Journal of Geophysical Research: Oceans
RESEARCH ARTICLE
10.1029/2017JC013635
Key Points:• Nitrate transport from south of Japanto the Kuroshio Extension is about
400 kmol/s regardless of the ﬂow
condition• Variation in eastward nutrienttransport at section 137E is mainly
attributed to the Kuroshio
recirculation• Kuroshio path meander and Kuroshio
Extension decadal oscillation are
dominant interannual variation
patterns at section 137E

Supporting Information:• Supporting Information S1
Correspondence to:
X.-H. Zhu,
xhzhu@sio.org.cn

Citation:
Long, Y., Zhu, X.-H., Guo, X., & Huang, H.
(2018). Temporal variation of Kuroshio
nutrient stream south of Japan. Journalof Geophysical Research: Oceans, 123,
7896–7913. https://doi.org/10.1029/
2017JC013635
Received 15 NOV 2017
Accepted 16 OCT 2018
Accepted article online 21 OCT 2018
Published online 6 NOV 2018

Temporal Variation of Kuroshio Nutrient Stream Southof JapanYu Long1,2, Xiao-Hua Zhu1,2, Xinyu Guo2,3,4, and Haocai Huang1

1

Ocean College, Zhejiang University, Zhoushan, China, 2State Key Laboratory of Satellite Ocean Environment Dynamics,
Second Institute of Oceanography, State Oceanic Administration, Hangzhou, China, 3Center for Marine Environmental
Study, Ehime University, Matsuyama, Japan, 4APL, Japan Agency for Marine-Earth Science and Technology, Yokohama,Japan

Abstract The temporal variation of the Kuroshio nutrient stream south of Japan was examined using
quarterly hydrographic data at section 137E from 2000 to 2013. During the seasonal cycle, nitrate ﬂux andtransport reach the maximum (minimum) in autumn (winter). The Kuroshio recirculation contributes about
60% to the mean eastward nitrate transport and the seasonal variations thereof. Empirical orthogonal
function analysis showed two dominant modes in nitrate ﬂux, nitrate concentration, and velocity at section
137E; these were associated with Kuroshio path meander and the Kuroshio Extension decadal oscillation. The
Kuroshio large meander induced a signiﬁcant increase in the eastward nitrate transport, from 609 to
1,723 kmol/s, at the section. Due to the modulation of the Kuroshio Extension decadal oscillation, both theeastward nitrate transport and the Kuroshio recirculation were stronger in an unstable state compared towhen in a stable state. The variation in the eastward nitrate transport in these dynamic patterns can be
attributed to the strength, location, and spatial expansion of the Kuroshio recirculation south of Japan. Nitratetransport from the south of Japan to the Kuroshio Extension was relatively stable at 400 kmol/s. There was astrong seasonal variation in the surface chlorophyll concentration over the subtropical gyre including Kuroshio
recirculation region, whereas variations in the Kuroshio region was controlled by the Kuroshio path shift. The
velocity-weighted nitrate concentration in the dense layer along the Kuroshio path increased throughout
the year while that in the light layer decreased in spring and autumn and increased in summer and winter.

Plain Language Summary The Kuroshio nutrient stream induces large poleward nutrienttransport in the North Paciﬁc and has a signiﬁcant impact on the low-trophic ecosystem. Historical datasuggested that the interannual variations of the Kuroshio nutrient transport south of Japan were controlled
by the Kuroshio large meander and the Kuroshio Extension decadal oscillation, which depend on thestrength, location, and spatial expansion of the Kuroshio recirculation gyre. However, nutrient transport
from the south of Japan to the Kuroshio Extension is relatively stable, and the local surface chlorophyll
concentration was also related to variations in the Kuroshio path. This study contributes to ourunderstanding of the nutrient cycle on both the local and basin scale.

1. Introduction
The western boundary current transports a large water mass along with heat and dissolved materials. It is an
important part of the global or basin scale substance cycle (Letscher et al., 2016; Williams et al., 2011). Some of
the most important dissolved materials are nutrients that are carried to the sea surface by multiple processes
such as geostrophic advection, Ekman transfer, vertical convection, and eddy diffusion (Williams & Follows,2003). These nutrients sustain primary productivity and thus maintain ﬁshery resources. Among them, thenutrient stream is a subsurface maximum of nutrient ﬂux deﬁned as the product of velocity and nutrient
concentration and was ﬁrst reported as a large conduit below the Gulf Stream (Pelegri & Csanady, 1991;
Pelegri et al., 1996). The Gulf Stream nitrate transport, which integrates the ﬂux over a section, was estimatedto be about 1,000 kmol/s.

©2018. American Geophysical Union.
All Rights Reserved.

LONG ET AL.

The subsurface maximum of the nutrient ﬂux results from a decreased velocity and a vertical increase in nutrient concentration. Similarly, the Kuroshio Current was also expected to be accompanied by a nutrient stream.
Chen et al. (1994, 1995) conﬁrmed this using in situ data from four sections east of Taiwan. The subsurface
maximum was at a depth of 500 m and had a multicore structure. Guo et al. (2012) studied the temporal

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and spatial variations of the Kuroshio nutrient stream in the East China Sea (ECS) using long-term data from88 cruises from 1987 to 2009. They reported that the nitrate ﬂux subsurface maximum was at a depth of
400 m and that the annual mean nitrate transport was 170.8 kmol/s. Guo et al. (2013) also presented dataon the spatial variation in the Kuroshio nutrient stream from the ECS to the south of Japan. They conﬁrmed
the presence of a subsurface maximum nitrate ﬂux core at sections PN, TK, ASUKA, and 137E across theKuroshio and section OK across the Ryukyu Current (Zhu et al., 2003) using hydrographic data from 39 cruises
covering a period from 2000 to 2009. They suggested that eastward intensiﬁcation of the Kuroshio nutrienttransport could be attributed primarily to the Kuroshio recirculation and that the nitrate transport of the
Ryukyu Current is twice that of the Kuroshio in the ECS, although the volume transported by the former isonly half that of the latter. However, the temporal variation of the Kuroshio nutrient stream south of Japan
has not been examined carefully.
The main purpose of our study was to reveal the temporal variations of the Kuroshio nutrient ﬂux and transport south of Japan. This area, where the Kuroshio in the ECS passes through the Tokara Strait and merges
with the Ryukyu Current, is characterized by a local recirculation gyre south of the Kuroshio (Kaneko et al.,
2001; Qiu, 1992) that induces an additional eastward volume transport to the Kuroshio. Such gyres are driven
by low potential vorticity values and are common near western boundary currents (Cessi et al., 1987; Ierley &Young, 1988). Guo et al. (2013) reported that the eastward nitrate transport was 1,230.4 kmol/s, of which the
Kuroshio recirculation accounted for 370.9 kmol/s from sections PN and OK to section ASUKA and 425 kmol/s
from section ASUKA to section 137E. Variations in nutrient transport in the Kuroshio in the ECS and Ryukyu
Current and the Kuroshio recirculation have a direct impact on eastward nutrient transport south of Japan.
The interannual variations of the Kuroshio in this area are also signiﬁcant. One variation is the Kuroshio pathmeander, a bimodal stream path categorized by Kawabe (1995) as a large meander state with an offshore
path and a nonmeander state with an onshore path. The other is Kuroshio Extension decadal oscillation(KEDO; Qiu & Chen, 2005, 2010), which dominates the area downstream of the Kuroshio south of Japan,
known as the Kuroshio Extension. It features two contrasting dynamic states: a stable one with a strong jet
ﬂow of the Kuroshio and a steady ﬂow path with a low eddy appearance and an unstable one which has
the opposite characteristics. Determining whether these dynamic patterns impact the nitrate transport southof Japan, and to what extent it affects the local and downstream surface chlorophyll, is essential for understanding the nutrient cycle south of Japan.
This paper is structured as follows. We provide details on the observations and process data and brieﬂy
describe the inverse method and the JCOPE-T model (Japan Coastal Ocean Predictability Experiment, T represents tide-resolving) in section 2. Section 3 presents the seasonal variations in average velocity, nitrate concentration, and nitrate ﬂux. Section 4 describes the empirical orthogonal function (EOF) analysis conductedon the interannual variations, and a discussion and summary are given in sections 5 and 6, respectively.

2. Materials and Methods
The hydrographic data for section 137E (Figure 1b; Oka et al., 2018), downloaded from Japan Meteorological
Agency website, consist of conductivity, temperature, and depth (CTD) proﬁles recorded from 1997 to 2017
and nitrate concentrations recorded from 1964 to 2017. Four hydrographical observations are typically
deployed each year, corresponding roughly to the four seasons. The period for each season is March toMay, June to August, September to November, and December to February for spring, summer, autumn,
and winter, respectively. The data before 2000 cannot be used to form the enclosed boxes needed by inverse
method, while those from after 2013 have a lower temporal resolution (one or two times a year). Therefore,
we used data from 2000 to 2013 to calculate absolute geostrophic velocity. The nitrate concentration data
were available at standard depths (0, 10, 20, 30, 50, 80, 100, 125, 150, 200, 250, 300, 400, 500, 600, 700,
800, 900, 1,000, 1,200, 1,250, 1,400, 1,500, 1,600, 1,750, 1,800, 2,000, 2,500, 3,000, 3,500, 4,000, 4,500, and
5,000 m). They were sparser than the CTD data (Figure 2) and therefore interpolated to the velocity data grid.
The observation was assumed to represent the related season in the EOF analysis.
Hydrographic data from sections PN, TK, 137E, ASUKA, and OK were used to obtain the absolute geostrophic
velocity following the inverse method (Wunsch, 1978; Figure 1). When using this method, only data that
could form enclosed boxes are selected. First, we set a reference level of 700 m for sections PN and TK(Wei et al., 2013) and 2,000 m for sections OK (Yuan et al., 1998), ASUKA, and 137E (Kaneko et al., 2001). For

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Figure 1. (a) Schematic of the study area. (b) The black dots denote the positions of hydrographic stations, and the blacklines and bars denote nitrate transport integrated over the section at 1 km wide intervals and its standard deviation(kmol/s), respectively. The red star and the broken red line denote the subsection dividing point and the virtual
boundary of the box model used in the nitrate budget calculation, respectively. The positive direction for the curve iseastward. The black dots connected by thin straight lines present zero values. PN, TK, OK, ASUKA, and 137E indicate
section names. The Tokara Strait is located at section TK. The colored shading denotes topography. KU = Kuroshio;
KR = Kuroshio recirculation; KE = Kuroshio Extension; RC = Ryukyu Current; ECS = East China Sea.

CTD casts shallower than the given depth, the reference level was set as the sea bottom. The water column wasthen divided into eight vertical layers using isopycnals at 24.80, 25.50, 26.50, 27.00, 27.30, 27.50, 27.60, and27.66σθ. It was assumed that there was no net water exchange between two adjacent layers, and mass wasconserved in the eight layers, while salt was conserved in the lower seven layers to obtain the optimal
velocity at the reference level. Finally, the absolute geostrophic velocity
perpendicular to a line connecting two hydrographic stations, at verticalintervals of 1 m, was obtained by adding these to the velocities calculated
from thermal wind relationships (Gill, 1982). Then the nitrate ﬂux, deﬁned
as the product of nitrate concentration and velocity, was integrated over asection to obtain the nitrate transport.

Figure 2. Hydrographic data from section 137E. The solid colored dots
denote the conductivity, temperature, depth data used to calculate velocity, black crosses denote raw nitrate concentration data, and the grayshading denotes the period when the Kuroshio large meander was observed.

LONG ET AL.

In addition, the latitudinal location of the Kuroshio path (taken from theJapan Meteorological Agency website) was compared to the EOF result.
We also used the weekly gridded Mapping of Absolute Dynamic
Topography (MADT), which has a spatial resolution of 1/4° × 1/4° andmerged multiple satellite altimeters including TOPEX/Poseidon,
European Remote Sensing Satellite 1 and 2, Geosat Follow On, andJason-1/2 from Archiving, Validation, and Interpretation of Satellite
Oceanographic data, to analyze the impact of the local recirculation gyreon the main stream of the Kuroshio. The surface geostrophic velocity
g ∂MADTwas computed as u ¼  gf ∂MADT∂y , v ¼ f∂x , where f is the latitudedependent Coriolis parameter; g is gravitational acceleration; u and v are
eastward and northward velocities, respectively; and x and y denote zonal
and meridional coordinates, respectively.

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Figure 3. (left column) Seasonally averaged absolute geostrophic velocity (black contour lines at 0.1/ms intervals) and nitrate concentrations (white contour lines
3
denoting 10, 20, 30, 40, 41, and 42 mmol/m ) at section 137E during (a) spring, (b) summer, (c) autumn, and (d) winter. (right column) Seasonally averaged
2 1nitrate ﬂuxes (black contours at intervals of 1 mmol·m ·s ) and the eight isopycnal layers (white contours) for (e) spring, (f) summer, (g) autumn, and (h) winter.
2 1
Thick black lines indicate a velocity of 0/ms in (a)–(d) and a nitrate ﬂux of 0 mmol·m ·s in (e)–(h). Positive values are for an eastward direction. The inverse
triangles in (e)–(h) denote hydrographic stations.

Monthly surface chlorophyll data were obtained from the Sea-viewing Wide Field-of-view Sensor satellite.
Its spatial resolution is 9 km, and it covers the period from 9 September 1997 to 12 September 2010.
Data were processed following Thomas et al. (2012), and we calculated both the average monthly climate
data (for 12 months from 1997 to 2010) and monthly climatic anomalies (the difference between the value
in a speciﬁc month of a speciﬁc year and the corresponding monthly climate data for at that month, from1997 to 2010).
We used a tide-resolving general circulation model (JCOPE-T; Varlamov et al., 2015) to conﬁrm upward velocity in the Kuroshio south of Japan. It has a horizontal resolution of 1/36°, 47 generalized vertical sigma layers,
and was developed based on the Princeton Ocean Model with a general coordinate of sigma (Mellor et al.,2002). JCOPE-T covers part of the Western North Paciﬁc (17–50°N and 117–150°E) and is nested in an outermodel, JCOPE2 (Miyazawa et al., 2009). The model is driven by ocean surface ﬂuxes (momentum, heat, andfresh water; provided every 6 hr by the National Centers for Environmental Prediction Global Forecast
System) and runoff from rivers in its domain.

3. The Seasonal Nitrate Flux Cycle
The seasonal mean velocity, nitrate concentration, nitrate ﬂux, and potential density at section 137E are
shown in Figure 3. The data recorded during the Kuroshio large meander period were excluded in thecalculation of the average of the seasonal mean. This is because when the eastward Kuroshio takes theoffshore path, the westward coastal counter current located north of the Kuroshio also shifts southward.
Including these data in the calculation of the average might have decreased the magnitude of Kuroshio
mean current.
Throughout the year, the Kuroshio main stream can be identiﬁed as a positive surface maximum at around
33.2°N and as a large area with a strong velocity spanning nearly 3° (Figure 3). The velocity of the Kuroshio

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Table 1
Volume Transport and Nitrate Transport Budget South of Japan

Kuroshio volume transport (Sv)
Kuroshio recirculation (Sv)
NT137E (kmol/s)
NTPN (kmol/s)
NTOK (kmol/s)
NTCC (kmol/s)
VTKRCKR (kmol/s)
VTPN(C137E  CPN) (kmol/s)
VTOK(C137E  COK) (kmol/s)
VTCC(C137E  CCC) (kmol/s)
VTKR(C137E  CKR) (kmol/s)

SpringSummerAutumnWinterMean ± STD

70.34
41.68961.29129.95112.2876.43
669.07
22.96
8.46
5.90
10.87

74.44
36.281,141.60
145.53211.18101.78644.01
12.59
11.22
8.48
23.78

77.44
41.28
1,102.31
167.41
194.20114.89633.57
33.15
3.02
4.40
32.80

61.55
28.06848.74134.0899.19
122.99448.25
5.86
7.18
1.57
32.76

70.94 ± 6.90
36.83 ± 6.34
1,013.48 ± 134.38
144.24 ± 16.79
154.21 ± 56.66
104.02 ± 20.36
598.73 ± 101.42
9.42 ± 22.09
1.73 ± 9.06
5.09 ± 2.89
25.05 ± 10.36

Note. VT, NT, and C denote volume transport (integration of velocity), nitrate transport (integration of nitrate ﬂux), and velocity-weighted nitrate concentration(nitrate transport divided by volume transport), respectively. The subscripts PN, OK, 137E, CC, and KR denote the Kuroshio in the East China Sea (section PN),
the Ryukyu Current (from the ﬁrst to ﬁfth station on the west side of section OK), the Kuroshio south of Japan (from the 3rd to 10th station on the north sideof section 137E), the counter coastal current (from ﬁrst to second station on the north side of section 137E), and the Kuroshio recirculation (from the 11th tothe southernmost station of section 137E), respectively. The division of the subsections is given in Figure 1b. Positive (negative) values denote inﬂow (outﬂow)
to the area for budget calculation area. STD = standard deviation.

main stream decreased vertically but still exceeded 0.4/ms at 500 m, showing a high potential for carrying
dissolved materials eastward. The westward Kuroshio recirculation, which lies south of the Kuroshio mainstream and reinforces its volume transport by ﬂowing back westward and remerging into the Kuroshio
main stream, can be recognized as the widely distributed negative value at section 137E. In addition, acounter coastal current exists north of the Kuroshio between the coast and the Kuroshio main stream.
The eastward maximum velocities were 0.87, 1.08, 1.26, and 1.20/ms for spring, summer, autumn, and winter(Figure 3), respectively. The velocity proﬁle changed little in the upper 1,000 m throughout the year, although
the area of eastward velocity deeper than 1,000 m decreased in spring and winter (Figures 3a and 3d). The
volume transport of the Kuroshio reaches a maximum in autumn and a minimum in winter (Table 1, row 1),
whereas the volume transport of the Kuroshio recirculation reaches a maximum in spring and autumn
and a minimum in summer and winter (Table 1, row 2). This inconsistency in the volume transport in the
Kuroshio and Kuroshio recirculation, which is based on seasonal variation, implies that although the
Kuroshio recirculation was a main factor, not the only one, affecting the variation of eastward volume transport
in the Kuroshio. The Kuroshio recirculation actually contributes more than half the annual mean volume transport of the Kuroshio (Table 1, column 6, rows 1 and 2).
The seasonally averaged nitrate concentration changed little (Figures 3a–3d), showing near depletion in thesurface layer and an increasing with depth. The maximum nitrate concentration was about 41–42 mmol/m3
between 1,000 and 1,500 m deep and deceased slightly toward the deeper layer. The subsurface maximum of
the nitrate ﬂux occurred in the isopycnal layers between 25.5 and 26.5 σθ, located roughly at 440, 430, 490,
and 490 m with values of 8.64, 10.52, 11.12, and 9.61 mmol·m2·s1 in spring, summer, autumn, and winter,
respectively (Figures 3e–3h). In addition, the structure of the nitrate ﬂux subsurface core was not horizontallysymmetrical; instead, it was inclined downward toward the offshore side of the Kuroshio.
The nitrate transports of Kuroshio and Kuroshio recirculation varied in a manner similar to their volume transport (Table 1; rows 1, 2, 3, and 7), which is consistent with the Kuroshio nutrient stream in the ECS (Guo et al.,2012). However, the Kuroshio nitrate transport at section 137E had four components: the Kuroshio in the ECS,
Ryukyu Current, counter coastal current, and the Kuroshio recirculation. The ﬁrst three can be derived directlyfrom the section or subsection (Figure 1b) integration of the nitrate ﬂux, but the last is not simply equal to the
Kuroshio nitrate transport minus the sum of the other three. Following Guo et al. (2013) and assuming theconservation of volume in the box shown in Figure 1b, we applied a nitrate budget calculation in eachisopycnal layer to quantify each component. According to our deﬁnition, nitrate transport (NT) equaled thevelocity-weighted nitrate concentration (C) multiplied by the volume transport (VT), where the subscript
PN and OK refer to the section names and CC, 137E, and KR correspond to the counter coastal current, theKuroshio, and the Kuroshio recirculation at section 137E, respectively.

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NT PN ¼ VT PN C PN ;(1)

NT OK ¼ VT OK C OK ;(2)

NT CC ¼ VT CC C CC ;(3)

NT 137E ¼ VT 137E C 137E :(4)

Applying the conservation of volume in each isopycnal layer, we obtainthe following:
VT 137E ¼ VT PN þ VT OK þ VT CC þ VT KR :(5)

Substituting (5) for (4) and adding (1)–(3), we get
NT 137E ¼ NT PN þ VT PN ðC 137E  C PN Þ þ NT OK þ VT OK ðC 137E  C OK Þþ NT CC þ VT CC ðC 137E  C CC Þ þ VT KR C 137E :(6)

Assuming that CKR can be represented by the velocity-weighted nitrateconcentration at subsection KR (from the southernmost red star to thesouthernmost part of section 137E in Figure 1b), we get
VT KR C 137E ¼ VT KR ðC 137E  C KR Þ þ VT KR C KR :(7)

NT KR ¼ VT KR C KR :(8)

NT 137E ¼ NT PN þ NT OK þ NT CC þ NT KR þ VT PN ðC 137E  C PN Þþ VT OK ðC 137E  C OK Þ þ VT CC ðC 137E  C CC Þ
þ VT KR ðC 137E  C KR Þ:(9)And ﬁnally,
Figure 4. (a) Time series of the principal component of velocity (black line),
nitrate concentration (blue line), and nitrate ﬂux (red line) of the ﬁrst
empirical orthogonal function. The green line denotes the latitude ofKuroshio path at section 137E, and the black-shaded area denotes theKuroshio large meander period. (b–d) The ﬁrst empirical orthogonal function
mode of (b) velocity, in 0.1/ms intervals; (c) nitrate concentration, at intervals
3
2 1
of 2 mmol/m ; and (d) nitrate ﬂux, at intervals of 1 mmol·m ·s . The
inverse triangles denote hydrographic stations, and the thick black lines
indicate a value of zero.

Although this formula worked in each isopycnal layer, we were mainlyinterested in the entire water column, for which we only showed integration over the section in Table 1. There are two terms are present: for example, NTPN denotes the downstream nitrate transport through section PN,
while VTPN(C137E  CPN) denotes the nitrate transport resulting from thedifference in the nitrate concentration between section PN and section 137E. Hence, the budget calculation
indicates that nitrate transport in the Kuroshio consists of nitrate input directly by inﬂow(NTPN + NTOK + NTCC + NTKR) due to differences in nitrate concentration between two
sections (VTPN(C137E  CPN), VTOK(C137E  COK), VTCC(C137E  CCC), VTKR(C137E  CKR)).
In terms of its annual mean, the Kuroshio nitrate transport (NT137E) was 1,013.48 ± 134.38 kmol/s(mean ± standard deviation), in which the Kuroshio recirculation (NTKR) accounted for598.73 ± 101.42 kmol/s. Nitrate transport in the Kuroshio was integrated over the section or subsection
instead of only the eastward nitrate transport. For this reason, our Kuroshio nitrate transports were smaller
than those in Guo et al. (2013). In addition, the Kuroshio in the ECS (NTPN) and the Ryukyu (NTOK) account for144.24 ± 16.79 and 154.21 ± 56.66 kmol/s, respectively. The large variation in the nitrate transport in the
Ryukyu can be attributed to frequent occurrences of mesoscale eddies in that area (Zhu et al., 2008).
This result implies that the Kuroshio recirculation not only accounts for approximately 60% of the eastward
nitrate transport of the Kuroshio south of Japan but also explains about 60% ((101.42/134.38)2) of the total
variance of nutrient transport in the Kuroshio. This suggests that the eastward nitrate transport minus thenitrate transport due to the Kuroshio recirculation, or the part that ﬂows to the downstream region insteadof recirculating in the gyre, was at 400 kmol/s. This value agrees with the results in Guo et al. (2013); theirbudget calculation showed that the eastward nitrate transport in the Kuroshio was 1,230.4 kmol/s at

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Figure 5. (a)–(f) Seasonally averaged absolute geostrophic velocity (black contour lines at 0.1/ms intervals) and nitrate con3
centrations (white contour lines denoting 10, 20, 30, 40, 41, and 42 mmol/m ) at section 137E from (a) spring 2004, (b)summer 2004, (c) autumn 2004, (d) winter 2004, (e) spring 2005, and (f) summer 2005 for the Kuroshio large meander
period over an interval of one season. (g)–(l) Seasonally averaged nitrate ﬂuxes (black contours at intervals of2 1
1 mmol·m ·s ) and the eight isopycnal layers (white contours) for (g) spring 2004, (h) summer 2004, (i) autumn 2004, (j)winter 2004, (k) spring 2005, and (l) summer 2005 but for the Kuroshio large meander period over an interval of one season.

section 137E. The Kuroshio recirculation contributes 370.9 kmol/s of nitrate transport from sections PN and
OK to section ASUKA and 425 kmol/s from section ASUKA to section 137E. The difference between theeastward nitrate transport in the Kuroshio at section 137E (1,230.4 kmol/s) and that due to the Kuroshio
recirculation (370.9 + 425 kmol/s) implies that the downstream nutrient transport was 434.5 kmol/s fromsouth of Japan to the Kuroshio Extension.
The nitrate budget also showed that Kuroshio in the ECS (NTPN) and the Ryukyu Current nitrate transport(NTOK) reached their maximum values in autumn and summer (Zhu et al., 2010), whereas nitrate transport
in the Kuroshio recirculation (NTKR) increased in spring and autumn. The distribution of the maximum nitratetransport in these components ﬁnally leads to summer and winter maxima in eastward nitrate transport in
the Kuroshio at section 137E. Therefore, the seasonal cycle of nutrient transport in the Kuroshio was affectedlargely by the Kuroshio recirculation, with the Ryukyu as the secondary factor. In addition, the difference inthe nitrate concentration between sections PN, OK, and 137E showed evident seasonal variation; this will
be discussed in section 5.

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Table 2The Position, Eastward Velocity Maximum, Eastward Volume Transport, Nitrate Flux Subsurface Maximum Depth, Eastward Subsurface Nitrate Flux Maximum, andEastward Nitrate Transport in the Kuroshio Path From Spring 2004 to Summer 2005
2004 spring

2004 summer

2004 autumn

2004 winter2005 spring2005 summer

33.67
1.22
58.83
290
9.14
764.12

32.33
1.19
51.51
420
9.20
608.76

31.67
0.83
84.83
300
8.29
1,522.21

31.33
1.57
83.64
380
11.57
1,502.67

31.67
1.50
82.31
380
15.36
1,536.68

33.33
1.40
97.51
400
15.42
1,723.35

Kuroshio path position (°N)
1
Eastward velocity maximum (ms )Eastward volume transport (Sv)
Nitrate ﬂux subsurface maximum depth (m)2 1Eastward subsurface nitrate ﬂux maximum (mmol·m ·s )Eastward nitrate transport (kmol/s)

4. EOF Analysis
Interannual variations of velocity, nitrate concentration, and nitrate ﬂux at section 137E were analyzed using
an EOF. The results consist of a principal component and its spatial pattern. The former represents ﬁeld variation in the time domain, whereas the latter shows a spatial structure. The ﬁrst ﬁve EOF modes explain 43%,
19%, 9%, 8%, and 3% of the total variance in the velocity; 64%, 12%, 7%, 4%, and 3% in the nitrate concentration; and 39%, 17%, 7%, 6%, and 4% in the nitrate ﬂux.
4.1. The First EOF Mode
The time series of the ﬁrst EOF principal component of velocity, nitrate concentration, and nitrate ﬂux
matched well with the Kuroshio path meander (Figure 4a), which is a meridional migration of the Kuroshio
axis south of Japan. Speciﬁcally, the Kuroshio large meander was clearly highlighted during 2004–2005 by
the trough in the time series. Previous studies also focused on the dynamic formation and disappearanceof the Kuroshio large meander, particularly in this time period (Miyazawa et al., 2008; Tsujino et al., 2006),
but paid little attention to its inﬂuence on material transport. An exception is Nagano et al. (2010), who madean effort to estimate the volume and heat transport of the Kuroshio by separating the contribution of theKuroshio recirculation to it in October 2004.
The spatial pattern of velocity (Figure 4b) of the ﬁrst EOF showed a horizontally reversed structure: the positive maximum was located at about 33.5°N, and the negative minimum was located at 31.5°N. The small value
south of 30.5°N implies that this spatial pattern is loosely associated with the Kuroshio recirculation and

Figure 6. Mapping of Absolute Dynamic Tomography in the Kuroshio large meander period from spring 2004 to summer
2005. (a)–(f) show data for 28 April 2004, 21 July 2004, 27 October 2004, 26 January 2005, 27 April 2005, and 27 July 2005,
respectively. The black arrows indicate the geostrophic velocity calculated from satellite data. Nitrate transport (blue line)
and volume transport (black line) were integrated over section 137E at intervals of 1 km. The black dots connected by thethin straight lines serve as reference for a value of zero. Positive transport is deﬁned as eastward.

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shows a horizontal shift in the maximum surface velocity at section 137E.
The spatial pattern of nitrate concentration (Figure 4c) of the ﬁrst EOF
showed a single negative maximum with a value of 20 mmol/m3 centered
at about 500 m. The spatial pattern of the nitrate ﬂux (Figure 4d) of the ﬁrst
EOF had a maximum at 350 m and 33°N and a minimum (maximum negative value) at 500 m and 31.2°N.
To visualize the results of the ﬁrst EOF mode, the detailed process of theKuroshio large meander from spring 2004 to summer 2005 is shown in
Figure 5. Using the meridional shift of the Kuroshio path and the eastward
volume transport ﬂuctuation of section 137E during this period (Table 2) asa basis, we divided the results into three stages: developing in spring andsummer 2004 (Figures 5a and 5b), prevailing in autumn and winter 2004
(Figures 5c and 5d), and decaying in spring and summer 2005
(Figures 5e and 5f).

Figure 7. (a) Time series of the principal component of velocity (black line),
nitrate concentration (blue line), and nitrate ﬂux (red line) of the second
empirical orthogonal function. The green-shaded area indicates the unstablestate period of the Kuroshio Extension. The black-shaded area indicates theKuroshio large meander period. (b)–(d) The second empirical orthogonal
function mode for (b) velocity, in 0.1/ms intervals; (c) nitrate concentration, at3
2 1intervals of 2 mmol/m ; and (d) nitrate ﬂux, at intervals of 1 mmol·m ·s .
The inverse triangles denote hydrographic stations, and the thick black lines
indicate a value of zero.

The surface maximum velocity was adjacent to the southeast coast ofJapan in spring 2004 (Figure 5a) and then shifted offshore, to about
32°N, in the following season. It reached its southernmost location in winter 2004 (Figure 5d) and returned to an onshore state in summer 2005
(Figure 5f). The upper layer eastward ﬂow (0–1,000 m in depth) weakened
in the developing stage and expanded in the prevailing and decaying
stages, whereas the deep eastward ﬂow (deeper than 1,000 m) weakened
in spring 2004 and turned westward that summer (Figure 5b). In addition,
the existence of several pairs of eastward-westward ﬂows south of theKuroshio in spring and summer 2004 implies high eddy activity and weak
Kuroshio recirculation at section 137E during the developing stage. The
uniform negative value south of 30°N from autumn 2004 (Figure 5c) tosummer 2005 (Figure 5f) indicates a stable Kuroshio recirculation gyre at
section 137E. The eastward volume transport decreased to 51.51 Sv inthe developing stage in summer 2004 and rose to 97.51 Sv in the decaying
stage during the next summer (Table 2). The vertical structure of nitrateconcentration changed little during the Kuroshio large meander period(Figures 5g–5l). Horizontally, the location of the maximum slope of thecontours shifted with variations in the Kuroshio path.

The latitudinal location of the nitrate ﬂux subsurface cores enclosed by the
7 mmol·m2·s1 contour had the same horizontal variation as the velocityof the surface maximum but varied vertically (Figures 5g–5l). It was located roughly between 25.5σθ and26.5σθ in spring, summer, and autumn 2004 and between 24.8σθ and 27.2σθ in winter 2004 and spring andsummer 2005 (Figures 5g–5l). It was shallower in spring, summer, and autumn 2004 than in winter 2004
and spring and summer 2005 in terms of the subsurface maximum depth (Table 2). Although the location
was 420 m in summer 2004, the nitrate ﬂux deeper than 1,000 m turned westward, while the eastward nitrate
ﬂux decreased to 608.76 kmol/s (half of its mean value) in summer 2004 but almost tripled to 1,723.35 kmol/sin summer 2005.

The MADT measured during the cruises (Figure 6) gave a plain view of the Kuroshio large meander from 2004
to 2005. The volume and nitrate transport per unit width matched the surface geostrophic velocity well. This
indicates that nitrate transport during the Kuroshio large meander can be represented by MADT. In the developing stage (Figures 6a and 6b), MADT had a chaotic surface geostrophic velocity ﬁeld and high eddy activity.
The Kuroshio recirculation moved to the west of section 137E, which made it difﬁcult to identify in the proﬁle(Figure 6a), and the eastward transport of the Kuroshio at section 137E weakened. In the prevailing anddecaying stages (Figures 6c and 6d and 6e and 6f, respectively), the area of Kuroshio recirculation gyre overlapped by section 137E grew larger, and both the Kuroshio and Kuroshio recirculation gyre were strengthened. This synchronous variation implies that the strength, size, and location of the Kuroshio recirculation
affected variations in the eastward volume and nitrate transport at section 137E.

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3

Figure 8. Composite maps of (a) velocity (in intervals of 0.1/ms ) and nitrate concentration (in intervals of 5 mmol/m )2 1and (c) nitrate ﬂux (in intervals of 1 mmol·m ·s ). (b) and (d) are the same as (a) and (c) but for the period when theprincipal component of the nitrate ﬂux of the second empirical orthogonal function was less than 1 standard deviation.
The inverse triangles denote hydrographic stations, and the thick black lines in (a)–(d) indicate a value of zero.

Interestingly, the results given by the section proﬁles of both velocity and nitrate ﬂux (Figure 5) and MADT
(Figure 6) differ from the spatial pattern of the ﬁrst EOF mode (Figure 4). The section proﬁles and MADT
showed that when the Kuroshio meandered, the eastward nitrate transport varied largely in relation to
the Kuroshio recirculation. However, the spatial pattern of the ﬁrst EOF mode showed a small value in thelocation of the Kuroshio recirculation. We explain this by noting that section 137E does not pass directly
through the center of the Kuroshio recirculation (Figure 6) that and therefore, variations in the Kuroshio
recirculation at that section are smaller compared to in the Kuroshio. In addition, variations in the
Kuroshio at section 137E were meridional (Figure 5), whereas the Kuroshio recirculation moved in a zonal
direction (Figure 6).
These results suggest that when the path of the Kuroshio meandered, nitrate transport from the Kuroshio
south of Japan to the Kuroshio Extension was relatively stable. This is consistent with the ﬁndings ofNagano et al. (2010), who suggested that the Kuroshio south of Japan had a stable volume transport ofabout 23.3–29 Sv regardless of the ﬂow condition. If the transport-averaged nitrate concentration was
16.4 mmol/m3 for the Kuroshio at section 137E (Guo et al., 2013), the nitrate transport of the Kuroshio southof Japan had a stable downstream nitrate transport range of between 382 and 475 kmol/s. Taking 430 kmol/s
as a rough estimate of the nitrate transport from the Kuroshio south of Japan to the Kuroshio Extension, thenitrate transport due to Kuroshio recirculation during the Kuroshio large meander period ranged from180 kmol/s (608.76 kmol/s in summer 2004 minus 430 kmol/s) to 1,300 kmol/s (1,723.35 kmol/s in summer
2005 minus 430 kmol/s). Compared to the mean state of the Kuroshio recirculation given in Guo et al. (2013;
i.e., 370.9 kmol/s from sections PN and OK to section ASUKA and 425 kmol/s from section ASUKA to section

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137E), there was a large ﬂuctuation in the variation of the westward nitratetransport due to the Kuroshio recalculation during the Kuroshio lager
meander period.
4.2. The Second EOF Mode
The time series of the principal component of the second EOF (Figure 7a)
showed a positive phase from 2000 to 2001 and 2006 to 2009 and a negative phase from 2002 to 2005 and 2010 to 2013. This was conﬁrmed as theperiods of stable (2002–2005 and 2010–2012) and unstable (1996–2001
and 2006–2009) states in the KEDO, as reported by Qiu et al. (2014).
When the Kuroshio Extension was in the stable state, the eastward volumetransport of both the Kuroshio and its southern recirculation gyre wasenhanced; the opposite was true for the unstable state. The spatial patternof velocity of the second EOF (Figure 7b) showed a surface maximum
located at 32.6°N and two surface minima at 33.6°N and 31.0°N. The spatial
pattern of nitrate concentration of the second EOF (Figure 7c) showed a
subsurface seesaw structure with a maximum located at 33.33°N, at
400 m, and a minimum at 32.0°N, at 500 m. The nitrate ﬂux (Figure 7d)
had a horizontal structure similar to that of velocity, although its maximum
varied at depths of 270, 430, and 800 m.
Figure 9. (a) The Mapping of Absolute Dynamic Topography (in intervals of
10 cm) output for the period when the principal component of the nitrate
ﬂux of the second empirical orthogonal function was greater than +1 standard deviation. (b) is the same as (a) for the period when the principal
component of the nitrate ﬂux of the second empirical orthogonal function
was less than 1 standard deviation. Thick black lines indicate contours of
110 cm.

Following Guo et al. (2012), we made composite maps of nitrate concentration, velocity, nitrate ﬂux, and MADT (Figures 8 and 9) corresponding
to the two states. In the unstable state, when the principal componentof the second EOF is larger than +1 standard deviation, the Kuroshio,
Kuroshio recirculation, and the counter coastal current north of theKuroshio were strengthened (Figure 8a). The eastward nitrate transport(Figure 8c) reached 1,250.95 kmol/s, with a subsurface maximum locatedat 32.7°N and a depth of 495 m. In the stable state, when the principal component of the second EOF is lessthan 1 standard deviation, the Kuroshio, Kuroshio recirculation, and counter coastal current north of theKuroshio were weakened (Figure 8b). The contours of the nitrate concentrations were ﬂatter than those in
the unstable state (Figure 8b), which explains why the spatial pattern of the second EOF had a dipole structure (Figure 7c). The eastward nitrate transport (Figure 8d) was 886.49 kmol/s; its subsurface maximum was
located at 33.3°N at a depth of 370 m. Therefore, the unstable state had a larger eastward nitrate transportwith a deeper subsurface maximum than the stable state. The MADT shows that the Kuroshio recirculation
south of Japan was enhanced (weakened) during the unstable (stable) state, and Kuroshio recirculation in
the Kuroshio Extension was weakened (enhanced) in similar conditions (Figure 9).

The net volume and nitrate transport over section 137E, modulated by the second EOF mode, can be derived
from one standard deviation of the principal component of the second EOF; particularly from the normalized
section of the integrated spatial patterns of velocity and nitrate ﬂux. This pattern had a net volume transportvariance of 0.88 Sv and a nitrate transport variance of 58.96 kmol/s. The low net volume transport reveals thatthe positive and negative volume transport at section 137E in this pattern were almost canceled, whichfurther implies that this pattern is controlled by the variation of the Kuroshio recirculation. It also shows thatthe variance in nitrate transport was affected by the horizontally asymmetrical distribution in the subsurface
maximum depth of the nitrate ﬂux. Thus, the velocity, eastward volume transport, nitrate ﬂux, and eastward
nitrate transport at section 137E were all enhanced during the unstable states of the modulation of the
KEDO, in 1996–2001 and 2006–2009. It also shows that the subsurface maximum of nitrate ﬂux occurredat deeper depths at these times. The opposite is true for the stable states of the modulation in 2002–2005
and 2010–2012.

5. Discussion
5.1. Variations in Surface Chlorophyll Levels
Both the seasonal cycle and the two dominant dynamic patterns suggest that the variation in primary productivity south of Japan may be related to local dynamic conditions, at least to the Kuroshio and Kuroshio

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Figure 10. (a) The ratio of the variance in the monthly climatic anomaly to the total variance in surface chlorophyll concentration. Surface chlorophyll concentrations(log10) for the (b) annual mean, (c) spring, (d) summer, (e) autumn, and (f) winter.

recirculation in the area, instead of to teleconnection. To verify this conclusion, we checked the satellitederived surface chlorophyll concentration in the area.
First, it was necessary to conﬁrm the extent to which the climatic anomaly corresponding to mesoscale andinterannual variations could explain the total variance, in the surface chlorophyll concentration (Figure 10a).
The extent was deﬁned as the ratio of the variance in the climatic anomaly of the surface chlorophyll concentration to the variance in surface chlorophyll concentration. This indicated the geographical importance ofthe climatic anomaly. In the Kuroshio recirculation region, the climatic anomaly explained less than 20% of
the total variance in surface chlorophyll concentration. This low value indicates that the seasonal variation
occurring in the subtropical gyre is important (Figures 10c–10f). In contrast, in the Kuroshio region along
the coast of Japan where this ratio is 40–80%, the climatic anomaly is dominant.
The annual mean surface chlorophyll concentration (Figure 10b) increased in the open ocean from the lower
to the higher latitudes. In the Kuroshio region, there was a shoreward increase in the surface chlorophyll concentration. The surface chlorophyll concentration also increased in spring and decreased in summer during
the seasonal cycle (Figures 10c–10f).
During the Kuroshio large meander period, the contour of the 0.4 log10 transformed surface chlorophyll
concentration showed an offshore bump in spring 2005; this was related to the Kuroshio path meander(Figure 6e), which did not appear in spring 2004 (Figures 11a and 11b). Moreover, during the modulationof the KEDO, the stable state had a lower surface chlorophyll concentration than the unstable state

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Figure 11. Surface chlorophyll concentration (log10) for (a) April 2004, (b) April 2005, the (c) stable state mean (including the mean surface chlorophyll concentrationin 2002–2005 and 2010), and the (d) unstable state mean (including the mean surface chlorophyll concentration during 1996–2001 and 2006–2009), respectively.

(Figures 11c and 11d). The meridional movement of the contours was comparable to that of the MADT
(Figures 8e–8f). The consistency in the location of the 1 log10 transformed contour between the stableand unstable states indicates that the modulation of KEDO was located in the region south of Japan.
Since the variation in the surface chlorophyll concentration likely results from variations in the local nutrient
supply, which could be related to the shift of the Kuroshio path, we checked nitrate concentrations at the bottom of the mixed layer (deﬁned as the depth where the temperature is lower than that at 10 m depth by
0.5 °C) at section 137E (Figure 12a). It increases during the winter on the north side of the Kuroshio (especiallyduring the large meander period), suggesting an upward nitrate supply to the mixed layer. To quantify thisupward nitrate ﬂux, we used the vertical velocity ﬁeld derived from the JCOPE-T model. The modeled
Kuroshio axis matches well with the observation made during the Kuroshio large meander period(Figures 12a and 12b). The vertical velocity at the bottom of the mixed layer intensiﬁes in winter. We calculated the mean vertical velocity in the area occupied by the counter coastal current (Figure 12c), shown as
the area between the two magenta lines in Figure 12b, with the line to the north denoting the boundaryof coastal water and counter coastal current and the line to the south denoting the boundary of theKuroshio and counter coastal current. It has a positive value of about 1–2 × 105 m/s. Assuming an averaged
nitrate concentration of 5 mmol/m3 at the bottom of the mixed layer in this area, the upward nitrate ﬂux isestimated at an order of O (104) mmol·m2·s1. This value is larger than the diffusive nitrate ﬂux (O(106) mmol·m2·s1; Kaneko et al., 2013) in the Kuroshio region by two orders of magnitude. This suggeststhat variations in the Kuroshio, not the turbulence upward nitrate ﬂux, control most of the variations in regional chlorophyll concentrations.
We then examined the vertical velocity at section 137E to ﬁnd a dynamic explanation (Figure 13). The
Kuroshio large meander in the JCOPE-T has a pattern similar to that in the MADT (Figure 6). The vertical displacement of the 18, 21, and 24 °C isotherms and the mixed layer depths represent the seasonal cycles, whilethat of the 15 °C isotherm represents dynamic variations in the interior of the Kuroshio. Generally, the 15 °C
isotherm and the bottom of the mixed layer have the smallest vertical distance in winter. During the Kuroshio
large meander period, a cyclonic eddy forms in the north side of the Kuroshio due to the downstream advection of the positive vorticity (Kuroda et al., 2013). The thermocline in the north side of the Kuroshio is

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upwardly displaced owing to the geostrophic balance and shows positive
vertical velocity in the north side of the Kuroshio. This allows the nutrientrich subsurface water to enter the mixed layer. Then the processes in thelatter bring these nutrients to the euphotic zone, where they become
available to biota and promote the production of chlorophyll.
Shiozaki et al. (2014) studied the factors controlling the onset of the springalgal bloom in the Kuroshio Extension and the Kuroshio-Oyashio transitionarea and concluded that light conditions and nitrate concentration wereinsufﬁcient for explaining the magnitude of the spring bloom. Instead,water mass advection was shown to be responsible on a regional basis.
Their results showed that the dominant mechanisms can be diverse evenin the Kuroshio Extension. Uchiyama et al. (2017) used the result of theirmodel to suggest that the downstream intensiﬁcation in primary productivity along the Kuroshio, from the Kuroshio recirculation to the Kuroshio
Extension, can be attributed to vertical nitrate transport induced by
eddies. They offered a climatological view of nutrient transport along theKuroshio and the related variations in primary productivity. However, several studies have shown that horizontal processes such as geostrophic
advection (Palter et al., 2005) and eddy advection (Lee & Williams, 2000)
may account for the supply of more nutrients than those accountable tovertical processes on different time scales. Our study shows that the shift
in the axis of the Kuroshio south of Japan results in a vertical displacementof the local thermocline and an upward nitrate ﬂux into the bottom of themixed layer. This vertical advection-induced nitrate ﬂux dominates theturbulent nitrate ﬂux and is closely related to the horizontal shift in the
Kuroshio axis.

Figure 12. (a) Nitrate concentration at the bottom of the mixed layer at section 137E as a function of latitude and time. The interval of the color shading3
is 2 mmol/m . The magenta line denotes axis of the Kuroshio, as taken fromthe website of the Japan Meteorological Agency. (b) Vertical velocity at thebottom of the mixed layer at section 137E, as a function of time and latitude.
The magenta lines denote the northern and southern boundaries of thecounter costal current to the south of the Kuroshio, and the thick black line
denotes the Kuroshio axis as derived from the JCOPE-T model. (c) Verticalvelocity averaged in the counter costal current area between the two
magenta lines in (b).

5.2. Seasonal Cycle in Nitrate Concentration Along the Kuroshio

The variation in nitrate concentration along the western boundary current
has been well discussed in studies on the Gulf Stream, but it is not as common with regard to the Kuroshio (Guo et al., 2013). It was traditionallyassumed that the nitrate concentration along the Gulf Stream increased
through diapycnal mixing (Pelegri & Csanady, 1991). However, Palter andLozier (2008) reported that the Gulf Stream had a higher nitrate concentration than that on either side of the current and concluded that the nutrient
concentration did not, therefore, increase along the Gulf Stream. In ourstudy (Table 3), the velocity-weighted nitrate concentration along theKuroshio, from upstream to downstream, decreased in spring and autumn
in the sequence CPN and COK > C137E > CKR and increased in summer and winter in the sequence
C137E > CPN > CKR and COK in the light layers (i.e., layers 1–2); in the intermediate and dense layers (3–5
and 6–8, respectively), it increased throughout the year in the sequence CKR > C137E > CPN and COK .
Williams et al. (2011) suggested that diapycnal mixing, isopycnal lateral exchange, and biologicalconsumption/regeneration had a role in the downstream variation of the nutrient concentration.
We used a mixing model to estimate the intensity of the lateral water exchange (supporting information
Table S1). In the dense layers, the lateral water exchange alone could generally explain the nutrient concentration at section 137E. In most intermediate layers, the concentration given by epipycnal mixing was larger
than the observed one. In the light layers, the model result matched the nitrate concentration of the Kuroshio
better in spring and autumn than in summer and winter. These results indicate that other processes should
be taken into consideration. The difference between the mixing model and observations also indicates that
there is a source term in the light layers and a sink term in the intermediate layers within the enclosed box.
Therefore, lateral water exchange is an important mechanism in the downstream variation of the velocityweighted nitrate concentration along the Kuroshio, especially in the intermediate and dense layers.

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Figure 13. (left) Monthly mean surface velocity (black arrows) and sea surface temperature (colored shading) from the JCOPE-T model output. The blue lines denotesection 137E. (right) Monthly mean vertical velocity (colored shading) at section 137E from the JCOPE-T model output. The black broken line denotes the bottom of
the mixed layer, the green line denotes isotherms, the red and blue contours denote eastward and westward velocities, respectively, and the thick black line
denotes a velocity of zero.

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Table 3
Velocity-Weighted Nitrate Concentration in Eight Isopycnal Layers (24.80,
25.50, 26.50, 27.00, 27.30, 27.50, 27.60, and 27.66σ θ)

Spring
1
2
3
4
5
6
7
8
Summer
1
2
3
4
5
6
7
8
Autumn
1
2
3
4
5
6
7
8
Winter
1
2
3
4
5
6
7
8

CPN

COK

CKR

C137E

1.80
7.47
15.91
27.54
37.16
N/A
N/A
N/A

1.05
6.40
16.45
30.99
38.19
40.73
38.88
38.50

0.47
3.96
14.51
29.79
38.90
41.49
41.50
40.55

1.01
5.40
14.77
28.15
37.32
40.83
41.13
40.34

2.19
7.26
15.21
27.91
36.79
N/A
N/A
N/A

0.99
5.41
15.13
29.33
36.71
39.15
37.97
39.90

0.65
4.77
15.08
30.40
38.91
41.19
41.05
40.19

2.71
7.56
16.18
27.84
36.55
40.46
40.51
39.56

2.18
9.61
19.11
29.78
37.25
N/A
N/A
N/A

1.78
6.47
15.81
30.80
38.00
40.38
40.18
39.73

0.78
4.77
14.53
29.56
38.72
41.26
41.04
39.82

1.97
6.33
15.90
28.77
37.50
40.78
40.98
40.12

1.71
7.40
16.35
28.68
36.75
N/A
N/A
N/A

0.77
5.28
14.27
29.86
37.37
39.90
42.77
39.53

0.88
4.99
14.34
29.18
38.45
41.33
41.31
40.32

2.51
7.16
15.99
28.84
37.38
40.70
41.03
39.89

10.1029/2017JC013635

In terms of diapycnal mixing, nitrate is transferred from nutrient-rich densewater to nitrate-depleted surface water. Kaneko et al. (2013) assessed thevertical diffusive nitrate ﬂux in the upper 300 m and gave a value of O
(106) mmol·N·m2·s1 on the north side of the Kuroshio; this is a larger
value than that assigned to the south side. If we apply the former to theentire Kuroshio area south of Japan that is 1,000 km long and 200 km wide,
the maximum vertical nitrate ﬂux over the entire area is 0.2 kmol/s. A
recent study (Tsutsumi et al., 2017) showed that the vertical diffusivity in
the Tokara Strait could be 100 times larger than that in the upstream
region owing to ﬂow-topography interactions. If we use an area 300 kmlong and 200 km wide for the Tokara Strait and a value 100 times the vertical diffusive nitrate ﬂux estimated by Kaneko et al. (2013), the verticalnitrate ﬂux could be 6 kmol/s, which is comparable to the variations in
nitrate transport induced by the difference in nitrate concentration along
the Kuroshio in the ﬁrst layer (Table S2). It must be noted that the Tokara
Strait is only ~700 m deep, and therefore, the strong vertical mixing there
has an effect on the light and intermediate layers at section 137E.
Therefore, the vertical diffusive nitrate ﬂux should be a good candidate
for the necessary source term in the light layers and the sink term in theintermediate layers.
The biogeochemical processes should be also considered potential candidates for the source terms. However, photosynthesis likely acts as a sinkterm in the light layers and remineralization as a source term in the deep
layers; its effects are opposite to those required by the source term in thelight layers and sink term in the intermediate layers. Yokouchi et al.
(2006) reported that the new photosynthesis and respiration rate in thisarea is 3.1  4.2 × 106 mmol·N·m2·s1. Applying the same analysis ofan area 1,000 km long and 200 km wide, the nitrate consumption in biological processes was estimated to be roughly 0.6–0.8 kmol/s in our study
area. This implies that the biological consumption is a relatively weak
explanation of the seasonal contrast in the nitrate concentrations along
the Kuroshio.

We also checked the ratio of the regenerated nitrate concentration to thenitrate concentration, which is also known as the extent of nitrate regeneration (Table S3) and is deﬁned as the ratio of nitrate concentration due
to remineralization to the total nitrate concentration (Creg/Ctotal). The
nitrate concentration at each section given by remineralization can be estimated using the expression Creg = AOU/R, where the Redﬁeld ratio R is taken as N:O2 = 16:170 (Redﬁeld
et al., 1963) and AOU stands for the apparent oxygen utilization. The remineralization rate is almost constantin layers 4–8 (all the dense layers and the two lower intermediate layers) throughout the year, varying with anamplitude of 1%. In layer 3 (the top of the intermediate layers), the remineralization rate ranges from 52% to60%, and it is the highest at section PN in summer and winter and relatively stable in the study area. The seasonal contrast is not clear in the Kuroshio recirculation. In layer 2 (the bottom of the light layers), it ranges
from 61% to 82% and varies among sections and seasons. There is no clear seasonality between section
PN and section 137E, indicating that remineralization might not be an important factor. The highest remineralization rate along the Kuroshio implies that the strong background stratiﬁcation in summer is a plausible
explanation for the source term in the light layer in that season.

Note. CKR, C137E, COK, and CPN are in millimole per cubic meter. Bold numbers denote increases along the Kuroshio from upstream to downstream.
N/A = not applicable.

The lateral water exchange is the dominant factor affecting variations in the velocity-weighted nitrate concentration in the dense layers along the Kuroshio, and it can explain more than 98% of the velocity-weighted
nitrate concentration at section 137E. The lateral water exchange accounted for more than 93% of the variations in the intermediate layers, while the diapycnal mixing is the second most important explanation andremineralization cannot be neglected in the upper intermediate layers. In the light layers, the seasonal

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variations are attributed mainly to the competition between seasonality of the lateral water exchange anddiapycnal mixing or remineralization. The lateral water mixing explains about half the total variation along
the Kuroshio, while the other mechanisms explain the remaining half. The lateral water exchange, in particular, is stronger than diapycnal mixing/remineralization in spring and autumn and weaker than these insummer and winter.

6. Summary
In this study, temporal variations in the Kuroshio nutrient stream south of Japan were examined using
seasonal hydrographic data collected from 2000 to 2013. The nitrate ﬂux was computed using the productof the nitrate concentration and absolute geostrophic velocity. The seasonal cycle and interannual variation
at section 137E of the Kuroshio nutrient stream south of Japan were studied using nitrate budget calculationand EOF analysis, respectively.
In the seasonal cycle, the eastward nitrate transport was the largest in autumn and the smallest in winter. The
calculations of the nitrate budget showed that about 60% of the mean eastward nitrate transport and its variations can be attributed to Kuroshio recirculation. Interpretations of the ﬁrst two EOF modes indicated that
the Kuroshio nutrient stream in the study area was modulated by two known patterns: the Kuroshio pathmeander (according to the ﬁrst EOF mode) and the KEDO (according to the second EOF mode). The spatial
pattern of the ﬁrst EOF was related to the meridional migration of the Kuroshio path south of Japan andthe zonal movement of Kuroshio recirculation in the Kuroshio large meander from 2004 to 2005. Although
the eastward nitrate transport ﬂuctuated between 608.76 kmol/s in summer 2004 and 1,723.35 kmol/s insummer 2005, downstream nitrate transport to the Kuroshio Extension region was stable. The spatial patternof the second EOF showed that the eastward nitrate transport was larger during the unstable state than in thestable state. This was also related to variations in the Kuroshio recirculation south of Japan.

Acknowledgments
This study was supported by the
National Natural Science Foundation ofChina (41576001, 41776107, 41621064,
41476020, 41606113, and 41576031);
the Scientiﬁc Research Fund of SIO
under grants JT1604, JG1621, andJT1801; the Project of State Key
Laboratory of Satellite Ocean
Environment Dynamics, SIO(SOEDZZ1701, SOEDZZ1806, and
SOEDZZ1804); the National Programme
on Global Change and Air-Sea
Interaction (GASIIPOVAI-01–02); and the
grants from the Ministry of Education,
Culture, Sports, Science andTechnology, Japan (MEXT) to a projecton Joint Usage/Research Center Leading Academia in Marine andEnvironmental research (LaMer). X. Guo
thanks support by JSPS KAKENHI grant
numbers (26287116/JPH05821). The
data used in this study were from JMA(http://www.data.jma.go.jp/gmd/
kaiyou/db/vessel_obs/data-report/
html/ship/ship.php), AVISO (ftp://ftp.
aviso.altimetry.fr/), and NASA (http://
oceandata.sci.gsfc.nasa.gov/).

LONG ET AL.

Surface chlorophyll concentration in the Kuroshio recirculation region, where the seasonal cycle was dominant and occurred in the subtropical gyre, was also examined. In the Kuroshio region, the climatologicalanomaly of surface chlorophyll concentration was dominant and related to interannual variation of theKuroshio path shift. When the Kuroshio took an offshore path, the surface chlorophyll concentration on its
north side increased owing to vertical displacement of the isopycnal layer-induced nitrate supply to themixed layer. The velocity-weighted nitrate concentration along the Kuroshio increased in the intermediate
and dense layer throughout the year; that in the light layer decreased in spring and autumn and increasedin summer and winter. Lateral water exchange and diapycnal mixing/remineralization were responsible forthis variation.

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