RESEARCH LETTER
10.1029/2018GL081497
Key Points:• Oyashio nitrate transport into theNorth Paciﬁc Intermediate Water inthe mixed water region isapproximately 110 kmol/s• Effective nitrate transport from thesubarctic gyre to the subtropic gyrealong with NPIW formation isapproximately 24 kmol/s• Oyashio is comparable to the
Kuroshio in nutrient transport but
has a shallower subsurface
maximum of nitrate ﬂux at 250‐m
depth
Supporting Information:• Supporting Information S1

Correspondence to:
X.‐H. Zhu,
xhzhu@sio.org.cn

Citation:
Long, Y., Zhu, X.‐H., & Guo, X. (2019).
The Oyashio nutrient stream and its
nutrient transport to the mixed water
region. Geophysical Research Letters, 46,
1513–1520. https://doi.org/10.1029/
2018GL081497
Received 7 DEC 2018
Accepted 9 JAN 2019
Accepted article online 16 JAN 2019
Published online 1 FEB 2019The Oyashio Nutrient Stream and Its Nutrient Transport
to the Mixed Water Region
Yu Long1,2, Xiao‐Hua Zhu1

, and Xinyu Guo1,3

1State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Ministry of Natural
Resources, Hangzhou, China, 2Ocean College, Zhejiang University, Zhoushan, China, 3Center for Marine Environmental
Study, Ehime University, Matsuyama, Japan

Abstract The Oyashio, a subarctic western boundary current in the North Paciﬁc, transports cool, fresh,and nutrient‐rich water equatorward. The Oyashio nutrient stream was assessed using long‐term meanhydrographic observation data. Its nitrate transport (about 350 kmol/s) is comparable to that of the Kuroshio
but has a much shallower subsurface core at 250‐m depth. In the layer ranging from 26.6 to 27.4 σθ, theOyashio nitrate transport to the mixed water region is about 110 kmol/s. This corresponds to the nitratetransport of the Oyashio to the North Paciﬁc Intermediate Water in the mixed water region. Along with thisprocess, the subarctic gyre nitrate export to the subtropic gyre reaches 24 kmol/s. This work ﬁrst quantiﬁes
the Oyashio nutrient transport and its intrusion into the mixed water region and suggests its importancein local/basin‐scale nutrient cycles.

Plain Language Summary The Oyashio carries cool, fresh, and nutrient‐rich water and issigniﬁcant in local/basin‐scale nutrient cycles. This study conﬁrmed the presence of a nitrate ﬂux
subsurface maximum in the Oyashio, which ﬁts the concept of nutrient stream. The Oyashio nutrient stream
acts as a subsurface giant conduit that continuously transports subarctic water with a high nutrient
concentration equatorward. This nutrient transport plays an important part in the formation of the North
Paciﬁc Intermediate Water and nutrient export from the subarctic gyre to the subtropical gyre.
1. Introduction
The mixed water region (MWR) between the Oyashio front and the Kuroshio Extension is the main location
at which the North Paciﬁc Intermediate Water (NPIW) forms (Talley et al., 1995; Yasuda, 2004). The cool,
fresh Oyashio water mixes with the warm, saline Kuroshio water (based on the water property, its intermediate water is referred to as “Old” NPIW), forms the intermediate salinity minimum centered at 26.8 σθ (or“New” NPIW), and circulates within the subtropical gyre. As it serves as an essential part of the global ocean
conveyor in the North Paciﬁc (Talley et al., 2011), the advection of NPIW plays an important role in regulating the meridional transport of heat, salt, and nutrients. From its origin, the nutrients are transported fromthe abyssal North Paciﬁc (Sarmiento et al., 2004) and Okhotsk Sea (Nishioka et al., 2013) to the nutrient‐depleted subtropical North Paciﬁc and replenishes the nutrient reservoir of the intermediate layer.
The signiﬁcance of subarctic thermocline nutrient export has been addressed in previous studies (Nishioka
et al., 2013; Sarmiento et al., 2004). Whitney et al. (2013) suggested that subarctic nitrate export along withNPIW formation had a value of 9.5 ± 3.2 kmol/s, which was derived from the product of Oyashio volumetransport (Talley et al., 1995; Yasuda, 2004) associated with NPIW formation (3 Sv) and the difference inthe nitrate concentration between the subarctic and subtropical gyres. This indicates effective nitrate transport (Ayers et al., 2014) that is based on volume conservation. However, an explicit and direct sectional
observation of nutrient ﬂux and transport is still required. Quantifying this nutrient transport would aid
our understanding on the basin‐scale nutrient budget (Whitney et al., 2013), nutrient supply in the
Kuroshio‐Oyashio transition region (Shiozaki et al., 2014), and nutrient transport between the subtropicaland subarctic gyre in the North Paciﬁc (Ayers & Lozier, 2010).

©2019. American Geophysical Union.
All Rights Reserved.

LONG ET AL.

Since Pelegrí and Csanady (1991) proposed the concept of the nutrient stream, various studies have discussed
the subtropical western boundary current (WBC) nutrient streams (such as the Kuroshio and Gulf Stream;
Chen et al., 1994; Guo et al., 2012, 2013; Long et al., 2018; Palter & Lozier, 2008; Pelegrí & Csanady, 1991;
Pelegrí et al., 1996) because of their signiﬁcance in the global nutrient cycles (Letscher et al., 2016; Williams
et al., 2011). However, the subpolar WBC has received less attention. The Oyashio (Kuroda et al., 2015;

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Uehara et al., 2004) transports nutrient‐rich subarctic water equatorward and directly impacts the local ecosystem (Sakurai, 2007). Hence, it is necessary to examine the Oyashio nutrient stream and its nutrient transport.
The following questions are addressed here: what is the long‐term mean nutrient transport of the Oyashio
and how does it distribute along isopycnal layers? How much nitrate does it transport to the NPIW in the
MWR? Based on repeated hydrographic observations from the Japan Meteorological Agency (JMA), we
accessed the long‐term mean nitrate ﬂux of Oyashio and its nitrate transport to the NPIW. The article is organized as follows: the data and method are provided in section 2; the results and discussion are in sections 4
and 5, respectively. A brief summary is presented in section 5.

2. Data and Method
The quarterly hydrographic observations (here after referred to as JMA‐line) from 2000 to 2015 consisted of
34 and 39 cruises of nutrient concentration and conductivity‐temperature‐depth data, respectively. The
inverse method (Wunsch, 1978) was applied to the conductivity‐temperature‐depth data to obtain the absolute geostrophic velocity, which is widely used in calculating velocity proﬁles in the WBC (Masujima et al.,2003, Zhu et al., 2003, 2008). In the calculation, we vertically divided the water column into ﬁve layers(bounded by the sea surface, 26.6, 27, 27.4 27.6, and 27.66 σθ) following the suggestion of Masujima et al.
(2003). By excluding six cruises of velocity data when the Oyashio axis was largely affected by eddies andselecting cruises with both velocity and nitrate concentration data, 26 nitrate ﬂux (the product of the velocity
and nutrient concentration, see supporting information S1 for more detail) cruises were obtained. The nutrient ﬂux was then integrated over the section to obtain the nutrient transport. The nitrate and phosphate concentrations were correlated well, with a N:P ratio of 14.27 and a cutoff of −1.52 mmol/m3 in a linear
regression of N to P. For convenience, we will focus on the results for nitrate in our description.
The average Mapping of Absolute Dynamic Topography heights and sea surface geostrophic velocity datafrom AVISO (Archiving, Validation, and Interpretation of Satellite Oceanographic data) were used to determine whether the Oyashio axis is affected by mesoscale eddies. The nitrate concentration at 26.8σθ (NPIW)
was obtained from the mapping data of the Global Ocean Data Analysis Project (Key et al., 2015; Lauvset
et al., 2016).

3. Result
The Oyashio ﬂows southwest along the continental slope of the Kuril Islands (Figure 1b). After reaching an
area southeast of Hokkaido, it meanders in a W shape and ﬂows east along the Oyashio front. Its typical pathconsists of four alternating ﬂows (Figure 1c). Following Kuroda et al. (2017) from north to south along thesection, we name these ﬂows SW‐1 (southwestward), NE‐2(northeastward), SW‐3 (southwestward), and NE‐
4 (northeastward), corresponding to the First Oyashio Intrusion and the Second Oyashio Intrusion (Kawai,1972), respectively. In the MWR, part of the Oyashio merges with the Kuroshio and ﬂows into the Kuroshio
Extension, while the rest ﬂows northeastward as NE‐4.
The JMA‐line captured all features of the four alternating ﬂows (Figure 1c). The mean nitrate transport perunit width matches the surface geostrophic velocity well, although there are discrepancies at the southern
stations (stations 17–22 from the north). This may result from the highly variable velocity ﬁeld induced by
eddies and can be understood from the large standard deviation of the nitrate transport per unit width.
The velocity proﬁle shows a surface maximum in the four alternating ﬂows (Figure 2a). The negative velocity at the northernmost area of the section represents SW‐1, and is relatively narrower and stronger among
the four alternating ﬂows, while its maximum surface velocity exceeds 0.2 m/s. Its return ﬂow, NE‐2, isrecognized as the positive value at the section south of SW‐1 and has a weak maximum surface velocity of
0.1 m/s. The ﬂow turns southwestward as SW‐3 and then northward as NE‐4, which has a larger surface
velocity maximum than SW‐3. The northeastward ﬂow in the southernmost area of the section could be
the Kuroshio jet ﬂow (Isoguchi et al., 2006).
The volume transports of the four alternating ﬂows from north to south are −9.3, 7.1, −5.8, and 11.3 Sv(positive values are northward). The net volume transport from SW‐1 to NE‐2 (First Oyashio Intrusion) issouthward and that from NE‐3 to SW‐4 (Second Oyashio Intrusion) is northward. As the Oyashio intrusion and merging of the Kuroshio water into it occurs concurrently, the volume transport of the Oyashio
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Figure 1. (a) A global view of the northwest Paciﬁc. (b) Nitrate concentration on 26.8σθ (color shading). The blue dots
denote the hydrographic stations used in the velocity calculation, and the thick black line is the JMA‐line. The magenta
boxes indicate the region where we used conductivity‐temperature‐depth data to obtain the typical T‐S relation of
Kuroshio and Oyashio. (c) Mean value (thick magenta line along the section) and standard deviation (thick magenta line
normal to the section) of nitrate transport per unit length across the section, surface geostrophic velocity (black arrows),
and its magnitude (color shading). The black dots and thin line through the dots represent the hydrographic stations
and the reference of zero. The white numbers (5, 10, 17, and 22) denote the numbering of the station from the north.

intrusion is not simply equal to the volume transport difference between the southwestward ﬂow minus
the northeast ﬂow.
The nitrate concentration of Oyashio is signiﬁcantly higher than that of other WBCs because the North
Paciﬁc has the highest nitrate concentration among the oceans on a global scale (e.g., Figure 1.5A in
Gruber, 2008). For example, the nitrate concentration at 100 m (about 20 mmol/m3) is comparable to thatat 600 m in the Kuroshio region (Guo et al., 2013). The nitrate concentration increases from 15 mmol/m3(1.5 mmol/m3 for phosphate) at the sea surface to a maximum of 43.5 mmol/m3 (3.15 mmol/m3 for phosphate) at a depth of 700–1,400 m (Figure 2b).
High nitrate concentrations result in a large surface nutrient ﬂux (Figure 2b) and a subsurface maximum at a
depth of 250 m, suggesting high potential for carrying nutrient‐rich water to the MWR in the surface layer.

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The subsurface extremes are −7, 3, −2, and 3 mmol·m−2·s−1 for SW‐1,
NE‐2, SW‐3, and NE‐4, respectively. The weakening of the nitrate ﬂux
from SW‐1 to SW‐3 can be attributed to the merging of Kuroshio withwater of a low nitrate concentration at the same isopycnal layer, that is,a dilution effect (Williams et al., 2011). The nitrate transports of the four
alternating ﬂows are −348.4, 249.4, −196.4, and 319.3 kmol/s, respectively. The phosphate ﬂux exhibits an analogous structure (Figure 2c),
and the phosphate transports of the four alternating ﬂows are −25.6,
18.3, −14.3, and 23.2 kmol/s, respectively.

−2 −1Figure 2. (a) Mean velocity (m/s), (b) nitrate ﬂux (2 mmol·m ·s ), and (c)−2 −1phosphate ﬂux (2 mmol·m ·s ) at the JMA‐line. The thick red line in (a),(b), and (c) denotes the isopycnal surface (σθ), nitrate concentration
33(mmol/m ), and phosphate concentration (mmol/m ), respectively. The
thick black line indicates a value of 0. The inverse black triangles denotehydrographic stations; the inverse red triangles denote the boundaries ofeach alternating ﬂow.

The nitrate ﬂux as a function of temperature and salinity shows the
Oyashio nutrient stream from a viewpoint of water property (Figure 3).
The typical T‐S relation of the two end members, the Oyashio and theKuroshio water, was obtained at JMA‐line and section 137E (magenta
box in Figure 1b), respectively. The distance between a speciﬁc point on
the TS diagram and the typical T‐S relation of the two end members on
the same isopycnal layer indicates the approximate extent of water mixing. For SW‐1(Figure 3a), the maximum nitrate ﬂux reached 7 mmol·m−2 −1·s , and its location on the T‐S frame is close to that of typicalOyashio water, suggesting a southwestward transport of the cooler,
fresher nutrient‐rich water. NE‐2 (Figure 3b) represents a positive nitrate
ﬂux, and its maximum reaches approximately 4 mmol·m−2·s−1. The T‐Sfeature of the water with the maximum nitrate ﬂux shifts slightly towardthat of typical Kuroshio water and suggests mixing between the Oyashio
and Kuroshio water. The nitrate ﬂux in the T‐S diagram exhibits little difference between NE‐2 (Figure 3b) and SW‐3 (Figure 3c), while NE‐4
(Figure 3d) exhibits a widespread, positive value between the typical T‐S
features of Kuroshio and Oyashio. The sectional T‐S diagram (Figure 3e)shows a negative nitrate ﬂux to typical Oyashio water, and a weak positive
value between the two typical water types. This suggests southward
nitrate transport of the Oyashio to the MWR and northward nitrate transport of the Oyashio‐Kuroshio mixing water.

We then calculated the water mixing ratio at the JMA‐line assuming thatisopycnal mixing (Shimizu et al., 2001; Figure 4a) occurred in the lower
four layers (26.6–27.66 σθ). The result shows that the Oyashio components
of SW‐1, NE‐2, and SW‐3 exceed 80% and there is a mixing front at NE‐4.
This suggests that the merging with the Kuroshio mainly occurs at NE‐4.
The nitrate transport contributed by the Oyashio water in each layer (Figures 4b–4e) decreases to almost
zero at the southern stations. The integrated nitrate transport contributed by the Oyashio water shows southward nitrate transport reaching approximately 50, 60, 60, and 30 kmol/s for the second to ﬁfth layers (26.6–
27.66 σθ), respectively (Figures 4b–4e). As the JMA‐line captures all four alternating ﬂows, the integrated
transport can be regarded as the Oyashio intrusion into the four layers. Therefore, the Oyashio intrusion
within the second to ﬁfth layers reaches approximately 200 kmol/s, 110 kmol/s of which occurs in the intermediate layers (second‐third layer, 26.6–27.4 σθ), which is the source of NPIW.

4. Discussion and Broader Implications
Oyashio water contributes approximately 110 kmol/s to the isopycnic layers of NPIW, with an uncertainty ofno more than 20% (a more detailed discussion of the nitrate transport uncertainty can be found in the supporting information S2; Isoguchi & Kawamura, 2003; Ito et al., 2004; Kuroda et al., 2015, 2017; Uehara et al.,2004). It should be noted that the density ranges of NPIW differed between previous studies. For example,
Yasuda (2004) used a range of 26.6–27.4 σθ, while Talley et al. (2011) used 26.7–27.2 σθ. The nitrate transport
within 26.6–27.4 σθ is 110 kmol/s, while that within 26.7–27.2 σθ is 60 kmol/s. Taking 85 kmol/s as an average, the transport of Oyashio water within the density range of NPIW is half of that of Kuroshio in the East
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Figure 3. Nitrate ﬂux (color shading, with an interval of 2 mmol·m ·s ) as a function of temperature and salinity in (a)SW‐1, (b) NE‐2, (c) SW‐3, (d) NE‐4, and (e) over the JMA‐line. The gray dots denote all data points of temperature and−2 −1salinity. The thick black contour denotes 0 mmol·m ·s . The thick red and blue lines denote the typical T‐S relation of
Oyashio and Kuroshio water, respectively. The black lines denote the potential density with values in (e).

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Figure 4. (a) Nitrate ﬂux (mmol·m ·s ) contributed by the Oyashio water. The thick red line denotes the mixing ratioof Oyashio water. The thick white line denotes the isopycnal layers (σθ). The top layer is blank due to the large uncertainties in its calculation. The inverse triangles indicate the hydrographic stations. (b)–(e) Nitrate transport due to Oyashio
water (blue line) and its integrated value from 43 N (red line) in the layers (b) 26.6–27σθ, (c) 27–27.4σθ, (d) 27.4–27.6σθ,and (e) 27.6–27.66σθ.

China Sea (170.8 kmol/s). Guo et al. (2012) stated that the Kuroshio Intermediate Water (KIW) is closely
related to the NPIW. Our estimation of Oyashio nitrate transport to the NPIW suggests that a considerable
portion of the Kuroshio nutrient transport in the intermediate layer is closely related to the contribution of
Oyashio, although the exact proportion of NPIW in the Kuroshio Intermediate Water is beyond the focus of
our study. Sarmiento et al. (2004) found that the formation of NPIW plays an essential role in the global thermocline nutrient cycle. Hence, the fate of subarctic‐origin nutrients is an interesting topic for further study.
The subarctic nitrate export along with the formation of NPIW can be estimated. The difference in thevelocity‐weighted nitrate concentration between the Kuroshio at section 137E (32.4 mmol/m3; in the density
range of 26.5–27.3; Guo et al., 2013) and Oyashio (40.4 mmol/m3) is 8 mmol/m3. Multiplying the volumetransport from the Oyashio in the formation of NPIW (3 Sv following Talley et al., 2011) indicates that the
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subarctic nitrate export is approximately 24 kmol/s (760 Gmol/year), which is twice of the value of
9.5 ± 3.2 kmol/s (300 ± 100 Gmol/year) given by Whitney et al. (2013). The reason for our result being larger
than that of Whitney et al. (2013) is that the difference in the nitrate concentration between the subarctic andsubtropic gyres used in their study was simply derived from the mean value over the entire gyre. However,this difference becomes larger if the nitrate concentration is used along with that of the Oyashio, which suggests that geostrophic nutrient transport should be carefully determined in the basin‐scale nutrient budget.
Similarly, this difference can also be applied to the transport of phosphate.
The eddy nutrient transfer cannot be neglected, although the complexity of the dynamics in this area prevents us from displaying long‐term variations using in situ data alone. The dominant dynamic pattern in thisarea is the Kuroshio Extension Decadal Oscillation (Qiu & Chen, 2005), which is unstable (stable) with more(less) eddy activity. In its unstable state, eddy transfer enhances cross‐gyre water/material exchange. As aresult, high (low) nutrient concentration water was transported southward (northward), along with high(low) potential vorticity water. This reduced the formation of subtropical‐mode water (Oka et al., 2015) inthe south and the primary productivity anomaly in the north. Palter et al. (2005) suggested that the advectionof mode water may introduce spatial and temporal variability in the subsurface nutrient reservoir andfurther affect the downstream primary productivity by altering the subsurface nutrient supply to the surface
layer. The extent to which these processes impact the mode water nutrient reservoir has not yet beenreported. Hence, studies focusing on the long‐term spatial‐temporal variation of the Oyashio nutrient stream
are expected.

5. Summary
We examined the long‐term mean nutrient ﬂux (production of velocity and nutrient concentration) andnutrient transport (integration of the nutrient ﬂux over the section) of Oyashio and its nutrient transport
in the formation of NPIW. The Oyashio nutrient stream has a maximum subsurface nitrate ﬂux of
7 mmol·m−2·s−1 at a depth of 250 m. It transports nitrate and phosphate southward at 348.39 and25.58 kmol/s, respectively. Variations in nitrate ﬂux between the four alternating ﬂows from SW‐1 to NE‐
4 were observed based on the section proﬁle and water properties, which suggests that the nitrate transportof Oyashio along with the NPIW formation is 110 kmol/s, and the effective nitrate transport from the subpolar to the subtropic region is 24 kmol/s. As the Oyashio nutrient stream plays an important part of theNorth Paciﬁc nutrient cycle, it should receive more attention in future studies.
Acknowledgments
This study was supported by the
National Natural Science Foundation ofChina (41576001, 41776107, 41806020,
and 41621064), the Project of State Key
Laboratory of Satellite Ocean
Environment Dynamics, SIO, MNR(SOEDZZ1901, SOEDZZ1806, and
SOEDZZ1804), the Scientiﬁc Research
Fund of SIO under grants JT1604 andJT1801, 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 (http://marine.copernicus.eu), and GLODAP(https://www.glodap.info). We appreciate J. Ayers and another anonymous
reviewer for their supportive commentson the original manuscript.

LONG ET AL.

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