPZD biogeochemical
model. A model-data comparison was conducted to demonstrate that the present ROMS-NPZD model
reproduces the mean upper-ocean dynamics and biological responses around the Kuroshio fairly well.
The model results indicate that the surface chlorophyll-a (chl-a) distribution is closely related to the eddy
activities. In general, chl-a is higher on the north side of the Kuroshio due to upwelling-favorable cycloniceddies and resultant upward eddy nitrate ﬂux. In particular, the standing cyclonic eddy developed between
the Kuroshio and the coastline in the KR helps to maintain abundant primary production. In contrast, weobserved, for the ﬁrst time in the area, that the productivity on the south side of the Kuroshio is considerably limited by the southward and downward eddy-driven nitrate transport and resultant deepening of thenitrate-rich water. The eddy-induced southward lateral nitrate transport appears almost everywhere along
the Kuroshio path and enhances the meridional transition. Although the higher EKE is produced around theKuroshio path both in the KR and KE, the associated eddy activities affect the primary production in quite
different ways. In the KR, the diapycnal, downward eddy nitrate ﬂux is pronounced near the surface along
the Kuroshio path to compensate the cyclone-induced upward nitrate transport. The energy conversion
analysis revealed that the topographically generated positive KmKe (barotropic conversation rate) and frontally intensiﬁed positive PeKe (baroclinic conversion rate) concurrently promote the eddy-driven downward
nitrate transport through shear instability and baroclinic instability, which suppress the biological production near the Kuroshio path. However, in the KE, the negative KmKe originated from the upstream ridge
topography and weak PeKe give rise to upward eddy nitrate ﬂux in the upper ocean, leading to the maintenance of the primary productivity around the Kuroshio path. The eddy-induced downward nitrate transport
occurs at depths greater than 400 m on the south side of the Kuroshio in the KE. Overall, the headland andridge topographies are found to be crucial for the changing roles of eddies in nutrient transport, which areresponsible for the upper-ocean primary production through the alternation of KmKe and PeKe. As a result,the primary production is generally higher in the KE by 23.5% (at 16.7% as the annual average) than thatin the KR.

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Acknowledgments
The authors appreciate the ﬁnancial
support from the JST-CREST program(Grant Number JPMJCR12A6).
Y. Uchiyama is also grateful for the
support from the JSPS grant-in-aid for
Scientiﬁc Research (15H00977,
15H04049, and 15KK0207) at Kobe
University. The ROMS-NPZD model isavailable from A. F. Shchepetkin uponrequest (alex@atmos.ucla.edu). The
JCOPE2 reanalysis data set is freely
available for scientiﬁc purposes uponrequest to APL/JAMSTEC (jcope@
jamstec.go.jp). The STRM30_Plus data
are available at ftp://topex.ucsd.edu/
pub/srtm30_plus/. The J-EGG500 data
set is distributed by JODC at http://
jdoss1.jodc.go.jp/cgi-bin/1997/
depth500_ﬁle. The MSLA (mapped
sea-level anomaly) in delayed-time
product was generated by Ssalto/
Duacs and distributed by AVISO (ftp.
aviso.altimetry.fr). The GPV-GSM
atmospheric product was generated
by JMA and is available from therepository at http://database.rish.
kyoto-u.ac.jp/arch/jmadata/data/gpv/
original/. The COADS and WOA05 data
are maintained and distributed by
NOAA at https://www.esrl.noaa.gov/
psd/data/gridded/data.coads.1deg.
html and https://www.nodc.noaa.gov/
OC5/WOA05/pr_woa05.html. The
gridded MODIS/Aqua satellite surface
chl-a product is available at https://oceandata.sci.gsfc.nasa.gov/MODISAqua/Mapped. The MGDSST data set isgenerated and distributed by JMA at
http://ds.data.jma.go.jp/gmd/goos/
data/rrtdb/ﬁle_list.php.

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