7
Contribution of Kuroshio Recirculation to Nutrient Transport Alongthe Kuroshio South of Japan: An Analysis of Model Results
Xinyu Guo1,2, Yingying Hu1, and Yoshikazu Sasai2

Abstract
In this chapter, a brief review is given of an estimation of nutrient transport along the Kuroshio from east of
Taiwan to south of Japan. Previous estimations were essentially based on observations that were limited in area
and time. To understand downstream transport by the Kuroshio more broadly, we present a new estimation ofnutrient transport using the results of an eddy‐resolving nitrogen‐based four‐component nitrate–phytoplankton–
zooplankton–detritus (NPZD) type model. Based on these calculations, we not only show the downstream
nutrient transport along the Kuroshio path but also pay particular attention to the contribution of recirculation
to nutrient transport increasing along the Kuroshio south of Japan as described in Guo et al. (2013) based on
observation data along several sections of the Kuroshio. From the viewpoint of the entire Kuroshio south ofJapan, recirculation of the Kuroshio contributes to nitrate transport only on the order of 45 kmol s−1, which ismuch smaller than the previously reported value by one order. The previous estimation considered only thejoining of recirculation into the Kuroshio but neglected the departure of recirculation from the Kuroshio.
7.1. I­ ntroduction
As the Kuroshio flows from low latitude to midlatitude
regions, it carries not only heat but also many dissolved
materials. Nutrients are one of the important materials
with horizontal nitrate transport reported to be on theorder of 100–200 kmol s−1 in the East China Sea (Chenet al., 1994, 1995, 2017; Guo et al., 2012, 2013) and,downstream, higher nitrate transport is reported to beon the order of 800–1300 kmol s−1 in the Kuroshio
south of Japan. The joining of the Ryukyu Current into
the Kuroshio south of Japan is partly responsible forthis increase because the Ryukyu Current carries nitratewith horizontal transport on the order of 300 kmol s−1(Guo et al., 2013). Another process is the joining of the
Shikoku Basin local recirculation (Nagano et al., 2013)
1
Center for Marine Environmental Studies, Ehime University,
Ehime, Japan
2
Japan Agency for Marine‐Earth Science and Technology,
Kanagawa, Japan(hereafter called the Kuroshio recirculation) into the
Kuroshio, which is estimated to make a large contribution to the downstream increase of horizontal nitratetransport within the Kuroshio between the two sections
across the Kuroshio south of Japan.
The question is whether intensification of the downstream nitrate transport within the Kuroshio by the
Kuroshio recirculation is applicable over the entire areaof the Kuroshio south of Japan. In the case that theKuroshio recirculation joins into the Kuroshio mainstream, it acts to increase the horizontal nitrate transport
within the Kuroshio. However, in the case that theKuroshio recirculation departs from the Kuroshio mainstream, such intensification is unlikely from the viewpointof water budget.
Currently available estimations by Guo et al. (2013) arebased on observations limited to the area west of 137°E.
To clarify the contribution of the Kuroshio recirculation
on downstream nitrate transport within the Kuroshio
main stream, we use the results of a biogeochemical
model that covers the entire area of the Kuroshio south

Kuroshio Current: Physical, Biogeochemical, and Ecosystem Dynamics, Geophysical Monograph 243, First Edition.
Edited by Takeyoshi Nagai, Hiroaki Saito, Koji Suzuki, and Motomitsu Takahashi.
© 2019 American Geophysical Union. Published 2019 by John Wiley & Sons, Inc.
127

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Kuroshio Current

of Japan. The spatial change of horizontal nitrate transport through several sections across the Kuroshio withdifferent locations relative to the Kuroshio recirculationsuggests that the contribution of the Kuroshio recirculation to net downstream nitrate transport within the entire
Kuroshio south of Japan is very limited.
We present model information and analysis methodof model results in Section 7.2. The volume andnitrate transport through several sections are given in
Section 7.3, and a discussion on the contribution of
Kuroshio recirculation on the nitrate transport within
the Kuroshio is given in Section 7.4, followed by a summary in Section 7.5.
7.2. ­Model and Mean Current Field
We have used the Ocean general circulation model
For the Earth Simulator (OFES) with sea‐ice process(Masumoto et al., 2004; Komori et al., 2005; Sasaki
et al., 2008) coupled to a simple nitrogen‐based Nitrate–
Phytoplankton–Zooplankton–Detritus (NPZD) pelagic
model (Oschilies, 2001). Details of the biological modelare described elsewhere (Sasai et al., 2006, 2010). The
OFES domain extends from 20°S in the South Pacific to
66°N in the North Pacific Ocean and from 100°E to
70°W. The OFES is 1/10° horizontal resolution and has
54 vertical levels from 5 m thickness at the surface to
330 m thickness at the maximum depth of 6065 m. The
OFES was integrated for 30 years under climatologicalforcing using the Japanese 25‐year Reanalysis (JRA25)(Onogi et al., 2007), from the observed climatologicalfields of temperature and salinity fields (World Ocean
Atlas, 2009 (WOA09)) (Locarnini et al., 2010; Antonov
et al., 2010) without motion. After 30 years of spin‐up
integration, the OFES was forced by six‐hourly JRA25
from 1979 to 1994. The last day of 1994 is used for the
initial physical fields for this simulation.
To reach equilibrated biological fields, the biologicalmodel was incorporated after 30 years spin‐up of the
OFES with JRA25 climatological forcing. The biologicalmodel coupled with the evolving physical fields was
integrated a further five‐year period under the climatological forcing. The biological fields in the last year of thecoupled five‐year integration were used as initial conditions for this simulation. The coupled physical‐biologicalmodel was forced by six‐hourly JRA25 from 1995 to2012. In this study, we used monthly mean of model
results for nitrate, phytoplankton, detritus, zooplankton,
and horizontal velocity covering 1995– 2012 fromcoupled physical‐biological model.
In the analysis, we define the Kuroshio south ofJapan as that from the Tokara Strait (section TN–TS,
Figure 7.1a) to the Izu‐Ogasawara Ridge (section IN–IS).
For the surface current velocity averaged over 1995– 2012,

80
34°N
70
31°N
60
28°N
50

(a) 3 m

25°N

40
34°N

30

31°N

20

28°N

10

(b) 461 m

25°N
130° E

135° E

140° E

0

Figure 7.1 Map of study area and the horizontal distribution ofmean currents (gray colors represent the magnitude of current
velocity (cm s−1) and arrows represent direction) at a depth of(a) 3 m and (b) 461 m. Land areas are shaded in black. We
choose two zonal sections (RW–RM–RE and TS–AS–IS) andthree meridional sections (TN–TM–TS, AN–AS–RE and IN–IS)
for the calculation for volume and nitrate transports. Black­triangles labeled with letters indicate endpoints of sections andpoints dividing the sections.

the Kuroshio west of the Tokara Strait shows a strong
current (0.7–0.8 m s−1) in the East China Sea (Figure 7.1a).
In the Tokara Strait and the area east of the Izu‐
Ogasawara Ridge, the Kuroshio also shows a strong
current with a mean velocity of over 0.8 m s−1. However,
the mean velocity of the Kuroshio between the Tokara
Strait and the Izu‐Ogasawara Ridge is relatively weak
and broad, which is likely due to meandering of theKuroshio path south of Japan.
An anticyclonic circulation with a center at point AS
can be found in the surface mean current (Figure 7.1a).
This anticyclonic circulation is the Kuroshio recirculationthat joins the Kuroshio west of point AS and diverges
from the Kuroshio east of point AS. A northeastward
current from the offshore side of the Ryukyu Islands also
joins the Kuroshio at the west edge of the anticyclonic
circulation.
To separate the contribution of Kuroshio recirculationand the northeastward current from eastern side of the
Ryukyu Islands to the Kuroshio south of Japan, weContribution of Kuroshio Recirculation to Nutrient Transport  129

designated several sections (Figure 7.1a) to estimate
volume transport and nitrate transport in various compartments. The Kuroshio recirculation appears mainly
as the westward current through section AS–RE andthe southward current through section AS–IS. The
northeastward current east of the Ryukyu Islands isapproximated by the current through section RW–RM.
Within the Kuroshio, flow through the Tokara Strait isrepresented by section TN–TM–TS; flow south of Japanby section AN–AS; and flow going to the Kuroshio
Extension by section IN–IS.
As shown in Figure 7.1b, the mean current at the depthof 461 m weakens but maintains a similar structure tothat in the surface layer (Figure 7.1a). Because of theconsistent current structure in the vertical direction, we
expect that these sections work well for the purpose of
capturing different currents in the Kuroshio south ofJapan.
7.3. ­Results
7.3.1. Vertical Profile of Mean Current Velocity
and Volume Transport Through Sections
Since nutrient flux depends closely on current velocity,
we first examine the vertical structure of current velocity
and then present transport through these sections. At theTokara Strait, the eastward current (>0.1 m s−1) is centeredat layers shallower than 500 m at section TM–TS andat shallow areas of section TM–TN (Figure 7.2a). Moving
downstream from the Tokara Strait, the eastward Kuroshio
occupies most of section AN‐AS with a significant current(0.3 ~ 0.4 m s−1) at the surface. Meanwhile, the westward
current (=Kuroshio recirculation) appears at section AS–
RE with a velocity of over 0.1 m s−1 at the surface layer(Figure 7.2b). Moving farther downstream, the eastward
current occupies most of section IN–IS, while the westward flowing recirculation of the Kuroshio is not observedat the section (Figure 7.2c).
At the two zonal sections (Figures 7.2d, 7.2e), we
observe a similar current structure: a northward current
is at the western side while a southward current is at theeastern side. The northward current has a subsurface
maximum core that is weak at section RW–RM but
strong at section TS–AS. The southward current has asurface maximum velocity that is weak at section RM–
RE but strong at section AS–IS. The stronger northwardand southward currents at section TS–IS than at section
RW–RE reflect the presence and absence of Kuroshio
recirculation at these two sections.
Volume transport through each grid point integrated
from sea bottom to sea surface has apparent horizontal
variation along the sections (Figure 7.3). At the Tokara
Strait, eastward transport is concentrated at section

TM–TS where total transport is 16.4 Sv while thatthrough section TN–TM is 1.1 Sv. The largest eastward
transport (74.1 Sv) is found at section AN–AS, where
the transport is large at the center of the section andgradually decreases to zero to the north and south. The
Kuroshio recirculation induces a westward transport
through section AS–RE (37.7 Sv) and a southward transport through section AS–IS (33.0 Sv), both of whichshow relatively large values at the segment half close topoint AS. Eastward transport through section IN–IS is41.1 Sv with peaks around 34°N and 32°N. This value(41.1 Sv) reflects the difference between the eastward
transport through section AN–AS and southward transport through section AS–IS.
The Kuroshio recirculation has little effect on section
RW–RE, where northward transport concentrates at
the western side between RW and RM. Therefore,
northward transport through section RW–RE can beconsidered as a net contribution to the Kuroshio southof Japan. Actually, the combination of northwardtransport through section RW–RE and westward transport through section AS–RE nearly balances with thenorthward transport through section TS–AS (the exactbalance needs an eastward volume transport of 1.1 Sv
through section RW–TS that has been confirmed by adirect calculation of volume transport). The westward
shift of the maximum northward transport along­section TS–AS is likely caused by the joining of thenorthward current through section RW–RE. Therefore,
eastward transport through section IN–IS is likely thetrue Kuroshio transport that comprises two components: one from the East China Sea and the other fromthe eastern side of the Ryukyu Islands. The Kuroshio
recirculation acts as a local acceleration engine to the
Kuroshio south of Japan.
The mean Kuroshio transport was estimated to be
42 Sv by Imawaki et al. (2001) by combining satellite
altimeter with in situ hydrographic and current
measurement data at a section south of Japan (ASUKA‐
line). In this estimation, the Kuroshio recirculation was
defined as a westward current across the section and has
a transport of 15 Sv. Sugimoto et al. (2010) reported amean Kuroshio transport of 33 Sv for another section
south of Japan (137°E) and a Kuroshio recirculation of
15.6 Sv there. Both the estimations given by Imawaki
et al. (2001) and Sugimoto et al. (2010) were performed ata spatially fixed section and such method has a possibility
not fully capturing the recirculation due to its variation inposition. Nagano et al. (2013) gave a physically reasonable definition on the Kuroshio recirculation south ofJapan by assuming its little contribution to the Kuroshiotransport flowing into the Kuroshio extension area.
Following their idea, we specified a point (WE) at section
AS–RE by setting the westward transport through130

Kuroshio Current

(a)
0m

TN

TM

TS

30° N

29°N

300m
600m
900m
1200m
1500m
31°N

(b)
0m

100
90
80
70
60
50
40
30
20
10
0
–10
–20

AN

AS

WE

RE

300m
600m
900m
1200m
1500m
33°N

(c)
0m

32° N

31° N

30° N

29° N

28° N

IN

27° N

26° N

IS

300m
600m
900m
1200m
1500m
35°N

(d)
0m

34°N

RW

33°N

32°N

31° N

30° N

RM

29° N
RE

300m
600m
900m
1200m
1500m
128°E

(e)
0m

129°E

130°E

TS

131°E

132° E

133° E

134° E

AS

IS

300m
600m
900m
1200m
1500m
131°E

132°E

133°E

134° E

135° E

136° E

137° E138° E

139° E

140°E

Figure 7.2 Temporally averaged current velocity (cm s−1) normal to the sections and nitrate concentration (mmol m−3)
at each section. Color with black contours represents velocity with an interval of 10 cm s−1. White contours indicate nitrate concentrations at intervals of 5 mmol m−3 with an exception of the maximum value of 41 mmol m−3.
Positive current velocity values are defined as having an eastward or northward direction, and thick black lines
indicate a velocity of zero. Triangles at the top of the profiles denote endpoints and intermediate points of thesections shown in Figure 7.1.

Contribution of Kuroshio Recirculation to Nutrient Transport  131

34°N
41.1
74.1

31°N

1.1
16.4

56.6
–33.0

28°N

–37.7
16.7

1.1

25°N
127°E

130°E

133°E

136°E

139° E

142° E

Figure 7.3 Volume transport (Sv) across the five sections. The
length of dark gray (light gray) lines indicates volume transport
across meridional (zonal) sections per horizontal distance of
0.1 degree. Dark gray (light gray) lines located at the right(upper) side of the section indicate an eastward (northward)
volume transport. Numbers in dark gray (light gray) indicate
total volume transport (Sv) through the section between adjacent triangles (e.g., 41.1 for section IN–IS). Positive values­represent eastward or northward transport.

section AS–WE equal to the southward transport
through section AS–IS (Figure 7.3). As will be shown in
Section 7.4, the difference in nitrate transports between
section AS–IS and section AS–WE will be treated as thecontribution of Kuroshio recirculation to the nitratetransport south of Japan.
7.3.2. Vertical Profile of Nitrate Concentration
and Nitrate Transport Through Sections
The nitrate concentration has a relatively simple
distribution along the sections (dark gray lines in
Figure 7.2), increasing with depth but showing slightvariability in the horizontal direction. The nitrateconcentration is less than 5 mmol m−3 in the upper layer(0–300 m), increases evenly to 40 mmol m−3 up to 900–
1000 m, and keeps an almost constant value around
40 mmol m−3 below 900–1000 m (Figure 7.2).
The mean nitrate flux, defined as the product ofmonthly nitrate concentration and monthly current
velocity and averaged over 1995– 2012, shows high variability in both the vertical and horizontal directions(Figure 7.4). A subsurface maximum core for eastward
nitrate flux is found at three meridional sections andfor the northward nitrate flux at two zonal sections.
Subsurface maximum cores were also found for the
westward nitrate flux at section AS–RE and for thesouthward nitrate flux at section AS–IS, both of whichcorrespond to the Kuroshio recirculation south of Japan.
At the Tokara Strait, a large flux is found in the bottom
layer south of point TM (9 mmol m−2 s−1) and the area
over the shelf break (1 mmol m−2 s−1) (Figure 7.4a), bothof which have a relatively strong current (Figure 7.2a). In
the area south of Japan, the eastward nitrate flux occupies
a large area with a maximum value over 3 mmol m−2 s−1 at~600 m depth, while the westward nitrate flux shows amaximum value over 2 mmol m−2 s−1 at ~800 m depth(Figure 7.4b). At the Izu‐Ogasawara Ridge, the large
eastward nitrate flux is concentrated mostly at the bottom layer with an exception at the surface layer near 35°N
(Figure 7.4c). At section RW–RE, most of the area has anorthward nitrate flux with a maximum value over
4 mmol m−2 s−1 near the shelf slope at 600 m depth(Figure 7.4d). Moving north, the northward nitrate flux
increases markedly at section TS–AS where the northward nitrate flux at a depth of 600 m covers a larger areaand has a larger maximum value (over 7 mmol m−2 s−1)
than at section RW–RE (Figure 7.4e). Meanwhile, thesouthward nitrate flux also appears at an area east of
point AS with a maximum value over 1 mmol m−2 s−1 at a
depth of ~800 m (Figure 7.4e).
Comparing the distributions of nitrate flux andcurrent velocity at each of the sections, we find that thezero lines for the nitrate flux and current velocity do
not always coincide with each other. This suggests thatthe average monthly nitrate flux does not always equal
the product of mean current velocity and mean nitrateconcentration. In other words, the product of the temporally variable components in nitrate concentrationand current velocity also contribute to the mean nitrate
flux through each section.
Similar to volume transport, we also present thehorizontal distribution of nitrate transport through eachgrid point integrated from sea bottom to sea surface
along the sections (Figure 7.5). At the Tokara Strait, thetotal eastward nitrate transport is 103.0 kmol s−1, of
which 99.4 kmol s−1 is attributed to section TM–TS.
Eastward nitrate transport through section AN–AS is979.7 kmol s−1, most of which is distributed around the
middle region of the section. Eastward nitrate transport
at the Izu‐Ogasawara Ridge decreases to 460.9 kmol s−1
and is characterized by several peak values along section
IN–IS. Northward nitrate transport is 219.8 kmol s−1 atsection RW–RM and 26.3 kmol s−1 at section RM–RE.
The westward shift of northward nitrate transport at
section RW–RE is also found at section TS–AS, where
nitrate transport totaling 868.3 kmol s−1 is obtained.
Meanwhile, southward nitrate transport is also found at
section AS–IS where nitrate transport totals 514.0 kmol s−1(Figure 7.5).

132

Kuroshio Current
(a)
0m

TN

TM

TS

30° N

29° N

300m
600m
900m
1200m
1500m

(b)
0m

31°N

12
10
8
6
4
2
0
–2
–4

AN

AS

WE

RE

300m
600m
900m
1200m
1500m
33° N

(c)
0m

32° N

31° N

30° N

29° N

28° N

27° N

26° N

IS

IN

300m
600m
900m
1200m
1500m
35°N

(d)
0m

34° N

33° N

32° N

31° N

30° N

RM

RW

29° N
RE

300m
600m
900m
1200m
1500m
128°E

(e)
0m

129°E

130°E

131° E132° E

133° E

134° E

AS

TS

IS

300m
600m
900m
1200m
1500m
131°E

132° E

133° E

134° E

135° E

136° E

137° E138° E

139° E

140°E

Figure 7.4 Temporally averaged nitrate flux (mmol m−2 s−1) and potential density (σθ) for the sections. Color withblack contours represent nitrate flux with an interval of 1 mmol m−2 s−1 and white contours show isopycnals of
24.80, 25.50, 26.50, 27.00, 27.30, 27.50 σθ. Positive nitrate flux values are defined as having an eastward or
northward direction, and the thick black lines indicate zero flux. The upper triangles are the same as those in
Figure 7.2. (See insert for color representation of this figure.)Contribution of Kuroshio Recirculation to Nutrient Transport  133

34°N
460.9
979.7

31°N

3.6

868.3

99.4

28°N

–616.6219.8–514.0

26.3

25°N
127°E

130° E

133°E

136°E

139°E

142° E

Figure 7.5 Nitrate transport (kmol s−1) across the five sections.
The length of dark gray (light gray) lines indicates nitrate transport
across meridional (zonal) sections per horizontal distance of 0.1
degree. Dark gray (light gray) lines located at the right (upper)
side of the section indicate an eastward (northward) nitrate transport. Numbers in dark gray (light gray) indicate total nitrate transport (kmol s−1) through the section between adjacent triangles(e.g., 460.9 for section IN–IS). Positive values represent eastward
or northward transport.

7.4. ­Contribution of Recirculation
to Variation in Nutrient Transport
Along the Kuroshio Path South of Japan
There is an apparent increase in nitrate transport fromthe Tokara Strait (103.0 kmol s−1) to south of Japan (979.7
kmol s−1 at section AN–AS). This increase is apparently
caused by the contribution of northward nitrate transport
across section TS–AS. The northward nitrate transport
across section TS–AS has two components: one is thenortheastward current east of the Ryukyu Islands that provides more than 246 kmol s−1 of nitrate transport to the
Kuroshio south of Japan, and the other is the Kuroshio
recirculation passing through sections AS–IS or AS–RE
that contribute more than 600 kmol s−1 of nitrate transport
to the Kuroshio south of Japan. Therefore, it is apparent
that the Kuroshio recirculation contributes to the eastward
nitrate transport south of Japan, which corroborates theconclusion of Guo et al. (2013).
On the other hand, there is also an apparent decrease innitrate transport from the south of Japan (979.7 kmol s−1
at section AN–AS) to the Izu‐Ogasawara Ridge (460.9
kmol s−1 at section IN–IS). This reduction is balanced by
the southward nitrate transport through section AS–IS.
The departure of the Kuroshio recirculation from theKuroshio main stream is likely to cause the reduction of
eastward nitrate transport from the south of Japan to the
Izu‐Ogasawara Ridge.
Taken together, the increase in eastward nitrate transport with the Kuroshio south of Japan from its entrance(Tokara Strait) to its exit (Izu‐Ogasawara Ridge) is in theorder of ~350 kmol s−1. The northeastward current eastof the Ryukyu Islands contributes to nitrate transport on
the order of 245 kmol s−1. This residual is generally­balanced by the difference (~100 kmol s−1) between southward nitrate transport across section AS–IS and westward
nitrate transport at section AS–RE. The exact balance inthe volume transport (Figure 7.3) but not in the nitratetransport among the sections (Figure 7.5) can be
explained by variation in nitrate concentration among
the sections (Guo et al., 2013).
If we consider that the Kuroshio recirculation has thesame input and output of volume transport as it joins
and leaves the Kuroshio main stream, its contribution tonitrate transport along the Kuroshio main stream isdetermined by the difference in nitrate concentration
within the input water from the Kuroshio recirculation to
the Kuroshio main stream and that within the output
water from the Kuroshio main stream to the Kuroshio
recirculation. The westward current through section
AS–RE (input water) has a mean nitrate concentrationof 16.35 mmol m−3 while the southward current throughsection AS–IS (output water) has a mean nitrateconcentration of 15.58 mmol m−3. Taking the difference(0.77 mmol m−3) and the southward volume transport of
33.0 Sv through section AS–IS, the Kuroshio recirculation contributes 24.1 kmol s−1 to nitrate transport along
the Kuroshio main stream. This value, however, is the contribution of mean current and mean nitrate concentration.
The contribution of temporally variable components incurrent velocity and nitrate concentration to the mean
nitrate transport should be also considered.
As defined in the last paragraph of section 7.3.1, we
identify a point WE at section AS–RE where the volumetransport through section AS–WE equals to that throughsection AS–IS. The westward mean nitrate transport
through section AS–WE, as averaged from monthly
values, is 559 kmol s−1 that is larger than the southward
mean nitrate transport through section AS–IS by 45 kmol
s−1. Therefore, the contribution of Kuroshio recirculation
to the mean nitrate transport along the Kuroshio mainstream is 45 kmol s−1, to which the mean current contributes 24.1 kmol s−1 while the temporally variable components contributes 20.9 kmol s−1.
The large temporal variation in the volume transport
through each section can be confirmed in Table 7.1. The
detailed processes responsible for such large temporal variations in the volume transport and nitrate transport will bereported in another work. Another issue is the transport ofbiogenic particles including phytoplankton, zooplankton,
and detritus. Because these biogenic particles are limited in
the upper ocean and its concentration is also not high,their transports are smaller than the nitrate transport
through the same section by 1–2 orders (Table 7.1).

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Kuroshio Current

Table 7.1 Mean and Standard Deviation of Transport for Water, Nitrate, Phytoplankton, Detritus and Zooplankton Throughthe Sections Shown in Figure 7.1. Northward and Eastward Transports are Defined as Positive Values.
Sections

Volumetransport (Sv)

Nitrate transport(kmol s−1)

Phytoplankton
transport (kmol s−1)

Detritus transport(kmol s−1)

Zooplankton
transport (kmol s−1)

TM–TS
TN–TM
AN–AS
AS–RE
IN–IS
TS–AS
AS–IS
RW–RM
RM–RE

16.4 ± 3.4
1.1 ± 0.4
74.1 ± 40.0
−37.7 ± 47.7
41.1 ± 9.4
56.6 ± 38.2
−33.0 ± 40.0
16.7 ± 17.8
1.1 ± 23.8

99.4 ± 39.6
3.6 ± 2.0
979.7 ± 645.8−616.7 ± 752.2
460.9 ± 165.4
868.3 ± 627.0−514.0 ± 625.9
219.8 ± 358.7
26.3 ± 452.0

1.8 ± 0.6
0.3 ± 0.1
3.7 ± 1.9
−0.9 ± 2.0
3.1 ± 0.9
1.3 ± 1.6
−1.0 ± 1.7
0.4 ± 0.6
−0.1 ± 0.7

1.6 ± 0.7
0.2 ± 0.1
2.8 ± 1.8
−0.3 ± 1.7
2.8 ± 1.1
0.7 ± 1.3
−0.3 ± 1.5
0.3 ± 0.4
−0.1 ± 0.5

1.2 ± 0.5
0.2 ± 0.1
2.0 ± 1.1
−0.2 ± 1.1
2.0 ± 0.7
0.5 ± 0.8
−0.3 ± 0.9
0.2 ± 0.3
−0.1 ± 0.3

7.5. ­Summary
Using monthly averaged output of an NPZD type modelover the period 1995– 2012, we estimated the volume andnitrate transports around the Kuroshio south of Japan.
There was an increase in nitrate transport of ~350 kmol s−1
from the Tokara Strait to the Izu‐Ogasawa Ridge. Joiningof the northeastward current east of the Ryukyu Islands
with the Kuroshio is the main reason for the increase innitrate transport over the entire area south of Japan. The
Kuroshio recirculation contributes only in the order of
45 kmol s−1 to the increase in nitrate transport.
­Acknowledgment
This study was supported by Grants‐in‐Aid for
Scientific Research (MEXT KAKENHI grant numbers: JP15H05821/26287116). Y. Hu thanks the China
Scholarship Council (CSC) for supporting her stay inJapan.
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