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Biogeosciences, 10, 6403–6417, 2013www.biogeosciences.net/10/6403/2013/
doi:10.5194/bg-10-6403-2013© Author(s) 2013. CC Attribution 3.0 License.

Spatial variations in the Kuroshio nutrient transport from the East
China Sea to south of JapanX. Y. Guo1,2 , X.-H. Zhu1,3 , Y. Long1 , and D. J. Huang1,3
1 State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, State Oceanic

Administration, Hangzhou 310012, China
2 Center for Marine Environmental Study, Ehime University, 2-5 Bunkyo-cho, Matsuyama 790-8577, Japan
3 Department of Ocean Science and Engineering, Zhejiang University, Hangzhou 310058, China
Correspondence to: X.-H. Zhu (xhzhu@sio.org.cn)
Received: 28 March 2013 – Published in Biogeosciences Discuss.: 16 April 2013
Revised: 23 August 2013 – Accepted: 4 September 2013 – Published: 10 October 2013

Abstract. Based on absolute geostrophic velocity, which was
calculated using repeated hydrographic data of 39 cruises
from 2000 to 2009 and nitrate concentrations measured in
the same areas from 1964 to 2009, we obtained the temporally averaged nitrate flux (the product of velocity and nitrate concentration) and nitrate transport (integration of flux
over one section) of four sections across the Kuroshio fromthe East China Sea (sections PN and TK) to an area southof Japan (sections ASUKA and 137E). In addition, we examined section OK east of the Ryukyu Islands in order tounderstand how the Ryukyu Current contributes to the transport of nutrients by the Kuroshio south of Japan. The mean
nitrate flux shows a subsurface maximum core with valuesof 9.6, 10.6, 11.2, 10.5, and 5.7 mol m−2 s−1 at sections PN,
TK, ASUKA, 137E, and OK, respectively. The depth of thesubsurface maximum core changes among these five sections
and is approximately 400, 500, 500, 400, and 800 m at sections PN, TK, ASUKA, 137E, and OK, respectively. The
mean downstream nitrate transport is 204.8, 165.8, 879.3,
1230.4, and 338.6 kmol s−1 at sections PN, TK, ASUKA,
137E, and OK, respectively. The transport of nutrients inthese sections suggests the presence of the Kuroshio nutrient stream from its upstream to downstream regions. The
deep current structure of the Ryukyu Current (section OK)
contributes to the same order of nitrate transport as does theKuroshio from the East China Sea (section TK) to the area
south of Japan; however, the former only has one-fifth thevolume transport of the latter. A budget calculation suggeststhat the downstream increase of transported nitrate along theKuroshio is mainly caused by the recirculation of nitrate

into the Kuroshio. This conclusion, however, depends on water depth. In the upper layers (< 26.5σθ ), the downstreamchange of nitrate concentration along the Kuroshio and thatfrom the recirculation of nitrate has a significant contribution to the downstream increase of nitrate transport along theKuroshio. In the deep layers (> 26.5σθ ), the change in nitrateconcentration is small and the Kuroshio recirculation dominates the downstream increase of nitrate transport.

1

Introduction

Strong western boundary currents carry large masses of water and amounts of heat as well as a variety of dissolved materials including nutrients, and have a significant influenceon the climate and marine ecosystems in the vicinity of theirpath. Quantitation of nutrient transport by the western boundary current is an important step to understand the nutrient element cycle on a basin scale. Pelegrí and Csanady (1991) and
Pelegrí et al. (1996) reported a subsurface maximum in the
nutrients flux (the product of velocity and nutrients concentration) and a nitrate transport (integration of flux over a section) with an order of 1000 kmol s−1 in the Gulf Stream, andfirst proposed the presence of a nutrient stream, a continuous
nutrient transport, along the Gulf Stream from its upstream
to its downstream areas.
As current speed is reduced, and since nutrient concentration increases as depth increases, the nutrient flux reaches
a high subsurface value (Pelegrí and Csanady, 1991). Basedon data from four cruises, Chen et al. (1994, 1995) calculated

Published by Copernicus Publications on behalf of the European Geosciences Union.

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X. Y. Guo et al.: Spatial variations in the Kuroshio nutrient transport

the nutrient flux of a section of the Kuroshio that is east of
Taiwan, and reported the presence of a subsurface maximum
core of nutrient flux at a depth of 500 m. Guo et al. (2012) obtained a long-term averaged nutrient flux at section PN (seeFig. 1b for its location) based on data from 88 cruises from1987 to 2009 and demonstrated a subsurface maximum coreof nutrient flux at 400 m depth in the long-term averaged nutrient flux. Comparing the nutrient fluxes of the section eastof Taiwan and at section PN, the depth of the subsurface maximum core of nutrient flux likely changes along the pathwayof the Kuroshio. However, except for these two sections, thevertical structure of the nutrient fluxes along the path of theKuroshio from the East China Sea to south of Japan is unknown.
Guo et al. (2012) reported a value of 170.8 kmol s−1 asthe long-term averaged amount of nitrate transported by the
Kuroshio along section PN in the East China Sea. This is
the only known value of the amount of nutrients transportedby the Kuroshio. It is smaller than that observed in the Gulf
Stream. As observed in the Gulf Stream, the amount of
nitrate that is transported may significantly increase along
this stream (Pelegrí and Csanady, 1991; Pelegrí et al., 1996;
Williams et al., 2011). Therefore, we also need to investigate
the downstream change of nutrient transport by the Kuroshioto (1) confirm the presence of the “Kuroshio nutrient stream”
from its upstream to downstream regions and (2) to understand the processes responsible for the downstream variations in nutrient transport by the Kuroshio. In this paper, thenitrate flux and nitrate transport are calculated through four
sections across the Kuroshio from its upstream region, theEast China Sea (sections PN and TK), to its downstream region, the area south of Japan (sections ASUKA and 137E). In
addition, we also examined section OK, which is east of the
Ryukyu Islands where the Ryukyu Current is located (Zhu
et al., 2004). While examining the nutrient flux and nutrient transport at five sections, our objectives are to confirm
the presence of the Kuroshio nutrient stream from the East
China Sea to the region south of Japan, reveal the downstream variations of the nutrient flux and nutrient transport
along the path of the Kuroshio, and to clarify the factors controlling the spatial variations of the nutrients transported by
the Kuroshio.

2

Data and analysis method

Conductivity Temperature Depth (CTD) profiler data fromwinter 2000 to fall 2009 and nutrient data from 1964
to fall 2009 at sections PN, TK, ASUKA, 137E andOK (Fig. 1b) were obtained from the Japan Meteorological Agency (JMA, http://www.data.kishou.go.jp/kaiyou/db/
vessel_obs/data-report/html/ship/ship.php); the nutrient data
were from water samples collected at standard depths andanalyzed using routine methods (see Table 1 in Aoyama etal. (2008) for details). Figure 2 summarizes the availability
Biogeosciences, 10, 6403–6417, 2013

Fig. 1. (a) Study area and schematic image of the Kuroshio path,
Kuroshio recirculation, and the Ryukyu Current. ECS: East China
Sea; TW: Taiwan. (b) Position of the hydrographic stations (blackdots), volume transport (red line, 1 Sv = 106 m3 s−1 ) and nitratetransport (blue line, kmol s−1 ) integrated from the sea surface to thedeepest layer with 25 km width. The positive direction for two transports is defined as the same direction as the Kuroshio or Ryukyu
Current. The thin straight lines connecting the black dots serve asa reference point for the transports. For the calculation method ofthese variables, see Eqs. (4)–(6) and their description in Sect. 2.
“PN”, “TK”, “OK”, “ASUKA”, and “137E” indicate the names ofthe sections. The Tokara Strait is located in section TK. The thincurved line denotes the 200 m isobath. The black asterisks separate
section OK into two parts: section OK-W to the west and section
OK-E to the east; section ASUKA into two parts: section ASUKAN to the north and section ASUKA-S to the south; and section 137E
into two parts: section 137E-N to the north and section 137E-S to
the south. There are four numbers for each section: the two red numbers indicate positive and negative volume transports through thesection within a unit of 0.1 Sv, and the two blue numbers indicate
positive and negative nitrate transports through the section within aunit of 0.1 kmol s−1 .

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X. Y. Guo et al.: Spatial variations in the Kuroshio nutrient transport

6405

Table 1. Along-stream variation of positive volume transport VT+ (Sv) and negative volume transport VT− (Sv), positive nutrient transport
NT+ (kmol s−1 ) and negative nutrient transport NT− (kmol s−1 ), positive transport-averaged nitrate concentration CT+ (mmol m−3 ) andnegative transport-averaged nitrate concentration CT− (mmol m−3 ), area-averaged nitrate concentrations for area with positive volumetransport CA+ (mmol m−3 ) and negative volume transport CA− (mmol m−3 ), net volume transport VT = VT+ +VT− (Sv) and net nutrienttransports NT = NT+ + NT− (kmol s−1 ) for each of and total 8 isopycnal layers. The first column is number of layer; the second to sixth
columns are one variable for sections PN, TK, OK, ASUKA, and 137E, respectively; the seventh to eleventh columns are another variable
for sections PN, TK, OK, ASUKA, and 137E, respectively.
PN

TK

OK

ASUKA

137E

PN

TK

VT+ (Sv)
1
2
3
4
5
6
7
8
Total

13.7
4.7
6.1
1.6
0.2
0.0
0.0
0.0
26.3

11.4
4.4
5.0
1.5
0.3
0.0
0.0
0.0
22.6

2.5
2.6
3.9
2.4
1.9
1.4
0.7
0.7
16.1

23.6
33.7
93.5
45.8
8.3
0.0
0.0
0.0
204.8

15.6
26.6
70.7
42.9
10.1
0.0
0.0
0.0
165.8

2.9
15.3
61.8
74.5
73.6
55.1
29.7
25.8
338.6

17.1
14.0
15.5
6.8
3.2
2.0
1.2
1.0
61.0

17.2
16.2
20.3
8.9
5.2
4.1
2.0
1.0
75.0

−1.2
−0.5
−0.6
−0.4
−0.2
0.0
0.0
0.0
−2.8

−0.5
0.0
0.0
0.0
0.0
−0.2
0.0
0.0
−0.7

1.7
7.2
15.4
28.6
36.0
0.0
0.0
0.0
7.8

1.4
6.1
14.1
28.1
34.9
0.0
0.0
0.0
7.3

1.1
5.9
15.9
30.8
38.2
40.4
40.3
39.4
21.0

43.3
86.0
249.2
202.2
123.2
83.3
50.5
41.6
879.3

35.3
111.5
338.4
262.5
194.8
168.4
81.4
38.2
1230.4

−1.3
−2.9
−8.7
−11.2
−7.8
0.0
0.0
0.0
−31.9

−0.3
−0.1
0.0
0.0
−1.0
−8.2
0.0
0.0
−9.5

2.5
6.2
16.0
29.8
38.1
40.9
40.9
40.2
14.4

1.9
7.2
16.3
29.9
36.4
0.0
0.0
0.0
14.5

1.4
6.2
13.6
28.7
35.3
0.0
0.0
0.0
9.7

0.9
5.8
16.6
31.5
39.0
40.7
40.2
39.4
30.8

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1.8
5.6
17.1
31.2
38.8
41.2
41.0
40.2
33.5

−2.9
−2.0
−2.2
−1.1
−0.9
−1.0
−0.6
−0.5
−11.2

−7.0
−9.3
−8.4
−3.5
−2.2
−1.6
−0.9
−0.9
−33.8

−8.7
−12.3
−11.8
−6.1
−4.6
−4.8
−3.3
−2.6
−54.2

−2.0
−10.4
−33.3
−34.5
−33.6
−40.3
−24.3
−19.7
−198.1−5.2−44.0−126.7−107.9
−84.1
−65.9
−37.7
−34.7
−506.2−6.6−62.1−185.0−186.3−181.0−198.8−133.8−103.3−1057.0

CT− (mmol m−3 )
2.1
6.9
16.6
29.4
37.4
40.7
40.8
40.0
16.4

1.1
6.2
15.6
29.4
36.2
0.0
0.0
0.0
11.3

0.5
4.9
0.0
0.0
36.3
39.1
0.0
0.0
12.7

CA+ (mmol m−3 )
1
2
3
4
5
6
7
8
Total

137E

NT− (kmol s−1 )CT+ (mmol m−3 )
1
2
3
4
5
6
7
8
Total

ASUKA

VT− (Sv)

NT+ (kmol s−1 )
1
2
3
4
5
6
7
8
Total

OK

0.7
5.2
15.2
30.6
38.7
40.5
40.2
39.2
17.7

0.7
4.7
15.2
30.4
38.6
41.1
41.0
40.0
15.0

0.8
5.0
15.7
30.7
38.9
41.2
41.1
40.1
19.5

CA− (mmol m−3 )
1.7
6.3
17.2
30.0
37.8
40.8
40.8
40.1
30.0

1.5
6.3
16.6
29.4
36.3
0.0
0.0
0.0
18.4

0.8
4.6
0.0
0.0
36.4
39.1
0.0
0.0
21.7

0.8
5.4
15.9
31.0
38.9
40.6
40.2
39.2
27.2

0.6
4.9
16.0
30.8
38.9
41.2
41.1
40.1
26.7

0.7
5.1
16.2
31.0
39.2
41.3
41.1
40.1
29.6

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X. Y. Guo et al.: Spatial variations in the Kuroshio nutrient transport

Table 1. Continued.
PN

TK

OK

ASUKA

137E

PN

TK

12.5
4.2
5.5
1.2
0.0
0.0
0.0
0.0
23.5

10.9
4.4
5.0
1.5
0.3
−0.2
0.0
0.0
21.8

−0.4
0.6
1.7
1.3
1.1
0.4
0.1
0.2
4.9

10.1
4.6
7.2
3.2
1.1
0.4
0.3
0.2
27.2

8.5
3.9
8.5
2.9
0.6
−0.7
−1.3
−1.6
20.7

of water temperature, salinity and nutrient data for all fivesections.
The CTD data at sections PN, TK, ASUKA, 137E and OK
from winter 2000 to fall 2009 (Fig. 2) were used in the inverse calculation of absolute geostrophic velocity. The reason that we limited our analysis to the last 10 yr was to use
a consistent average time for all the sections. As shown in
Fig. 2, the data at sections ASUKA and OK are highly concentrated during the last decade.
The inverse method (Wunsch, 1978) was applied to thethree areas that were enclosed by five sections, the line connecting the two stations at the north end of sections PN and
TK, and the coastline of southern Japan (Fig. 1b). The waterexchange through two gaps between the islands at the southwestward extension of section TK was neglected. The watercolumn was vertically separated into eight layers boundedby the sea surface and eight isopycnals (24.80, 25.50, 26.50,
27.00, 27.30, 27.50, 27.60, 27.66σθ ). An approximate massconservation was assumed within each of the eight layers,
while an approximate salt conservation was assumed within
the lower seven layers. By allowing small mass and salt unbalances in each layer, an optimum velocity was obtained at
the reference level, which was set at a depth of 2000 m for
sections OK, ASUKA, and 137E, 700 m for sections PN and
TK, and at sea bottom if the water depth was shallower thana depth of 2000 m or 700 m. The optimum reference level forthe inverse method to calculate the absolute geostrophic velocity was suggested to be 2000 dbar in the region southeastof the Ryukyu Islands (Yuan et al., 1998; Zhu et al., 2008)
and in the Kuroshio region south of Japan (Kaneko et al.,2001), and to be 700 dbar at sections PN and TK (Guo et al.,2012; Wei et al., 2013). After obtaining the velocity at thereference level, the absolute geostrophic velocity normal to
the line connecting the two stations at one-meter intervals inthe vertical direction was obtained from the thermal winds
relation. The absolute geostrophic velocity at one section isdenoted as V (x, z, t), where x is the horizontal distance fromleft side of the section, z is water depth, and t is the time ofobservations.

Biogeosciences, 10, 6403–6417, 2013

ASUKA

137E

NT (kmol s−1 )VT (Sv)
1
2
3
4
5
6
7
8
Total

OK

22.3
30.8
84.8
34.5
0.5
0.0
0.0
0.0
172.9

15.3
26.5
70.7
42.9
9.1
−8.2
0.0
0.0
156.3

0.9
4.9
28.5
40.0
40.0
14.7
5.4
6.0
140.4

38.1
42.0
122.4
94.4
39.1
17.4
12.8
6.9
373.0

28.7
49.4
153.3
76.2
13.8
−30.4
−52.5
−65.1
173.5

To calculate the nutrient flux, the product of velocity andnutrients concentration, the nutrient concentration at the definition point of velocity is also required. The nutrients in the
JMA data set have nitrate and phosphate concentrations but
not silicate. Since nitrate and phosphate concentrations havea good linear correlation (Guo et al., 2012), the focus of thisstudy is on nitrate. The nitrate concentrations at standard levels were first linearly interpolated at one-meter intervals at
the hydrographic stations along each section and were thenlinearly interpolated horizontally to the definition point ofvelocity at the same depth, which were set at the middle oftwo hydrographical stations. The nutrient flux, F (x, z, t) was
calculated by using F (x, z, t) = V (x, z, t)C(x, z, t), whereC(x, z, t) is nitrate concentration.
It should be noted that the large meander in the
Kuroshio significantly changes the nitrate concentration
at section 137E. This large meander was observed toform from August 1975 to March 1980, November 1981
to May 1984, December 1986 to July 1988, December
1989 to December 1990, and from July 2004 to August 2005 (http://www.data.kishou.go.jp/kaiyou/shindan/b_
2/kuroshio_stream/kuroshio_stream.html). During these periods, the area between the meander and coast became large
and the nitrate concentration in this area was high. A simple
average, without considering the path of the Kuroshio, may
induce a pseudo higher nitrate concentration at section 137E.
Therefore, the data that were collected during the period of
this large meander at section 137E were removed (Fig. 2).
The purpose of this study is to obtain the value of themean nutrient flux of the Kuroshio at each section during
all cruises. As typically done, the direct average of the nutrient flux from each cruise was computed as shown in thefollowing equations:
F0 (x, z) =

N
N
1 X
1 X
F (x, z, tk ) =V (x, z, tk )C(x, z, tk ), (1)
N k=1
N k=1

where F0 (x, z) is the mean nutrient flux, subscript k is thetime index, and N is the total number of cruises. To calculate
F (x, z, tk ), both the current speed and nutrient concentrationwww.biogeosciences.net/10/6403/2013/

X. Y. Guo et al.: Spatial variations in the Kuroshio nutrient transport

6407

Fig. 2. Data distribution of the five sections. Blue dots denote the CTD data used to calculate velocity; grey circles denote the bottle data
for water temperature and salinity; red dots denote the raw nitrate concentration; black circles denote the raw nitrate concentration used to
calculate the nitrate flux; black crosses denote the temporally averaged nitrate concentration used to calculate the nitrate flux when the raw
nitrate concentration was not available. The grey shading for section 137E denotes the periods when the large meander of the Kuroshio was
observed and the data collected during periods were not used in this study.

from the same cruise are needed. However, nitrate concentration was not sampled for section TK from winter 2000 to fall2009, and was not sufficiently sampled for section ASUKA
from winter 2000 to fall 2009 (Fig. 2). Therefore, it is difficult to use Eq. (1) to calculate F (x, z, tk ) for all cruises fromwinter 2000 to fall 2009 at these two sections. Instead, nitrateconcentrations were temporally averaged at the two sections
for all cruises that reported a nitrate concentration by using
the following equation:
C0 (x, z) =N0
1 X
C(x, z, tk ),N0 k=1(2)where C0 (x, z) is the mean nitrate concentration and N0 is
the total number of cruises for the nitrate data at sections
TK or ASUKA. C0 (x, z) was then used to replace C(x, z, tk )in Eq. (1) and F (x, z, tk ) was obtained for all cruises fromwinter 2000 to fall 2009 at the two sections.
However, this replacement introduces an error in themean nitrate flux F0 (x, z). The nutrient concentration at a
fixed place during a cruise, C(x, z, t), contains the mean
component C0 (x, z) and its temporally varying component C 0 (x, z, t), i.e., C(x, z, t) = C0 (x, z) + C 0 (x, z, t).
Similarly, the current speed at a fixed place during a cruise, V (x, z, t), also has a mean component, V0 (x, z), and a temporally varying component,
V 0 (x, z, t), i.e., V (x, z, t) = V0 (x, z) + V 0 (x, z, t) andNP
V (x, z, tk ). With such separation, the mean
V0 (x, z) = N1
k=1

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nutrient flux becomes
F0 (x, z) = V0 (x, z)C0 (x, z) +N
1 X
V 0 (x, z, tk )C 0 (x, z, tk ). (3)N k=1

According to Eq. (3), the mean nutrient flux at a fixed point
comprises two components. One is the product of the meancurrent speed and mean nutrient concentration. The other isthe temporal average of the product of temporally varying
component of current speed and nutrient concentration. The
calculation of F0 (x, z) at sections TK and ASUKA neglects
the contribution of the second term on the RHS of Eq. (3).
This is the first problem when calculating the mean nutrient
flux at sections TK and ASUKA. The second problem is thedifferent period used for obtaining the mean nutrient concentration (before 2000) from that used for mean velocity(from winter 2000 to fall 2009) at section TK. As discussed
in Sect. 4, these problems do not interfere with the results ofthis study.
The spatial integral of F0 (x, z) at a section used to define the nutrient transport through the section. In order toavoid a pseudo spatial variation of nutrient transport along
a section due to the different horizontal distances of the hydrographic stations, C0 (x, z), V0 (x, z), and F0 (x, z) were resampled by dividing a section using a unit width of 25 km
in the horizontal direction and a 1 m interval in the verticaldirection. The resampled variables are denoted as C0 (i, j ),V0 (i, j ), and F0 (i, j ), where i and j are indices in the horizontal and vertical directions, respectively. Based on the resampled variables, three variables were obtained at each of
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X. Y. Guo et al.: Spatial variations in the Kuroshio nutrient transport

the eight isopycnal layers (l) for each section, i.e., spatiallyaveraged nitrate concentrations were obtained for each unit
width C0 (i, l), volume transport per unit width VT0 (i, l), andnitrate transport per unit width NT0 (i, l), by using the following equations:
C0 (i, l) =j2
X1
C0 (i, j )j2 − j1 + 1 j =j(4)

1

VT0 (i, l) =j2
XV0 (i, j )1x(5)F0 (i, j )1x,

(6)

j =j1

NT0 (i, l) =

j2
X
j =j1

where l is the index for eight isopycnal layers, j1 and j2
are two depth indices for the upper end and lower end of
one isopycnal layer, and 1x is the unit width (25 km). The
vertical sum of VT0 (i, l) and NT0 (i, l) for l = 1, 8 produces
the volume transport VT(i) and nutrient transport NT(i)within one unit width along a section. The horizontal sumof VT0 (i, l) and NT0 (i, l) for i = 1, M (M is the grid point
number of a section in the horizontal direction) produces thetotal volume transport VT(l) and nutrient transport NT(l)for one isopycnal layer of a section; the horizontal sum of
VT(i) and NT(i) for i = 1, M or the vertical sum of VT(l)and NT(l) for l = 1, 8 produces the total volume transport
VT and total nutrient transport NT through a section.
As presented in Sect. 3, the recirculation south of theKuroshio is very important to the eastward nutrient transport
by the Kuroshio south of Japan. To determine the contribution of this recirculation, positive values for the nutrient fluxand nutrient transport are defined at a section if they have thesame direction (approximately eastward) as the Kuroshio or
Ryukyu Current; negative values are used for the nutrient fluxand nutrient transport if they are opposite in direction (approximately westward) compared to the Kuroshio or Ryukyu
Current. Superscript “+” (“−”) is used to represent positive(negative) velocity, nutrient flux and nutrient transport.
3
3.1

Results
Current fields at the five sections

The plots of temporally averaged velocity present a consistent image of the Kuroshio from the East China Sea (Fig. 3aand b) to south of Japan (Fig. 3d and e). In the East China
Sea, the surface current speed exceeds 1 m s−1 above theshelf break at section PN and exhibits a structure with asingle maximum speed core around 28.3◦ N. The current reduces its speed in proportion to the distance from the maximum speed core in both horizontal and vertical directions.
The current speed exceeds 0.1 m s−1 at 600 m depth, showing high potential to carry dissolved material downstream in
the subsurface layer.
Biogeosciences, 10, 6403–6417, 2013

At the Tokara Strait, which is where the Kuroshio exits
the East China Sea, the current shows two maximum speed
cores at the southern and northern channels (Fig. 3b). The
surface maximum current speed at the Tokara Strait is lower
at the northern channel than at the southern channel, and bothchannels are lower than those at section PN. Although thesurface current is weaker at the Tokara Strait than at section
PN, the reduction of current speed with depth is also smallerin the Tokara Strait than at section PN. Consequently, a current speed of 0.3 m s−1 is maintained at 600 m depth in thesouthern channel of the Tokara Strait.
At section ASUKA (Fig. 3d), the Kuroshio is significantly
intensified, which may correspond to the large area with acurrent speed larger than 0.6 ms−1 and the offshore extension of an area with a current speed larger than 0.1 m s−1 .
The eastward volume transport at section ASUKA is 61.0
Sv, which is much larger than that at section PN (26.3 Sv)
and section TK (22.6 Sv) (Table 1). On the other hand, a current with an opposite direction to the Kuroshio can be easily
identified south of the Kuroshio. The speed of this countercurrent is much weaker than the Kuroshio but its presence
covers a large area.
The current structure at section 137E (Fig. 3e) is similar with that at section ASUKA, i.e., a strong and robust
eastward Kuroshio and a weak but wide westward countercurrent. However, the distance between the Kuroshio andthe coastline at section 137E is greater than that of section
ASUKA. A counter coastal current appears at the inshoreside of the Kuroshio at section 137E (Fig. 3e).
The current is weak in the surface layer but strong in thesubsurface at section OK (Fig. 3c). This is a well-known feature of the Ryukyu Current (Zhu et al., 2004), which is abranch current of the Kuroshio in the East China Sea (Andres et al., 2008). Although it is not very clear in Fig. 3d, amaximum speed core can be identified at 600 m depth. The
eastward volume transport is 16.1 Sv at section OK.
The eastward volume transport of the Kuroshio is 26.8 Sv
at section PN and 23.5 Sv at section TK, and changes only
slightly in the East China Sea. The combination of the eastward volume transport at sections TK and OK is ∼ 40.0 Sv,
which is less than that at section ASUKA (61.0 Sv). Although sections ASUKA and 137E are both located southof Japan and not far from each other, the eastward volumetransport increased by 14.0 Sv from section ASUKA to section 137E (75.0 Sv; Table 1).
The increasing eastward volume transport of the Kuroshio
south of Japan is caused by the counter current south of theKuroshio, i.e., the Kuroshio recirculation. The eastward volume transport of the Kuroshio south of Japan consists of
two parts: one is related to basin-scale wind stresses andthe other is related to the anticyclonic recirculation gyre,which is a local current system south of the Kuroshio (Hasunuma and Yoshida, 1978). Imawaki et al. (2001) indicatedthat the recirculation played a significant role in the eastward volume transport of the Kuroshio measured at sectionwww.biogeosciences.net/10/6403/2013/

X. Y. Guo et al.: Spatial variations in the Kuroshio nutrient transport

6409

Fig. 3. Temporally averaged absolute geostrophic velocity (ms−1 ) and nitrate concentration (mmol m−3 ) for all five sections. Colored areas
and black contours show velocity with an interval of 0.1 m s−1 . Positive values are defined as the direction of the Kuroshio or Ryukyu
Current. Red contours indicate nitrate concentrations at an interval of 5 mmol m−3 , with the exception of 41 mmol m−3 . Thick black lines
indicate a speed of zero. The inverse triangles denote the hydrographic stations where water temperature, salinity, and nitrate concentrations
are available.

ASUKA. Ichikawa et al. (2004) also noted its influences on
the Ryukyu Current and attributed the increase in volumetransport from section OK (Zhu et al., 2004) to a section
southeast of Amami island to the presence of the Kuroshio
recirculation.
3.2

Nitrate concentrations and nitrate fluxes at the fivesections

The temporally averaged nitrate concentration has a relatively simple distribution at the five sections (Fig. 3).
In general, nitrate concentration increases with depth:
approximately zero at the sea surface, 5 mmol m−3 at
100–200 m depth, 35 mmol m−3 at 700–800 m depth, and∼ 40 mmol m−3 below 1000 m depth. The vertical varying
rate of nitrate concentration is small in the surface layer(0–200 m), large in the subsurface layer (200–1000 m), andsmall in the deep layer (> 1000 m). The horizontal variations
in mean nitrate concentration at these sections are found only
near the shelf slope where the contour lines of nitrate concentration raise toward the shallow shelf region (Fig. 3).
The temporally averaged nitrate flux has a consistent subsurface maximum core at all five sections (Fig. 4). The depthof the subsurface maximum core is relatively stable along thepath of the Kuroshio: 400 m at section PN, 500 m at section
TK, 500 m at section ASUKA, and 400 m at section 137E.
However, the corresponding isopycnal layer of the subsurface maximum core slightly changes along the path of theKuroshio: 25.5–26.5σθ at section PN, ∼ 26.5σθ at section
TK, ∼ 26.5σθ at section ASUKA, and 25.5–26.5σθ at section 137E. The subsurface maximum core of nitrate flux atwww.biogeosciences.net/10/6403/2013/section OK is located at a depth of 800 m, with a corresponding an isopycnal layer of 27.0σθ .
The nitrate flux at its subsurface maximum core is
9.6 mmol m−2 s−1 at section PN, 10.6 mmol m−2 s−1 atsection TK, 11.2 mmol m−2 s−1 at section ASUKA, and10.5 mmol m−2 s−1 at section 137E, indicating that the values along the path of the Kuroshio are stable. The nitrate flux
is 5.7 mmol m−2 s−1 at section OK. The high nitrate concentration at 800 m depth is the reason why the Ryukyu Current properly transports nutrients to regions south of Japan,
since the current is weak at section OK. In fact, the mean
nitrate concentrations at the subsurface maximum core of nitrate flux are 18.3, 26.6, 24.6, 19.7, 36.0 mmol m−3 at sections PN, TK, ASUKA, 137E, and OK, respectively, while
the mean speeds at the subsurface maximum core of nitrate
flux are 0.50, 0.40, 0.45, 0.53, 0.16 m s−1 at sections PN, TK,
ASUKA, 137E, and OK, respectively.
Since the maximum nitrate flux does not significantly
change along the Kuroshio, the nitrate transport through eachsection should be proportional to the area covered by thesame contour of nitrate flux. Furthermore, the area with a nitrate flux larger than 1 mmol m−2 s−1 significantly increases
at sections ASUKA and 137E. Consequently, the eastward
nitrate transports are higher at sections ASUKA and 137E
than at sections PN, TK, and OK (Table 1).
Up to now, we have only discussed the positive nitrate flux
in Fig. 4. However, negative nitrate flux can also be observedat all sections (Fig. 4). This is particularly apparent for sections ASUKA and 137E, where an increase in negative nitrate flux in the subsurface layer is widely observed. This

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6410

X. Y. Guo et al.: Spatial variations in the Kuroshio nutrient transport

Fig. 4. Temporally averaged nitrate flux (mmol m−2 s−1 ) for all sections. Colored areas and black contours indicate the nitrate flux with aninterval of 1 mmol m−2 s−1 . Positive values of velocity are defined as in Fig. 3. Red contours indicate the eight isopycnal layers. Thick blacklines indicate zero nitrate flux. The inverse triangles are the same as those in Fig. 3.

negative nitrate flux is caused by a weak but broad Kuroshio
recirculation south of the Kuroshio (Fig. 4), and this westward nitrate transport also shows a downstream intensification (Table 1).
3.3

Horizontal distributions of nitrate transport
through a defined unit width

In order to obtain a general image of the horizontal distribution of nutrient transport in the study region, we determined the averaged volume transport VT0 (i, l) and nitrate transport NT0 (i, l) defined by Eqs. (4)–(6) for a defined unit width (25 km) within each of eight isopycnal layers(Fig. 5). The eastward volume transport is large in the upper layers (Fig. 5a–c) but small in the lower layers (Fig. 5f–h). The counter current is apparent at sections ASUKA and
137E, and is located just south of the eastward Kuroshio(Fig. 5a–c). The nitrate concentration varies mainly in thevertical direction, as determined by the difference between
the volume transport and nitrate transport in Fig. 5.
The eastward nitrate transport can be confirmed in the
1st layer (Fig. 5a), appears in the 2nd layer (Fig. 5b) to
the 6th layer (Fig. 5f), and weakens in the two heaviest
layers (Fig. 5g–h). In the first layer (Fig. 5a), the downstream intensification of the eastward nitrate transport along
the path of the Kuroshio is observed from section TK tosection ASUKA, but not from section ASUKA to section
137E. However, the downstream intensification of the eastward nitrate transport from section TK to section ASUKA
and from section ASUKA to section 137E is evident for layers 2 through 5, while that of section ASUKA to section 137E
remains constant for the 6th to 8th layers. On the other hand,
Biogeosciences, 10, 6403–6417, 2013

the westward nitrate transport is evident for layers 2 through8. Being similar to the eastward nitrate transport, the westward nitrate transport also intensified from section ASUKA
to section 137E.
Because the horizontal variations of nitrate transport are
similar among the eight layers, the integration from the 1st
layer to the 8th layer (Fig. 1b) does not provide a surprising result. The increase in eastward nitrate transport fromthe East China Sea to south of Japan as well as the downstream intensification of both the eastward and westward nitrate transports from section ASUKA to section 137E are apparent in Fig. 1b.
4
4.1

Discussion
Error caused by using mean nitrite concentration

As we described in Sect. 2, the mean nitrate flux at sections
TK and ASUKA were obtained by calculating the productof the mean current speed and mean nitrate concentration fordifferent periods. We now use section 137E as an example toexamine the error caused by this method.
Figure 6a shows the nitrate flux obtained by calculating
the product of the mean current speed and mean nitrate concentration for the same period (2000 to 2009). The difference
between Figs. 6a and 4e is presented as Fig. 6b; the difference
shown is the contribution of the second term on the RHS of
Eq. (3). The spatial structure of the nitrate flux in Fig. 6a issimilar to that in Fig. 4e. The maximum difference between
the two mean nitrate fluxes is ∼ 0.9 mmol m−2 s−1 (Fig. 6b),which is less than 10 % of the nitrate flux at the same location
shown in Fig. 4e. This result suggests that the contribution ofwww.biogeosciences.net/10/6403/2013/

X. Y. Guo et al.: Spatial variations in the Kuroshio nutrient transport

6411

o

36 N
172
328
(−1)
(−2)

o

Latitude (°N)

33 N
30oN
o

27 N

171
459
(0)
(0)

114
156
(−5)
(−3)
137
240
(−12)
(−14)

0
1
(−70)
(−55)

24oN

15
21
(−1)
(−2)

21oN

(a) <24.80σθ

11
10
(−28)
(−19)

0
1
(−86)
(−58)

44
266
(0)
(−1)
47
354
(−5)
(−29)

2 Sv
20 kmol s−1

162
1064
(−1)
(−9)

140
861
(0)
(−1)
0
2
(−93)
(−417)

16
86
(0)
(0)

10
62
(−20)
(b) 24.80−25.50σθ(−97)

00(−122)(−568)

2 Sv
20 kmol s−1

50
707
(0)
(0)
61
955
(−6)
(−83)

24
382
(0)
(0)

203
3210
(−4)
(−80)

155
2485
(0)
(0)
0
0
(−83)
(−1235)

14
234
(−22)
(c) 25.50−26.50σθ(−322)00(−114)(−1669)

15
429
(0)
(0)
16
465
(−4)
(−107)

2 Sv
20 kmol s−1

15
460
(0)
(0)

89
2546
(−5)
(−161)

68
2015
(0)
(−1)
0
6
(−35)
(−1064)
9
287
(−11)

(d) 26.50−27.00σθ(−342)

0
0
(−55)
(−1652)2 Sv
20 kmol s−1

o

36 N
52
1937
(−4)
(−165)oLatitude (°N)

33 N

3
101
(0)
(−10)

30oN
o

27 N
24oN
o

21 N

2
83
(−2)
(−77)

11
420
(0)
(0)

31
1187
(−1)
(−28)
1
47
(−21)
(−810)

8321(−9)(e) 27.00−27.30σθ(−337)

0
0
(−42)
(−1635)

1.5 Sv
15 kmol s−1

0
0
(−2)
(−82)
0
0
(0)
(0)

6
222
(−1)
(−25)

3
134
(−15)
(−612)

8332(−8)(f) 27.30−27.50σθ(−337)

o

41
1679
(−5)
(−203)

17
699
(−1)
(−45)

0
0
(−43)
(−1788)

1.5 Sv
15 kmol s−1

0
0
(0)
(0)
0
0
(0)
(0)

2
66
(0)
(−1)

20
800
(−3)
(−132)

6
263
(−2)
(−78)
6
242
(−7)
(−298)

6233(−6)(−243)(g) 27.50−27.60σθ

0
0
(0)
(0)

0
2
(−29)
(−1207)

0
0
(0)
(0)

1.5 Sv
15 kmol s−1

1
39
(0)
(−4)

8
328
(−4)
(−153)

158(−4)(−150)
9358(−5)(−196)
6221(−5)(−194)(h) 27.60− 27.66σθ

1
55
(−22)
(−878)

1.5 Sv
15 kmol s−1

126oE 130oE 134oE 138oE 142oE 126 E 130oE 134oE 138oE 142oE 126oE 130oE 134oE 138oE 142oE 126oE 130oE 134oE 138oE 142oE
Longitude (°E)

Longitude (°E)

Longitude (°E)

Longitude (°E)

Fig. 5. The same as Fig. 1b, but for each of the eight isopycnal layers: (a) sea surface–24.8σθ , (b) 24.8–25.5σθ , (c) 25.5–26.5σθ ,(d) 26.5–27σθ , (e) 27–27.3σθ , (f) 27.3–27.5σθ , (g) 27.5–27.6σθ , and (h) 27.6–27.66σθ .

the second term on the RHS of Eq. (3) to the mean nitrate flux
is smaller than that of the first term on the RHS of Eq. (3) by
one order. Therefore, the approximation using only the firstterm on the RHS of Eq. (3) at sections TK and ASUKA is
acceptable.
The second problem with calculating the mean nitrate flux
at sections TK and ASUKA is that different periods wereused for averaging the current speed and nitrate concentration data. This is not a major problem for section ASUKA
but is a significant problem for section TK, where the averaging period for the current speed (2000 to 2009) and thatfor the nitrate concentration (prior to 1990) are completely
different. This treatment not only neglects the second termon the RHS of Eq. (3) but also changes the mean nitrate concentration used in the first term on the RHS of Eq. (3). Figure 6c shows the nitrate flux that was obtained by calculating
the product of the mean current speed from 2000 to 2009
and the mean nitrate concentration from 1970 to 1999 at section 137E. The difference between Figs. 6c and 4c is also
small (Fig. 6d), suggesting that the influence of using different periods for the mean nitrate concentration and the current
speed does not cause a significant change in the mean nitrateflux. Therefore, the mean nitrate fluxes for sections TK and
ASUKA, which are presented in Fig. 4b and c, are acceptable
as a first approximation to the true values.
4.2

Contribution of the Ryukyu Current and Kuroshio
recirculation to the downstream nitrate transport of
the Kuroshio

To quantitatively estimate the nitrate transport at the five sections of this study, the totals of the following items, which arewww.biogeosciences.net/10/6403/2013/

listed in Table 1, were defined for each of the eight isopycnal layers: positive volume transport (VT+ ), positive nutrienttransport (NT+ ), positive transport-weighted
mean nitrate
concentration (CT+ = NT+ VT+ ), area-weighted mean nitrate concentration for the areas with positive volume transport (CA+ ), negative volume transport (VT− ), negative nutrient transport (NT− ), negative transport-weightedmean
nitrate concentration (CT− = NT− VT− ), area-weighted
mean nitrate concentration for the areas with negative volume transport (CA− ), net volume transport (VT = VT+ +VT− ), and the net nutrient transports (NT = NT+ + NT− ).
Since nitrate concentration generally increases with depth
while velocity decreases with depth, CT+ (CT− ) is usuallysmaller than CA+ (CA− ) due to positive vertical shear. In
other words, the difference between CT+ and CA+ or thedifference between CT− and CA− reflects the effects of thevertical shear of the current on the nitrate transport.
Sections PN and TK are very similar in structure for all theterms shown in Table 1. The positive volume transport (VT+ )and nitrate transport (NT+ ) are of the same order for all the
isopycnal layers at the two sections, while the negative volume transport (VT− ) and negative nitrate transport (NT− )are smaller than the positive volume transport and positive
nitrate transport, respectively, by one order. A consistent reduction of VT+ , NT+ , CT+ , CA+ from section PN to section
TK can be confirmed in all the isopycnal layers listed in Table 1. The reduction in VT+ is consistent with the separationof a northward branch current from the Kuroshio southwestof Kyushu (Lie et al., 1998; Guo et al., 2006). The reductionin NT+ from section PN to section TK is caused not onlyby the reduction in VT+ but also by the reduction in CT+ .
The primary reason for the reduction in CT+ is the reduction
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6412

X. Y. Guo et al.: Spatial variations in the Kuroshio nutrient transport

Fig. 6. Nitrate flux (mol m−2 s−1 ) for section 137E calculated using the temporally averaged speed from 2000 to 2009, (a) temporally
averaged nitrate concentration before 2000, and (c) temporally averaged nitrate concentration from 2000 to 2009. (b) Difference between
Fig. 4e and (a); (d) difference between Fig. 4e and (c). Colored areas and black contour lines indicate the nitrate flux with an interval of
1 mol m−2 s−1 in (a) and (c), and with an interval of 0.3 mol m−2 s−1 in (b) and (d). Positive values indicate an eastward flux. Red contour
lines indicate the eight isopycnal layers. Thick black lines indicate zero nitrate flux. The inverse triangles are the same as those in Fig. 3.

in CA+ from section PN to section TK. The weaker vertical
shear of the current at section TK than at section PN (Fig. 3)
induces a smaller difference of CT+ between the two sections than that of CA+ .
The Ryukyu Current plays a more important role in thedownstream transport of nitrate than the downstream transport of volume. The positive volume transport (VT+ ) is
16.1 Sv at section OK, which is smaller than that at section
TK (22.6 Sv; Table 1). However, the positive nitrate transport(NT+ ) is 338.6 kmol s−1 at section OK, which is almost two
times that at section TK (165.8 kmol s−1 ; Table 1). The sameconclusion is also held for the net nitrate transport (NT) (Table 1). As stated previously, the direct reason for the large
nitrate transport at section OK is the deep location of relatively strong current speed, where the nitrate concentration
is high (Fig. 3).
Another important issue is the contribution of recirculation, as shown from the values for section OK in Table 1. Differing from sections PN and TK, the negative volume transport (VT− ) and negative nitrate transport (NT− ) at section
OK are of the same order as the positive volume transport(VT+ ) and positive nitrate transport (NT+ ), respectively.
From the distribution of the nitrate transport for the definedunit width (Fig. 1), we know that the positive nitrate transport
is located adjacent to Okinawa Island, which corresponds to
the Ryukyu Current, while the negative nitrate transport is
widely distributed far from Okinawa Island. Therefore, thesecond reason that the Ryukyu Current brings more nutrientsto the area south of Japan than the Kuroshio in the East China
Sea is the contribution of recirculation.

Biogeosciences, 10, 6403–6417, 2013

The net volume transport (VT) at section ASUKA is similar to the sum of those at sections TK and OK, but the net
nitrate transport (NT) at section ASUKA is larger than thesum of those at sections TK and OK (Table 1). This result indicates an increase of nitrate concentration from section TK
and section OK to section ASUKA, which will be examinedin the next subsection. Layers 1 through 5, especially layer 3(25.50–26.50σθ ) and layer 4 (26.50–27.00σθ ,), have a major
contribution to the net nitrate transport at section ASUKA.
The positive and negative nitrate transports are significantly
higher at section ASUKA compared to their sum at sections
TK and OK, indicating again the apparent contribution of
the Kuroshio recirculation to the negative nitrate transport(NT− ) as well as positive nitrate transport (NT+ ). As shownin Fig. 4 and Table 1, the recirculation-induced negative nitrate transport is large in the subsurface of the current (layers
3–5). As a response to the recirculation-induced negative nitrate transport, the positive nitrate transport in these layers
also increases. Consequently, the positive nitrate transport at
section ASUKA reaches > 1.7 times the sum of those at sections TK and OK.
Although sections ASUKA and 137E are in close proximity to each other, the positive nitrate transport (NT+ ) increased by 50 % from section ASUKA to section 137E (Table 1). Since the positive volume transport (VT+ ) only increased by 25 % from section ASUKA to section 137E, theincrease of nitrate concentration from section ASUKA tosection 137E is expected. However, such a change in nitrate concentration (CT+ and CA+ ) was only confirmed fromthe 2nd layer to the 3rd layer. The negative nitrate transport(NT− ) is larger at section 137E than at section ASUKA.
www.biogeosciences.net/10/6403/2013/

X. Y. Guo et al.: Spatial variations in the Kuroshio nutrient transportThis indicates that a part of the negative nitrate transport
returns to the Kuroshio between these two sections. Therefore, a clear image can be drawn for the contribution of theKuroshio recirculation area that merges into the Kuroshio
main stream and increases the positive nitrate transport along
the Kuroshio.
4.3

A budget calculation for water and nutrienttransports within each isopycnal layer

As described in the previous subsection, the Kuroshio recirculation is an apparent cause for the downstream increase ofnitrate transport (Table 1). However, the along-stream changeof nitrate concentration (Table 1), which is caused by diapycnal mixing and biological processes, must also play a role inthe downstream increase of nitrate transport. To quantitate
the contributions of the Kuroshio recirculation and downstream change in the nitrate concentration, a budget calculation is conducted for the water and nitrate transports in anarea enclosed by two or three sections.

A budget calculation for an area enclosed by section
137E-N and section ASUKA-N
For this calculation (Fig. 7a), section 137E is divided into twoparts and is separated at approximately 31.5◦ N (9th stationfrom the northernmost station in Fig. 1) where the eastward
Kuroshio almost disappears. Its northern component (section 137E-N) represents the eastward Kuroshio and its southern component (section 137E-S) represents the westward
Kuroshio recirculation. We further divided section 137E-N
into two parts: a small northern component (section 137E-NN) that corresponds to a westward coastal current and a large
southern component (section 137E-N-S) that corresponds tothe Kuroshio. Similarly, section ASUKA is also divided into
two parts: a northern component (section ASUKA-N) and asouthern component (section ASUKA-S), which is separated
at approximately 31◦ N (11th station from the northern moststation in Fig. 1).
We denote the volume transport within an isopycnal layer
as V1 , V2 , and VC for sections ASUKA-N, 137E-N-S, and137E-N-N, respectively (Fig. 7a), and the nitrate transports
within the same isopycnal layer are denoted as NT1 , NT2 ,and NTC for sections ASUKA-N, 137E-N-S, and 137E-N-N,
respectively. The ratio of NT1 to V1 (i.e., transport-weighted
mean nitrate concentration) is denoted as C1 , that of NT2 to
V2 as C2 , and that of NTC to VC as CC .
We assume that the difference in the volume transports
through sections ASUKA-N and 137E-N is due to the
Kuroshio recirculation in the southern region between sections 137E-S and ASUKA-S. The volume transport of theKuroshio recirculation (VR ) is therefore V2 − V1 − VC and
its nitrate transport is given by the production of VR and thetransport-weighted mean nitrate concentration (CR ) of wawww.biogeosciences.net/10/6403/2013/6413

ter carried by the Kuroshio recirculation. Although the exact
value of CR is unknown, it is reasonable to assume that it
is equal to the transport-weighted mean nitrate concentrationof the westward water in the same isopycnal layer of section
137E-S, which is the origin of the Kuroshio recirculation water in the area between sections ASUKA-N and 137E-N. The
following equation was then obtained:
NT2 − NT1 − NTC = VR CR + V1 (C2 − C1 ) + VR (C2 − CR )(7)
+VC (C2 − CC ) ,where the first term on the RHS, VR CR , is the nitrate transport via the Kuroshio recirculation, and the other terms arise
from the difference in nitrate concentration between eachpair of sections and can be attributed to the diapycnal mixing and biological processes occurring over the area between
two sections. The values of these terms within each isopycnal
layer and for the entire water column are given in Table 2.
According to Table 2, the nitrate transport by the Kuroshio
recirculation accounts for more than 95 % of the downstreamincrease in nitrate transport for the entire water column. This
result, however, does not mean that we can neglect the role ofthe change in nitrate concentration between two sections. For
example, the terms arising from the change in nitrate concentration are more important than the Kuroshio recirculation in
the upper two isopycnal layers. In the third isopycnal layer,
the Kuroshio recirculation becomes dominant but the terms
arising from the change in nitrate concentration between two
sections still have a considerable positive contribution. In the
deep layers (layers 4–8), the downstream increase of nitratetransport is almost entirely due to the Kuroshio recirculationand the terms arising from the change in nitrate concentration between two sections that have a negative or negligible
contribution.
The transport-weighted mean nitrate concentration (Table 2) shows a reduction of nitrate concentration along theKuroshio from section ASUKA-N (C1 ) to section 137E-NS (C2 ) in the first layer. This change is related to the biogeochemical processes that occur over the area between two
sections. The nitrate concentrations in the Kuroshio recirculation (CR ) and in the westward coastal current (CC ) are also
very different than those (C1 and C2 ) in the Kuroshio, but
their contribution to the change in nitrate transport is verysmall because their volume transport in this layer is small.
The transport-weighted mean nitrate concentration (Table 2) shows an increase in nitrate concentration from section ASUKA-N (C1 ) to section 137E-N-S (C2 ) and from theKuroshio recirculation (CR ) to section 137E-N-S (C2 ) in the
2nd and 3rd layers. This increase may be related to the diapycnal mixing that occurs over the area between section
ASUKA-N and section 137E-N-S, due to the presence of thedownstream reduction of nitrate concentration in the 4th and5th layers between each pair of sections. For layers 4 through7, the nitrate concentration is higher in the Kuroshio recirculation (CR ) than in the two sections of the Kuroshio (C1 andBiogeosciences, 10, 6403–6417, 2013

6414

X. Y. Guo et al.: Spatial variations in the Kuroshio nutrient transport

V C , C C , NT C
V 2, C 2, NT 2

V 1, C 1, NT 1

V 2, C 2, NT 2
V 1, C 1, NT 1

V R , C R , NT R

V R , C R , NT R

V 0, C 0, NT 0
(a)

(b)

Fig. 7. Schematic of a budget model for an area enclosed by (a) sections ASUKA-N and 137E-N-S and (b) by sections ASUKA-N, TK and
OK-W. V: volume transport; NT: nitrate transport; C: ratio of NT to V. (a) Subscript “1” denotes section ASUKA-N; subscript “2” denotes
section 137E-N-S; subscript “C” denotes section 137E-N-N where a westward coastal current flows into the area enclosed by sections
ASUKA-N and 137E-N-S; and subscript “R” denotes a section (red dashed line) where the Kuroshio recirculation flows into the area
enclosed by sections ASUKA-N and 137E-N-S. (b) Subscript “1” denotes section TK; subscript “2” denotes section ASUKA-N; subscript“0” denotes section OK-W; and subscript “R” denotes a section (red dashed line) where the Kuroshio recirculation flows into the area
enclosed by sections ASUKA-N, TK and OK-W.
Table 2. Budget of nitrate transport (kmol s−1 ) for each isopycnal layer between sections 137E-N-S and ASUKA-N.
Layer

1
2
3
4
5
6
7
8
All

NT2 -NT1 NTC(kmol s−1 )
−8.2
24.8
82.7
46.8
65.3
87.0
50.8
29.9
379.2

VR CR
(kmol s−1 )V1(C2 -C1 )(kmol s−1 )VR(C2 -CR )(kmol s−1 )VC(C2 -CC )(kmol s−1 )

0.0
10.7
69.0
51.6
70.2
88.2
51.1
30.0
370.9

−8.1
10.2
9.3
−2.6
−2.0
0.0
0.0
0.0
6.6

0.0
4.0
4.6
−2.4
−3.0
−1.2
−0.3
0.0
1.7

−0.1
−0.1
−0.2
0.2
0.1
0.0
0.0
0.0
0.0

C2 ), indicating that remineralization plays an important rolein the Kuroshio recirculation.

A budget calculation for an area enclosed by sections
ASUKA-N, TK and OK-W
In a similar way, section OK was divided into two parts at
approximately 129◦ E (5th station from the western most station in Fig. 1). The current in its western component (sectionOK-W) is the Ryukyu Current and the current in its easterncomponent (section OK-E) is considered to be the Kuroshio
recirculation. The budget is calculated in an area surroundedby sections TK, OK-W and ASUKA-N (Fig. 7b). The residual component of the volume transport through three sections
Biogeosciences, 10, 6403–6417, 2013

C1

CR

CC

C2

(mmol m−3 )(mmol m−3 )(mmol m−3 )(mmol m−3 )

2.5
6.1
16.0
29.8
38.1
40.7
40.8
40.6

0.7
5.0
15.6
30.8
39.1
41.3
41.1
40.1

3.0
7.5
17.1
28.9
37.3
40.8
40.8
39.8

2.1
6.9
16.6
29.4
37.4
40.7
40.8
40.1

is supplied by the Kuroshio recirculation from the area between sections OK-E and ASUKA-S.
The volume transport through sections OK-W, TK and
ASUKA-N are denoted as V0 , V1 , and V2 , respectively. The
volume transport of the Kuroshio recirculation (VR ) is obtained by VR = V2 − V0 − V1 . The nitrate transport throughsections OK-W, TK and ASUKA-N is denoted by NT0 , NT1 ,and NT2 , respectively, and their corresponding transportweighted mean nitrate concentrations are C0 , C1 , and C2 .
The nitrate concentration of the Kuroshio recirculation into
the area (CR ) is assumed to be equal to the transportweighted mean nitrate concentration in the westward water
current of section ASUKA-S and that in the eastward water
current of section OK-E. An equation for the budget in thiswww.biogeosciences.net/10/6403/2013/

X. Y. Guo et al.: Spatial variations in the Kuroshio nutrient transport

6415

Table 3. Budget of nitrate transport (kmol s−1 ) for each isopycnal layer between sections TK, OK-W, and ASUKA-N.
Layer

1
2
3
4
5
6
7
8
All

NT2 -NT1 NT0(kmol s−1 )
26.0
49.9
140.0
113.2
65.5
60.5
19.7
−1.2
473.7

VR CR
(kmol s−1 )V0(C2 -C0 )(kmol s−1 )V1(C2 -C1 )(kmol s−1 )VR(C2 -CR )(kmol s−1 )

3.6
38.6
123.7
114.8
65.1
60.8
19.6
−1.3
425.0

1.6
0.2
0.9
−0.9
0.7
0.4
0.1
0.1
3.2

12.1
0.4
9.9
2.5
0.9
−0.3
0.0
0.0
25.4

8.6
10.7
5.6
−3.3
−1.1
−0.4
0.0
0.0
20.1

area was also obtained:
NT2 − NT1 − NT0 = VR CR + V0 (C2 − C0 ) + V1 (C2 − C1 )
(8)
+VR (C2 − CR )According to Table 3, in which the values for each term in
Eq. (8) are given, the advection of the Kuroshio recirculation (VR CR ) is more important than the terms arising fromthe difference in nitrate concentration between the pairs ofsections and the downstream increase of nitrate transport(NT2 − NT1 − NT0 ). The contribution of the Kuroshio recirculation to the downstream increase of nitrate transport for
all eight layers is approximately 90 %. As with the area enclosed by sections ASUKA-N and 137E-N-S, this does notmean that the role of the terms related to the difference innitrate concentration between a pair of sections is negligible. For example, the terms that depend on the difference innitrate concentration between a pair of sections are more important than the Kuroshio recirculation in the first layer (Table 3). In the 2nd and 3rd layers, the terms that depend on
the difference in nitrate concentration between a pair of sections are still important. It is only below the 4th layer that theKuroshio recirculation becomes the only contribution to thedownstream increase of nitrate transport.
The downstream increase of nitrate concentration along
the pathway of Kuroshio (from section TK to section
ASUKA-N) and that from the Kuroshio recirculation to the
Kuroshio (section ASUKA-N) in the upper three layers, aswell as the downstream reduction of nitrate concentrationfrom the Kuroshio recirculation to the Kuroshio in the deep
layers (layers 4–7), were also found for this area. The result
is likely a consistent feature in the Kuroshio and Kuroshio
recirculation.
Williams et al. (2011) reported that the same downstream
intensification of nutrient transport occurred along the Gulf
Stream and concluded that the primary cause for the increasein nutrient transport was the recirculation-induced increasing
downstream volume transport of the Gulf Stream, while thesecond cause was the downstream increase of nutrient concentration in the water carried by the denser layers of the Gulf
www.biogeosciences.net/10/6403/2013/

C0

C1

CR

C2

(mmol m−3 )(mmol m−3 )(mmol m−3 )(mmol m−3 )

1.4
6.0
15.7
30.4
37.5
39.6
40.1
39.3

1.4
6.1
14.1
28.1
34.8
39.1
0.0
0.0

0.8
4.8
15.3
30.6
38.8
41.0
40.7
39.7

2.5
6.1
16.0
29.8
38.1
40.7
40.8
40.6

Stream. The importance of recirculation is similar with our
results according to the budget calculation for the two areasin the Kuroshio region. However, the downstream increase innutrient concentration in denser layers was found only fromsection Tokara to section ASUKA, and was not found fromsection OK-W to section ASUKA and from section ASUKA
to section 137E. In fact, the downstream changes of nitrateconcentration in the denser layers from section OK-W tosection ASUKA, from section ASUKA to section 137E, andfrom the Kuroshio recirculation to section ASUKA and section 137E are generally small or negative.
Williams et al. (2011) reported a downstream decrease innutrient concentration in the water carried by the lighter layers of the Gulf Stream and confirmed that the lateral waterexchange in the same isopycnal layers were the cause for thedownstream increase and decrease of nutrient concentrationin the water carried by the Gulf Stream. We can confirm thatthe nitrate concentration is lower in the Kuroshio recirculation than in the light layers (layers 1–3) of the Kuroshio, andthat such a dilution effect is not strong enough to induce alower nitrate concentration in the downstream region than in
the upstream region of the Kuroshio main stream due to therelatively smaller volume transport of the Kuroshio recirculation.
Pelegrí and Csanady (1991) also reported a downstreamincrease of nitrate transport along the Gulf Stream and attributed it to the diapycnal mixing that occurred along the
Gulf Stream. Since the results by Pelegrí and Csanady (1991)
are similar to the results of our budget calculations (Tables 2
and 3) (i.e., nitrate concentration increased in the light layers and decreased in the dense layers), we believe that diapycnal mixing should be an important factor in controllingthe change of nitrate along the Kuroshio. Recently, Kaneko
et al. (2013) calculated the vertical nitrate flux in the mainstream of the Kuroshio from measured nitrate concentrations
and the vertical viscosity coefficient estimated from field turbulent measurements. Their results indicated an upward nitrate flux from the depth of 300 m to the surface layer. This

Biogeosciences, 10, 6403–6417, 2013

6416

X. Y. Guo et al.: Spatial variations in the Kuroshio nutrient transportmeasurement supports our result of a downstream increase ofnitrate concentration along the light layers of the Kuroshio.

5

Summary

Using repeated hydrographic data from 2000 to 2009 and an
inverse method, we obtained the absolute geostrophic current speed at four sections across the Kuroshio from the East
China Sea (sections PN and TK) to south of Japan (sections ASUKA and 137E) and at a section southeast of the
Ryukyu Islands (section OK). The vertical structures and volume transports of temporally averaged velocity in these sections were generally consistent with previous results regarding the Kuroshio and Ryukyu Current in such locations. In
addition, the presence of the Kuroshio recirculation was confirmed for sections OK, ASUKA and 137E.
Using absolute geostrophic current speed and the nitrateconcentration measured at all five sections, we then obtained
nitrate fluxes and their temporal mean values. The presenceof a subsurface maximum core in the nutrient flux, as reported in the Gulf Stream (Pelegrí and Csanady, 1991; Pelegrí et al., 1996) and at section PN (Guo et al., 2012), wasconfirmed at all sections and their depths were found to be
400, 500, 500, 400, and 800 m for sections PN, TK, ASUKA,
137E and OK, respectively, indicating the presence of a stable nutrient stream along the Kuroshio from the East China
Sea to south of Japan. The eastward nitrate transport carriedby this stable nutrient stream is 204.8, 165.8, 338.6, 879.3,
and 1230.4 kmol s−1 for sections PN, TK, OK, ASUKA, and137E, respectively.
The deepest subsurface maximum core of nitrate flux at
section OK was induced by the vertical structure of the
Ryukyu Current, whose maximum current speed is in its subsurface (Zhu et al., 2004). This in turn results in a large downstream nitrate transport at section OK, suggesting that a weak
but deep Ryukyu Current contributes to the same order ofnitrate transport as the strong but shallow Kuroshio that is
located in the East China Sea to the region south of Japan.
The Kuroshio recirculation has a significant contribution to the amount of nitrate transport through sections
ASUKA and 137E. The eastward nitrate transport from section ASUKA to section 137E increased by a factor of 1.5,
the eastward nitrate transport at ASUKA is larger than thesum of those at sections TK and OK by a factor of 1.8. The
importance of the Kuroshio recirculation to the downstreamincrease of nitrate transport, however, does not mean that thephysical processes (e.g., diapycnal mixing) and biogeochemical processes that change the nitrate concentration along
the pathway of Kuroshio are negligible. A budget calculation within each of the isopycnal layers suggested that thedownstream change of nitrate concentration along the pathway of Kuroshio and that from the Kuroshio recirculation to
the Kuroshio has a significant contribution to the downstream
Biogeosciences, 10, 6403–6417, 2013

increase of nitrate transport in the upper layers (< 26.5σθ )along the Kuroshio main stream.
The downstream intensification of the nutrient transport isa common feature in the nutrient streams of the Gulf Stream
and the Kuroshio. The recirculation and its contribution to
the nutrient transport occur in both streams. A special featurein the northwest region of the Pacific is the Ryukyu Current,
which has a deep current structure and transports the sameorder of nutrients as the Kuroshio does from the East China
Sea to the region south of Japan. A difference between the
Gulf Stream and the Kuroshio is the downstream variation ofnutrient concentration in the water carried by the two western boundary currents. Nitrate concentration decreases in thelight layers but increases in the dense layers along the Gulf
Stream (Williams et al., 2011). However, the downstream
variation of the nitrate concentration in the Kuroshio region
is not as simple as that of the Gulf Stream. Therefore, more
complex processes are expected to occur in the Kuroshio andshould be investigated in future studies.

Acknowledgements. This work was supported by the National
Natural Science Foundation of China (41176021, 41276028 and41321004) and the National Basic Research Program of China(2011CB409803 and 2012CB956004). X. GUO is thankful for
support from JSPS KAKENHI (21310012) and the Open Fund of
Key Laboratory of Marine Ecosystem and Biogeochemistry, SOA(LMEB201107).
Edited by: C.-K. Kang

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