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Multiscale Routes to Supply Nutrients Throughthe Kuroshio Nutrient Stream

ABSTRACTO

Takeyoshi Nagai1, Sophie Clayton2, and Yusuke Uchiyama3

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In this study, the role of the Kuroshio as a nutrient conduit is discussed using results from recent in‐situ observations and numerical simulations. It is shown that nutrient concentrations in the Kuroshio are elevated compared to those in ambient waters of the same density, similar to the Gulf Stream. The source of the Kuroshio
nutrient stream appears to be the tropical western North Pacific on the south side of the North Equatorial
Current. Along its path, widespread vigorous subsurface turbulence with an average eddy diffusivity of O(10−4m2s−1) associated with near‐inertial waves is observed in the Tokara Strait, where the upstream Kuroshio flows
over shallow topography and seamounts. In the Kuroshio Extension, subinertial shear forms thermohalineinterleaving structures, which induce double‐diffusive convection with an average subsurface thermal diffusivityof O(10−3 m2s−1). These very strong diapycnal mixing processes appear to be important for the upward nutrient
supply. An eddy‐induced nitrate flux is directed mostly downward and offshore, removing nutrients in the region
between the Kuroshio and the Japanese coast off Honshu Island. In contrast, on the north side of the Kuroshio
Extension, where warm streamers and warm core eddies emanate from the Kuroshio Extension, the eddy‐
induced nitrate flux is upward and northward.
6.1. ­INTRODUCTION

One of the most compelling scientific puzzles for many
oceanographers, including those in fisheries, has been tounderstand why many pelagic fish populations in thewestern North Pacific utilize the Kuroshio region for
spawning, fixing the destinies of their juveniles to be carried along the Kuroshio, a warm nutrient‐poor westernboundary current of the North Pacific. The key to unravelling the mystery of the “Kuroshio paradox” (Saito,
Chapter 1 this book), which we introduce in this chapter,
may lie under the surface of the Kuroshio waters.

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1
Department of Ocean Sciences, Tokyo University ofMarine Science and Technology, Tokyo, Japan
2
Ocean, Earth & Atmospheric Sciences Department, Old
Dominion University, Norfolk, VA, USA
3
Department of Civil Engineering, Kobe University, Hyougo,Japan

At the ocean surface, satellite remote sensing of phytoplankton pigment concentrations clearly indicates thatthe western North Pacific subtropical region is basicallyan ocean desert, sustaining very low concentrations ofchlorophyll a (Figure 6.1). The Kuroshio and theKuroshio Extension flow along the western and northernedges, respectively, of this ocean desert, which separates
the high chlorophyll waters of Kuroshio–Oyashio confluence to the north and the region between the Kuroshio
and the Japanese archipelago. However, taking a closer
look at the regions along the Kuroshio Extension in thewinter and spring seasons, in which the deepening of thesurface mixed layer in winter mixes the subsurface nutrients, followed by restratification in spring, the chlorophyll a concentrations are substantially elevated along
the northern edge of the Kuroshio Extension (35°N, 145–
180°E). One explanation for this increase is that the chlorophyll a is transported from the nutrient‐rich Oyashio
water, but it is not clear why chlorophyll a concentrations

Kuroshio Current: Physical, Biogeochemical and Ecosystem Dynamics, Geophysical Monograph 243, First Edition.
Edited by Takeyoshi Nagai, Hiroaki Saito, Koji Suzuki, and Motomitsu Takahashi.
© 2019 American Geophysical Union. Published 2019 by John Wiley & Sons, Inc.
105

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KUROSHIO CURRENT

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Figure 6.1 MODIS satellite seasonal climatological sea surface chlorophyll a concentrations in the western North
Pacific subtropical region (mg m−3) for winter (a) and spring (c). Average chlorophyll a within the regions boundedby thick black curves in (a) and (c) are shown for winter (b) and spring (d), respectively, as a function of distancefrom the Kuroshio Axis (positive northward), defined at the AVISO climatological sea surface height of 0.9 m,
which is indicated by black contours in (a) and (c). Shading in (b) and (d) is 95% confidence interval. Note that,in (b) and (d), the mean chlorophyll concentrations mostly on the north side of the Kuroshio Extension are shown.

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are higher at the northern edge of the Kuroshio Extension
than that in adjacent regions further north, closer to the
Oyashio. Another potential explanation for the highchlorophyll a associated with the northern flank of thefront is advection of high chlorophyll water from theJapanese coast by the Kuroshio, and/or lateral transportof nutrients from coastal waters. These lateral advections
should certainly contribute to the elevated chlorophyll aconcentrations observed along the Kuroshio Extension.
However, it is not clear why the coastal waters havehigher chlorophyll a concentrations over such a fairly
wide area despite the narrow continental margin along theJapanese coast, or how the nutrients are supplied to maintain relatively high phytoplankton concentrations near
the coast. This might be explained by an e­ddy‐induced

0004279817.INDD 106

nutrient upward flux (McGuillicuddy et al., 2003, Oschiles
et al., 2003) caused by mesoscale eddies emanating fromthe Kuroshio and the Kuroshio Extension. However,assuming a zonal front with abundant nutrients on thenorthern side of the front, the southward horizontal shiftof the preexisting domed nutrient‐rich cold water in a
cyclonic eddy may experience average subduction due tostretching of the water column (Levy et al., 2001) and may
not be effective in causing an increase in primary production. The nonlinear mesoscale eddies generated throughbaroclinic instability advects the shallow pycnocline fromthe northern side to the southern side of the front, and the
deep pycnocline from the southern to the northern side of
the front, which acts to restratify the front. Also, as thesemesoscale eddies are more frequently found in the

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Multiscale Routes to Supply Nutrients Through the Kuroshio Nutrient Stream  107

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counterintuitive, but consistent with the idea that theKuroshio acts as a subsurface nutrient stream.
Although the Kuroshio is considered a nutrient stream
similar to the Gulf Stream, it is not clear whether thenutrient concentrations along constant density surfaces
are elevated within the Kuroshio nutrient stream, as has
been found in the Gulf Stream. The mechanisms which
form and maintain the elevated nutrient concentrations
within these nutrient streams are not well understood.
Mesoscale along‐isopycnal stirring tends to homogenize
any tracer anomalies formed along density surfaces. The
timescale at which mesoscale stirring diffuses the along‐isopycnal nutrient anomaly can be estimated using a ratioof squared width of the anomaly δx2 to the along‐isopycnal
diffusivity Ke caused by mesoscale eddies, δx2/Ke. With thepreviously reported Ke of O(104 m2s−1) (Wolfram et al.,2015) in the western boundary current regions, andassuming 100 km for the widthδx of the nutrient stream,
the timescale can be estimated as a few tens of days,
which is much shorter than the advection timescale along
the Kuroshio nutrient stream ~100 days (see section 6.3).
Accordingly, the along‐isopycnal nutrient anomaly,
which persists even in its downstream regions, implies
that the across‐density surface, i.e., diapycnal flux, shouldbe important in generating and/or maintaining theobserved elevated nutrient concentrations within the
nutrient streams. A previous study speculated that diapycnal mixing caused by vertical shear of the Gulf Stream
generates elevated nutrient concentrations along thestream (Pelegrí et al., 1996). However, coarse lateral resolution microstructure measurements made during the1970s and 1980s did not show any enhanced turbulence at
the Gulf Stream front. In contrast, recent microstructure
surveys have observed relatively large turbulent dissipation rates in the thermocline of the Gulf Stream (Inoueet al., 2010) and the Kuroshio, which can be attributed topropagating near‐inertial internal waves (Nagai et al.,2009, 2012, 2015a; Kaneko et al., 2012, 2013). While a
simple induction process was proposed and evaluated as
the ultimate supply process of the Gulf nutrient stream in
subpolar regions, many recent studies have shown that
submesoscale processes in the upper layers of the fronts
and eddies play a role in vertical nutrient supply(Mahadevan & Archer, 2000), the onset of the phytoplankton blooms (Mahadevan et al., 2012), and carbon
export (Omand et al., 2015). Because the supply regionsof the Kuroshio nutrient stream, i.e., the Kuroshio
Extension front and Kuroshio–Oyashio confluence
region, are full of these rich submesoscale streamers andfilaments, it is likely that these submesoscale processes
play a very important role for nutrient supply, in addition
to the simple induction process.
In this study, we analyze climatological nitrate data andhistorical in situ observation data to show for the first

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Kuroshio–Oyashio confluence region, this mechanism
does not explain the elevated chlorophyll a concentrations
observed along the Kuroshio Extension. In addition,
recent numerical studies on the role of mesoscale eddies in
the coastal upwelling region in the California Current
System showed that the mesoscale eddies tend to subductthe upwelled nutrients, suppressing primary production(Gruber et al., 2011; Nagai et al., 2015c). Assuming abundant nutrients on the cold and dense northern side of theKuroshio Extension, which is a similar situation to that in
the coastal upwelling regions where abundant nutrientsare in cold and dense upwelled water near the coast, we
speculate that the eddy‐induced circulations also tend to
subduct the nutrients in the Kuroshio region (Lee &Williams, 2000). The role of mesoscale eddies in nutrienttransport and primary production in the Kuroshio andthe Kuroshio Extension regions has thus been elusive.
On the other hand, previous studies of the Gulf Stream,a western boundary current in the North Atlantic, foundthat it is a nutrient stream in its subsurface layer, whichtransports a large amount of nutrients from the south tosubpolar regions downstream (Pelegrí & Csanady, 1991;
Pelegrí et al., 1996, Chapter 3 this book). In addition, thenutrient concentrations are elevated along the stream,
compared to those found in the ambient water of thesame density (Pelegrí et al., 1996). Further analyses using
a numerical model have suggested that nutrients carriedby the Gulf nutrient stream might originate in the tropicalocean, and even farther south in the Southern Ocean(Williams et al., 2006, 2011). It has also been shown thatthe ultimate supply of nutrients to the euphotic zone
through this Gulf nutrient stream can be achieved byinduction as the stream encounters a deeper mixed layer
toward the north, which enables the nutrients to be carried laterally through the mixed layer base to mix with thesunlit surface layers (Williams et al., 2006). In the North
Pacific, the observations off southeast Taiwan by Chenet al. (1995) and those in the Okinawa Trough (Chen,
1996; Guo et al., 2012) found that the subsurface layersof the Kuroshio also transport a large amount of nutrients to the north, and that nutrient concentrations
increase in the downstream direction, probably suppliedby recirculation gyres (Guo et al., 2013). Accordingly, thesubsurface Kuroshio nutrient stream appears to be a conduit for nutrient enrichment in the region between the
Kuroshio and the Japanese coast, and the Kuroshio
Extension region. This is supported by a number ofobservational studies on the Kuroshio‐influenced coastalseas, which have reported that bottom intrusions of cold
saline nutrient‐rich water into the Bungo Channel, Japan,
are accompanied by Kuroshio water intrusions (Kaneda
et al., 2002; Arai, 2005; Kuroda, Chapter 8 this book).
Because the Kuroshio is known as an oligotrophic
nutrient‐poor warm current, these findings are somewhat

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KUROSHIO CURRENT

6.2. ­ELEVATED NUTRIENTS ALONG THE
KUROSHIO NUTRIENT STREAM

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A previous study focusing on nitrate concentrations
across a repeated section, the PN line in the upstream
Kuroshio, in the Okinawa Trough, showed that theKuroshio transports a large amount of nitrate at a rate of
170.8 k mol N s−1 (Guo et al., 2012). This study focused on
advective nitrate transport driven by a combination of astrong and deep Kuroshio current and increasing nitrateconcentrations with depth, resulting in a subsurface
nitrate conduit. However, it has yet to be determined
whether the nutrient concentrations along the Kuroshio
nutrient stream are elevated, as they are in the Gulf
Stream. To address this, the annual mean nitrateconcentration from World Ocean Atlas 2013 data (Garcia
et al., 2014) is sliced on the density surface at σθ = 25 kg m−3(Figure 6.2). The concentration is clearly elevated along
the western boundary of the North Pacific where theKuroshio flows from the southwest to the northeast(Figure 6.2a). The relatively high nitrate concentrations
found along the Kuroshio and in the adjacent waters­between the Kuroshio and Japanese coast may explain
why high chlorophyll a concentrations are seen in theseregions. The depth of the density layer of σθ = 25 kg m−3
becomes shallower toward the downstream of theKuroshio nutrient stream (Figure 6.2b). During thewinter, the climatological mean density suggests that thedensity layer of σθ = 25 kg m−3 outcrops in regions from
30–32°N to the north, suggesting that nitrate on thisdensity layer is entrained into the surface mixed layer
where it fuels phytoplankton growth (Figure 6.2c).
To investigate this elevated nitrate on a regional scale,in situ nitrate data from 1997 to 2015 along the PN line
(Guo et al., 2012) were bin averaged as a function of σθand the distance from the Kuroshio axis, defined here as
the location of the maximum surface velocity magnitude.
The bin‐averaged nitrate concentration shows relatively
high values along the same density surface near the
Kuroshio axis (Figure 6.3), consistent with that seen in
the World Ocean Atlas (Figure 6.2) and with the Gulf
nutrient stream (Pelegrí et al., 1996). Downstream, thesame bin‐averaged nitrate data obtained in October 2009

across the Kuroshio Extension front (Nagai et al., 2012;
Clayton et al., 2014) also show elevated nitrate concentrations in the Kuroshio Extension front (Figure 6.4) (Nagai& Clayton, 2017). In this case, the elevated nitrate is seen
in relatively less dense water (σθ = 23.5–25 kg m−3) compared to that found in the PN Line (σθ = 25.5–24.5 kg m−3;
Figure 6.3). This suggests that there are nutrient diapycnal transports caused by microscale mixing processes(see section 6.5) that occur between upstream regionsin Okinawa Trough and the downstream Kuroshio
Extension region. The high nitrate concentrations in theσθ = 26 kg m−3 density layer are also seen on the northernside of the Kuroshio Extension (σθ = 26 kg m−3 0‐60 km in
Figure 6.4), which is considered to be a deeper part of thenutrient stream, because this density layer has previously
been reported as the core of the Kuroshio nutrient stream
in the upstream PN Line (Guo et al., 2012).

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time that nutrient concentrations are indeed elevatedalong density surfaces within the Kuroshio nutrient
stream. Then, using the new results from a numerical simulation, we investigate the source of the nutrients in the
Kuroshio nutrient stream. Based on our previous results,
the role of eddies in nutrient transport is summarized.
Finally, a series of previous in situ observations of theauthors showing the important diapycnal mixing
processes for nutrient diffusion and supply in the
upstream and downstream Kuroshio is discussed.

O1080004279817.INDD 108

6.3. ­SOURCE OF THE NUTRIENTSIN THE KUROSHIO NUTRIENT STREAM

The World Ocean Atlas data show high nitrate concentrations following the path of the Kuroshio density­surface, upstream and at its origin, Luzon Strait and off
the Philippine islands (Figure 6.2a). Because water movements in the ocean interior tend to follow density surfaces, the likely source regions for the elevated nitrate
along the Kuroshio might be the southern edge of thesubtropical gyre, which could be connected to the tropicalwestern North Pacific, as well as the South China Sea,
given a clockwise upper layer circulation of the North
Pacific subtropical gyre. It is suggested that the Kuroshio
nutrient stream transports nitrate from these southern
regions to the Kuroshio Extension and the Kuroshio–
Oyashio confluence region.
To identify the source regions for the elevated nitrate,the Regional Oceanic Modeling System (ROMS) coupledwith a simple nitrogen‐based, nitrate‐ammonium‐phytoplankton‐zooplankton large and small detritus N2PZD2model (Gruber et al., 2006; Nagai & Clayton, 2017) wasused to simulate the Kuroshio nutrient stream. The model
was eddy permitting with a slightly higher lateral resolution of ~10 km in the Kuroshio region, with 64 verticallevels, and forced with monthly Comprehensive Ocean
Atmosphere Data Set (COADS) (da Silva et al., 1994)­climatology for the first five years; for the following 16
years the model was forced with six‐hourly NCEP CFS
Reanalysis from 1980 to 1995 (NCEP, 2015), with lateralboundary conditions set from World Ocean Atlas (Garcia
et al., 2014) at the open boundaries. The simulation wasfor 21 years in total, and the output from the final year
was used in this study.
The model successfully reproduces the satellite‐observed
sea surface height, chlorophyll a concentrations, and the

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Multiscale Routes to Supply Nutrients Through the Kuroshio Nutrient Stream  109(a)Annual mean NO3 on σθ = 25

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Mean winter NO3 on σθ = 25

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Figure 6.2 World Ocean Atlas 2013 (Garcia et al., 2014) for (a) annual mean nitrate concentrations (m mole m−3)
sliced at σθ = 25 kg m−3, (b) annual mean depth of the density layer (m), and (c) nitrate concentrations (m mole m−3)
sliced at σθ = 25 kg m−3 during winter. Dashed contour represents outcropping positions. (See insert for color­representation of this figure.)

nitrate in situ data across the Kuroshio Front (Nagai &Clayton, 2017). Similar to the result shown in Figure 6.2,
the modelled nitrate concentration on 30 June sliced at thedensity surface of σθ = 25 kg m−3 shows that nitrate concentrations are elevated along the western boundary of
the North Pacific following the Kuroshio path (Figure 6.5a)0004279817.INDD 109

and are comparable to concentrations found in the World
Ocean Atlas (Figure 6.2a). The meridional nitrate gradient at around 15°N in the western to the central North
Pacific is also reproduced well. On 15 March, the density
layer of σθ =25 kg m−3 outcrops at around 30°N, similar to
the World Ocean Atlas, suggesting that the modelled

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110

KUROSHIO CURRENT

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Distance (km)Figure 6.3 Bin‐averaged nitrate concentration (m mole m−3) obtained along the PN Line in the Okinawa Troughduring 1997 through 2015 as a function of σθ and the distance from the Kuroshio axis defined at the maximum
surface velocity is shown as shading. The gray contours show depth (m).

Rnitrate transported to the Kuroshio Extension is also
entrained into the surface mixed layer during winter time.
To investigate the source of the nitrate transported
along the path of the Kuroshio, a passive tracer initialized with the same distribution as the modeled nitrate on
30 December of the previous year, but filled with zero
values from 17–18°N, south of Luzon Strait to the north,and from 120°E to the west, was released on 1 January.
After 15 days, the tracer on the σθ = 25 kg m−3 density
layer flows across Luzon Strait and approaches Taiwan

0004279817.INDD 110(Figure 6.6a). A large fraction of the tracer passes
through the East China Sea and reaches south of Honshu
Island after 225 days (7–8 months; Figure 6.6b). The
results on the σθ = 25 kg m−3 layer show that the Kuroshio
can transport the tracer from the Philippine islands to the
Kuroshio Extension region in roughly one year (345 days)(Figure 6.6c). The same tracer experiment was repeatedfor nitrate originating only from the South China Sea
with the initial tracer released from 120°E to the west.
The fraction of the South China Sea origin tracer in the

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Multiscale Routes to Supply Nutrients Through the Kuroshio Nutrient Stream  111

23 E

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Figure 6.4 The same figure as Figure 6.3 but for nitrate (m mole m−3) measured during October 2009 in the
Kuroshio Extension. The gray contours show depth (m), and the alphabet characters A–E represents the origins in
the five north–south transects shown in Figure 6.15a.

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Model march 15 NO3 on σθ = 25

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Figure 6.5 Modeled nitrate concentrations (m mole m−3) on the density surface at σθ = 25 kg m−3 for (a) 30 June and(b) 15 March. The dashed contours show the outcropping positions.

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112

KUROSHIO CURRENT(a)Model tracer at day 15 on σθ = 25

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Figure 6.6 Passive tracer distributions released from the tropical regions of North Pacific at (a) 15 days, (b) 225
days, and (c) 345 days after the release. The passive tracer initial condition is taken from modeled nitrate on
1 January (m mole m−3). The dashed contours show the outcropping positions.

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downstream Kuroshio regions after 345 days is found to
be only 1–3% relative to the contribution from thesouthwestern North Pacific. Some of the tracer released
from the South China Sea enters the East China Sea (notshown). Previous studies have pointed out that theKuroshio water intrudes significantly into the South
China Sea (Nan et al., 2015) and our results suggest thatonce the nutrients in the Kuroshio nutrient stream enter
the South China Sea, most of them are subsequently nottransported to the downstream Kuroshio regions.
6.4. ­EDDY‐INDUCED NITRATE FLUXIN THE KUROSHIO AND THE
KUROSHIO EXTENSIONAs mentioned in the introduction, the role of mesoscale eddies in transporting nutrients in the Kuroshio
region is not well understood. To quantify the eddy

0004279817.INDD 112

nitrate flux, a ROMS simulation was conducted with ~3‐km resolution in the Kuroshio region (Uchiyama et al.,2017). The eddy nitrate flux was then computed by
Reynolds decomposition as follows:
FH

Nv

Nv

Nv

Fv

Nw

Nw N w(6.1)

where the last terms on the right‐hand side of theequations are eddy‐induced nitrate flux along the meridional N v and vertical directions N w , respectively; theoverbar represents a low‐pass filter at 90 days; the prime
is a fluctuation from the low‐pass filter; N is nitrate; andv and w are meridional and vertical velocities, respectively. The fluctuations decomposed by equation (6.1)
include meso and submesoscale variabilities. The resultant eddy vertical flux averaged within the upper 200 m

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Multiscale Routes to Supply Nutrients Through the Kuroshio Nutrient Stream  113

(a)

(b)

Horizontal flux

Vertical flux

48° N

44° N

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40° N

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36° N

28° N
125° E–10130° E–5135° E

140° E0(mmolN m–1 s–1)

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145° E
5150° E

10125° E130° E–10–5135° E

140° E

0145° E
5150° E10(μmolN m–1 s–1)

Figure 6.7 Eddy‐induced nitrate flux (a) along the meridional direction and (b) along the vertical direction­averaged within the upper 200 m depth. White (positive) represents southward and upward directions in (a) and(b), respectively, and vice versa for black (negative). The black curves indicate the approximate Kuroshio Current.

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shows dominant downward flux, especially in themeander troughs of the Kuroshio south of Honshu
Island (Figure 6.7b). In this region, the eddy meridional
flux points mostly southward (Figure 6.7a), suggestingthat the eddies and filaments near the Kuroshio off
Honshu Island tend mostly to remove abundant nitrate in
the regions between the Kuroshio and southern coast to
the subsurface layers in the offshore regions (Figure 6.8aand 6.8b). This result is similar to that obtained by thesame analyses conducted on the California Current
System (Gruber et al. 2011; Nagai et al. 2015c) and isconsistent with the results of Lee & Williams (2000), in
which the nutrient concentration was set on the northside of the zonal front. Because along‐isopycnal meridional gradient of nitrate is mostly positive between the
coast of Honshu Island and the Kuroshio (Figure 6.5a),with a large amount of the nitrate transported by the subsurface Kuroshio nutrient stream, eddy‐induced circulations, which tend to restratify the front, result insouthward (offshoreward) and downward eddy‐induced
nitrate fluxes (Figure 6.7, 6.8a and 6.8b).
In contrast, the large upward and northward eddy‐
induced nitrate fluxes are seen north of the first meandercrest of the Kuroshio Extension, with southward anddownward eddy fluxes produced on the south side
(Figure 6.7, 6.8c and 6.8d). This tendency continues

0004279817.INDD 113

toward east along the Kuroshio Extension (Figure 6.7).
As seen in the modeled nitrate concentrations (Figure 6.5)
and in the World Ocean Atlas (Figure 6.2), on theσθ = 25 kg m−3 density surface, and the bin‐averaged
nitrate as a function of distance from the Kuroshio
Extension axis and σθ (Figure 6.4), the along‐isopycnal
nitrate increases and then decreases with latitude acrossthe Kuroshio Extension. The positive along‐isopycnal
meridional gradient of nitrate on the south side of theKuroshio Extension and the negative gradient on thenorth side correspond to the cases of high nitrateconcentration on the north and on the south of the zonal
front, respectively (Lee & Williams, 2000). In the former
case, the eddy‐induced flux is southward and downward.
On the contrary, the eddy‐induced flux in the latter case isdirected to northward and upward, consistent with themodel results (Figure 6.7, 6.8c and 6.8d). It should be
noted that both fluxes, directed northward and upwardon the north, and southward and downward on the south
side of the Kuroshio Extension nutrient stream act tohomogenize the positive along‐isopycnal nitrate anomaly
advected by the Kuroshio nutrient stream.
On the north side of the Kuroshio Extension nutrient
stream, filaments of warm water, i.e., warm streamers,
are frequently generated and transport heat to warm core
eddies propagating to the north of the Kuroshio Extension

4/15/2019 7:58:32 PM

114

KUROSHIO CURRENT

(a)

(c)

Vertical flux : 138° E

0(μmolN m–2 s–1)

Vertical flux : 145° E

0.1

–200

0.05

(m)

–400

0

–600

–0.05

–800

FS

–0.1

–1000

(b)

(d)

Horizontal flux : 138° E

0Horizontal flux : 145° E

100

50

O

–200
–400

(m)

(μmolN m–2 s–1)

–800
–1000
34.7° N

34.1° N

33.5° N

32.9° N

32.2° N

PR
O

–600

31.6° N

38° N

37.2° N

36.4° N

35.6° N

0
–50
–100

34.8° N

34° N

Figure 6.8 Vertical sections of eddy‐induced nitrate flux along (a–b) 138°E and (c–d) 145°E for (a and c) verticaland for (b and d) meridional flux. Blue with thin broken contours represents northward and downward directions.
Thick solid contours show mean current velocity. Magenta horizontal curves indicate the mixed layer depths.
(See electronic version for color representation of this figure.)
143° E
145° E
Oct. 18 2009 SST

147° E

28

EV
IS
ED

141° E

39° N

38° N

37° N

26
24
22

downwelling (McWilliams et al., 2009). On the other hand,the frontogenetic deformation at the two fronts bounding
a warm streamer generates upwelling in the warm streamers(Figure 6.10; Klein & Lapeyre, 2006). We speculate thatthe northward and upward eddy‐induced nitrate flux
shown in Figure 6.7 and 6.8c–d includes these warm
streamer effects, and that warm streamers in the Kuroshio
Extension region are one of the final supply routes to thesurface for nutrients transported by the Kuroshio.

20

36° N

35° N

34° N

18
16
14

R

12

Figure 6.9 A warm filament or warm streamer observed in thesatellite sea surface temperature data during the October 2009
cruise by the R/V Natsuushima. The black contour represents
AVISO sea surface height averaged during the observation period.

(Yasuda, 1995) (Figure 6.9). These thin filaments are
induced by large mesoscale deformations. For cold
streamers, the frontogenetic deformation at the two
fronts bounding the cold filament produces pronounced

0004279817.INDD 114

6.5. ­DIAPYCNAL MIXING PROCESSESALONG THE KUROSHIO
As opposed to the eddy‐induced along‐isopycnal stirring
effects, diapycnal upward nutrient transport acts to enhanceand maintain the along isopycnal nutrient anomaly along
the Kuroshio nutrient stream. One of the most likely mixing “hotspots” along the Kuroshio is in the Kuroshio
upstream, in the Okinawa Trough and the Tokara Strait. In
these regions, the Kuroshio has to flow over shallow topography and narrow straits between islands and steep­seamounts. Downstream, there is also a large seamountwith a number of islands in Izu‐Ogasawara Ridge south ofTokyo (Hasegawa, Chapter 10 this book). No such topographic obstacles are present in the Gulf Stream except the
Bahama Banks (Gula et al., 2016) and the New England
Seamounts (Cornillon, 1986; Ezer, 1993).

4/15/2019 7:58:33 PM

Multiscale Routes to Supply Nutrients Through the Kuroshio Nutrient Stream  115

Current
UpwellingO

Vorticity

FS

Confluence

6.5.1. Observations of Diapycnal Mixing
in the Tokara Strait

PR
O

Figure 6.10 Schematics of the warm streamer under the frontogenetic confluence.

R

EV
IS
ED

In a previous study of the Kuroshio, large amplitude
near‐inertial internal waves of high vertical wavenumber
were observed in the Okinawa Trough and the Tokara
Strait near the Kuroshio, and in the region between the
Kuroshio and the continental shelf. These internal waves
likely induce strong turbulent mixing (Rainville & Pinkel,
2004). Recent microstructure experiments showed strong
turbulent mixing associated with near‐inertial internal
waves of very small vertical wavelength, O(50 m) nearislands and seamounts (Tsutsumi et al., 2017). However,direct turbulence measurements are still scarce in thisregion. Nagai et al. (2017) conducted a quasi freefall tow‐
yo microstructure experiment, using an Underway‐VMP(UVMP) in the Tokara Strait during November 2016
aboard the R/T/V Kagoshima‐maru. The measured zonal
velocity shear using a shipboard Acoustic Doppler Current
Profiler (ADCP, 75 kHz 30° beam angle, Ocean Surveyor,
Teledyne RDI), which is back‐rotated at the local inertial
frequency to a reference time, shows banded structures ofhigh vertical wavenumber alternating positive and negativevalues following closely along the isopycnal. This ­suggests
the propagation of near‐inertial internal waves (Figure 6.11a).
Hodographs and rotary spectra indicate that the energyof these waves propagate in the upward and downward
directions (Nagai et al., 2017).
In this large amplitude high vertical wavenumber
near‐inertial internal wave field, we have succeeded in
making direct turbulence measurements at very highlateral resolution, ~1–2 km, down to 300 m depth.
The measured turbulent kinetic energy dissipation
rates show bands of coherent strong turbulent layers

0004279817.INDD 115(Figure 6.11b) clearly associated with the observed
banded near‐inertial internal wave shear (e.g.,
129.35‐129.55°E along σθ = 23 kgm−3; 129.35‐129.45°E
along σθ = 25 kgm−3; 129.47‐129.65°E along σθ = 24.5‐25
kgm−3; 129.56‐129.65°E along σθ = 26 kgm−3;
Figure 6.11a, 6.11b). The banded turbulent layers
spread horizontally with a lateral scale of over O(10 km).
These results suggest that the frequently observed dominant near‐inertial internal wave shear in previous
studies is actively breaking into three‐dimensional turbulence. This creates widespread subsurface active mixing layers in the Kuroshio upstream. The upper 500 m
averaged relative vorticity shows that the large amplitudeinternal wave shear and associated strong turbulence
are in the region of negative relative vorticity of theKuroshio (Figure 6.12), similar to previous studies(Kunze, 1985). This suggests that the Kuroshio acts as acatalyst to dissipate near‐inertial internal waves propagating near the Tokara Strait. The estimated eddy diffusivities using measured turbulent kinetic energy
dissipation rates with background buoyancy frequency
show average values of O(10−4 m2s−1) below 100 m depth.
Although our turbulence measurements only reach theupper edge of the North Pacific Intermediate Water(NPIW) density layer, the average subsurface diffusivityof O(10−4 m2s−1) is consistent with the previously estimated values (Nakamura et al., 2014) for diapycnal diffusivity for water mass modification of the NPIW inthis region using long‐term salinity records. In addition
to water mass mixing, the observed turbulent mixing isimportant for driving upward nutrient fluxes, since theKuroshio carries a large amount of nutrients in its subsurface layer (σθ = 24–27 kg m−3) in this region (Chen,
1996; Guo et al., 2012).

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116

KUROSHIO CURRENT(a)0

129.35129.4

129.45129.5

129.55129.6

129.65

100
0.01

300

0.005

400

0

FS

Depth (m)

200

500

–0.005

600
700
0

O

(b)

–0.01

100

Depth (m)

300
400
500
600
700

129.35

129.4

PR
O

–6.5

200

129.45129.5
129.55Longitude

129.6

129.65

–7
–7.5
–8
–8.5

EV
IS
ED

Figure 6.11 ADCP vertical shear of the zonal current, back‐rotated with the local Coriolis frequency (s−1) isshown in (a) (white solid contour) positive and (white dashed contour) negative values. (b) Measured turbulent
kinetic energy dissipation ratesεin log scale (W kg−1) in the Tokara Strait ­during November 2016 R/T/V Kagoshima‐
maru cruise. Black contour represents isopycnal.

1.1

1.05

1

feff/f

0.95

0.9

0.85

R

0.8

0.75
129.3

129.35129.4

129.45129.5

129.55129.6

129.65Longitude (°E)

Figure 6.12 Normalized effective Coriolis frequency (Kunze,1985) by local Coriolis frequency averaged within the upper
500 m as a function of longitude. Shading represents 95 %confidence interval for each average value. Horizontal dashedline indicates unity.

0004279817.INDD 116

Wind energy is the main source of these near‐inertial
internal waves. The estimated wind power input to­produce near‐inertial oscillations using a slab model suggests that wind input is sufficient to produce the observednear‐inertial internal wave vertical energy flux ofO(0.1‐1 m Wm−2) (Nagai et al., 2017). However, it isunclear how the observed high vertical wavenumber
near‐inertial internal waves are generated. The study
region is close to the so‐called critical latitude, 28.9°N,
where the semidiurnal M2 internal tide energy can be
efficiently converted to that of the near‐inertial internal
waves of high vertical wavenumber through parametricsubharmonic instability (PSI) (MacKinnon & Winters,2005). Relatively long‐term ADCP observations in thesame region during 2001‐2002 suggest that near‐inertial
shear amplitude modulates fortnightly similar to that oftidal forcing. This suggests that PSI may be a source forthe observed high vertical wavenumber near‐inertial
internal waves (Qiu et al., 2012; Nagai et al., 2017).
However, the near‐inertial shear amplitude is correlated
better with the external diurnal tidal forcing (D1 = K1 + O1)

4/15/2019 7:58:35 PM

Multiscale Routes to Supply Nutrients Through the Kuroshio Nutrient Stream  117y(km)–80 –60 –40 –20 0

R

EV
IS
ED

Even without wind, tides, and topography, meanderingstrong fronts can generate spontaneously high verticalwavenumber near‐inertial internal waves (Snyder et al.,
1993; Plougonven & Snyder, 2005, 2007; Alford et al.,2013; Nagai et al., 2015a; Shakespeare & Taylor, 2014;
Shakespeare & Hogg, 2017). Results from numerical simulations of the meandering Kuroshio Extension without
surface forcing reproduce high vertical wavenumber near‐inertial internal waves in the thermocline of the Kuroshio,
consistent with observations (Nagai et al., 2015a).
However, semi‐Lagrangian analyses for an internal wave
energy equation suggest that most of the generated
internal wave energy is reabsorbed into the mean flow
due to a strong mesoscale confluence (Nagai et al.,2015a). Using the observation data, Nagai et al. (2009;2012; 2015a) showed that the banded shear is frequently
observed in the thermocline of the Kuroshio Extension;
it is attributed to the propagating internal waves(Figure 6.13a). Simultaneously measured microscale
turbulent kinetic energy dissipation rates show very large
values of O(10−7–10−6 W kg−1), 100‐1000 times greaterthan typical open water thermocline values (Figure 6.13c).
These observed banded shears could be caused by trapped
and amplified wind‐induced internal waves (Kunze, 1985;
Whitt & Thomas, 2013; Whitt et al., 2018), and are notnecessarily due to spontaneously generated near‐inertial
internal waves. However, regardless of their generation
mechanisms, the microstructure observations suggestthat these banded internal wave shears enhance diapycnal
turbulent mixing in the thermocline of the Kuroshio.
Internal wave induced mixing in the Kuroshio thermocline can be an important process for the upward diffusion of nutrients transported through the Kuroshio
nutrient stream.

0004279817.INDD 117

20

40

60

×10–3

5

–100
–200

0

–500
–600
140° E

39° N

36° N

33° N

(b)

–5

(a)

0

146° E

O

–400

FS

–300

PR
O

6.5.2. Banded Shear and Strong Turbulencein the Thermocline of the Downstream Kuroshio
Extension0z(m)than that of semidiurnal components (D2 = M2 + S2 + N2).
The location of the study site is at ~30°N, north of thecritical latitude, where the diurnal internal tides are alsonear‐inertial internal waves. Thus, observed high verticalwavenumber near‐inertial internal waves might also be
generated by diurnal tides over nearby seamounts.
Previous studies have also shown that the geostrophic
flow over topography can generate near‐initial internal
waves (Nikurashin & Ferrari, 2011). Globally e­ stimated,
this lee wave generation rate in the Kuroshio upstream isO(0.1–1 m Wm−2) (Nikurashin & Ferrari, 2011), consistent with our estimates above. Because the Kuroshio
flows over shallow topography in the Okinawa
Trough and the Tokara Strait, this mechanism cannot be
ruled out.

–100
–200

(c) –300
0

20

40

60

y(km)

-9 -8 -7 -6
log10 ϵ(Wkg-1)

Figure 6.13 (a) The residual shear with (white solid contour)
positive and (white dashed contour) negative values, computedby subtracting the geostrophic shear from the total shear measured during August 2011 along the meridional direction as in(b). Measured TKE dissipation rates ε in log scale (W kg−1) are
shown in (c). Note that panel (a) and (c) are aligned with thesame lateral position to compare the locations of the strong
shear and dissipation rates.

6.5.3. Double‐Diffusive Convection in and below
the Thermocline of the Kuroshio Extension
Even without turbulence, seawater can be mixed diapycnally by double‐diffusive convection. Oschlies et al.
(2003) estimated the salt‐finger induced nitrate diapycnal
vertical flux using a salt‐finger diffusivity parameterization in an eddy permitting model of the Atlantic. The
results suggest that the basin‐wide average nitrate flux
due to salt‐fingers accounts for 0.08 mmol N m−2 day−1,
which is equivalent to the average turbulent and mesoscale eddy‐induced vertical nitrate fluxes estimated by
Oschlies (2002). The parameterized salt‐finger induced
diffusivities in the model are O(10−5 m2s−1) on average(Table 1 of Oschiles et al., 2003). These results suggestthat salt‐fingers could be an important diapycnal mixing
process for vertical nutrient flux. However, because theresolution of the numerical model was eddy permitting at

4/15/2019 7:58:37 PM

118

KUROSHIO CURRENT

(a)

1

2

3

4

5

6

7

8

9

10

0

Depth (m)

0.1
0.05

250

0
–0.05
–0.1

0

500
1

Depth (m)

2

3

0

4

5

EV
IS
ED

250

PR
O

250

(c)

20

O

Depth (m)

(b)

FS

500

6

7

8

15
10
5

9

10

34.5
34
33.5

500

1

2

3

4

5
Day

6

7

(e)

(d)

146° E

9

10

(f)

–9

140° E

8

–8

–7

–10

–9

–8

–7
0

152° E

100

36° N

Depth (m)

39° N

200
300
400

30

R

33° N

25 0.5

1

1.5 2
Day

2.5

0.5

1

1.5 2
Day

2.5

500

Figure 6.14 (a) Across front velocity measured by the EM‐APEX Float deployed along the Kuroshio Extension with(white solid contour) positive and (white dashed contour) negative values. The trajectory of the float is seen in (d).
Measured temperature and salinity are shown in (b) and (c). Microscale turbulent kinetic energy dissipation rates
ε (W kg−1) and thermal variance dissipation rates χ(K2 s−1) are shown in log scale in (e) and (f), respectively.

0004279817.INDD 118

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Multiscale Routes to Supply Nutrients Through the Kuroshio Nutrient Stream  119

R
Sz

1

ui S
z xi

vi S
z yi

Mixing Terms
(6.2)

K

2

2

(6.3)

z

where χ is the microscale thermal variance dissipation
rate, and z is the mean temperature vertical gradient(Osborn & Cox, 1972). The effective thermal diffusivity
shows O(10−3 m2s−1), which is 100 times greater than theeddy diffusivity for density (Nagai et al., 2015b) based on
the measured turbulent kinetic energy dissipation rates,
illustrating that double‐diffusion is more important than
turbulence below 150 m depth along the Kuroshio
Extension. Additionally, the thermal diffusivities based on
the microstructure measurements are two orders of magnitudes larger than those estimated by the eddy permitting
model of Oschiles et al. (2003). These subsurface layers
correspond to the density layers of σθ = 25–27 kg m−3, where
the core of the Kuroshio nutrient stream flows. Our measurements clearly show that the active double‐diffusive
convection, observed below the Kuroshio Extension, plays
a very important role in mixing the nutrients upwards into
the euphotic layer. Coherence analysis between theobserved density ratio Rρ and the shear measured using the
EM‐APEX float showed a high correlation at subinertial
and inertial frequencies (Nagai et al., 2015b). High correlations at the subinertial frequency correspond to a
combination of mesoscale shear and thermohaline variations caused by the meandering of the Kuroshio Extension.
High correlations at the inertial frequency correspond to
the dominant near‐inertial wave shear (Figure 6.14a) andassociated fine scale thermohaline interleaving (Figure 6.14,
6.14c). These results suggest that the vertical shear inequation (6.2) caused by both subinertial flows due tomeso and submesoscale meandering of the Kuroshio
Extension, and that of near‐inertial internal waves, act tocatalyze active microscale double‐diffusive convection.

R

EV
IS
ED

PR
O

where xi and yi are the zonal and meridional coordinates
on the density surface, respectively, z the vertical coordinate, S salinity, Sz the salinity vertical gradient, and Rρ isthe density ratio defined as R
z / S z , where α is thethermal expansion coefficient, β the salinity contraction
coefficient, θz the vertical temperature gradient, andthe Mixing Terms represent all of the diffusion terms(Schmitt, 1990). Equation (6.2) illustrates that the
products of the vertical shear and salinity along isopycnal gradients are the critical terms to tilt the existing thermohaline anomaly to change Rρ. Accordingly,
the model still needs to resolve fine‐scale vertical shear
associated with mesoscale and submesoscale flows
and internal waves to evaluate the contributions fromdouble‐diffusive convection to tracer mixing.
Along the Kuroshio nutrient stream, the along‐isopycnal
thermohaline anomaly is enhanced after it reaches theKuroshio Extension, where cold low‐salinity water of the
subpolar region is advected to the south by the Oyashio
current to encounter the warm salty Kuroshio waters. In
July 2013, an Electro‐Magnetic Autonomous Profiling
Explorer Float (EM‐APEX float) was deployed along theKuroshio Extension axis from the R/V Kaiyo (Nagai et al.,2015b) to measure current velocity, temperature, salinity,and pressure down to 500 m depth every two hours.
An autonomous microstructure profiling float (Navis‐
MicroRider: Navis‐MR) equipped with two shear probes,
two FP07 thermistors, and CTD sensors was also deployed,along the same trajectory as the EM‐APEX float, to record
upper 500 m microstructure data every three hours (Nagai
et al., 2015b). The EM‐APEX float and the Navis‐MR float
measured these data over 900 km in 10 days and over 300 km
in 3‐4 days, respectively, along the Kuroshio Extension.
The temperature and salinity data obtained by the
EM‐APEX float showed fine‐scale vertical inversions
below 150 m depth over the entire 900‐km trajectory(Figure 6.14b, 6.14c). These thermohaline interleaving
structures are found to be favorable for active salt‐finger
and diffusive‐convection of double‐diffusion, based on thecomputed Turner angle (Figure 5 in Nagai et al., 2015b).
The measured microscale turbulent kinetic energy dissipation rates were small, O(10−10–10−9 W kg−1) with a few
moderate values of O(10−8 W kg−1) (Figure 6.14e). On
the other hand, microscale dissipation rates of temperature

FS

DR
Dt

variance measured by the Navis‐MR, showed very large
values of O(10−7‐10−6 K2s−1) below 150 m depth over
300 km, where fine‐scale thermohaline interleaving structures were formed (Figure 6.14f). The combination of
weak turbulence, strong thermal dissipation rates, anddouble‐diffusion favorable thermohaline conditions areindicative of the active occurrence of double‐diffusive
convection. The average effective thermal diffusivity
below 150 m depth is computed byO~0.3°, the fine‐scale thermohaline interleaving structuresin the simulation might be largely underestimated. The
Lagrangian rate of change of the density ratio Rρ can be
expressed as:0004279817.INDD 119

6.6. ­NUTRIENT SUPPLY PROCESS OF THE
KUROSHIO NUTRIENT STREAM ON THE SOUTH
SIDE OF THE KUROSHIO EXTENSION
As noted in the Introduction (section 6.1), the large
amount of nutrients transported by the Gulf nutrient
stream is supplied to the surface mixed layer throughinduction. The induction process is also thought to

4/15/2019 7:58:38 PM

KUROSHIO CURRENT

FS

is found in the high PV stratified layer below the mixed
layer, downfront wind induced subduction is an unlikely
candidate as a generation mechanism for the nitrate interleaving. The observed vigorous turbulence caused by the
downfront wind in the mixed layer on the south side of
the Kuroshio Extension, above the nitrate interleaving
structures, can entrain nitrate into the mixed layer. The
estimated turbulent vertical diffusive nitrate flux is
O(1 mmol N m−2 day−1), consistent with the satellite chlorophyll‐based estimates of primary production in thisregion during October 2009 (Nagai & Clayton, 2017).
To investigate the mechanisms responsible for forming
nitrate interleaving structures, a numerical simulation was
conducted using the North Pacific configurations of
ROMS coupled with the N2PZD2 ecosystem model (Nagai& Clayton, 2017), similar to that described in Section 6.2.
A nested grid with lateral resolution of 2 km was embeddedin the Kuroshio Extension region and the model was forced
with JRA‐55 6‐hourly reanalysis winds, including thosefrom the observation period. The simulation for October
2009 reproduced prominent warm streamers extending
from the first meander crest to the warm core ring on thenorth of the Kuroshio Extension. This compared favorably with the October 2009 observations (Nagai et al.,2012; Nagai & Clayton, 2017). On the southern side of thefront, the simulations showed nitrate interleaving structures extending from the north below the mixed layer, withlength scales similar to those of the observations. The
Lagrangian rate of change in the vertical nitrate gradient,which may be a measure of nitrate interleaving tendency,
can be expressed similar to equation (6.2):

(b)

37

B

Leg E

NO3
z

u NO3
z
x

v NO3
z
y

(c)

Leg D

C

Depth (m)

E

36

R

Latitude

D

146

35.2

35.4
35.6
Latitude

–20

8

–40

6

–80

35

145
Longitude

0

–60

35.5

144

M B

(6.4)

A

36.5

w NO3
z
z

35.6

35.8
36
Latitude

4

–100

2

–120

0

36.2

Nitrate (mmol N m–3)

(a)

D
Dt

Depth (m)

EV
IS
ED

PR
O

supply nutrients to the surface euphotic layer of theKuroshio nutrient stream (Komatsu, Chapter 5 thisbook). However, as mentioned in sections 6.1 and 6.2, thefinal supply regions of the Kuroshio nutrient stream are
the Kuroshio Extension region and the Kuroshio–
Oyashio confluence, where meso and submesoscaleeddies and filaments likely stir nutrients in the lateral
direction. The microscale turbulence and inorganic
nutrient measurements were made across the Kuroshio
Extension aboard the R/V Natsushima (Nagai et al.,2012). These observations were made in the fall, when themixed layer was developed as deep as 60–70 m on thesouthern side of the Kuroshio Extension, whereas on thenorthern side, the mixed layer was much shallower at~10 m. One of the most striking features observed was a
tongue‐like structure in the nutrient concentrations,extending for approximately 30 km at ~60 m depth from
north to south, below the mixed layer on the southernside of the Kuroshio Extension (Clayton et al., 2014;
Figure 6.15b). This interleaving structure originated fromthe along‐isopycnal maximum concentration of nitrateformed to the north of the Kuroshio Extension nutrient
stream (see also Figure 6.4).
The microstructure data also showed strong turbulence
within the surface mixed layer on the southern side of theKuroshio Extension. This was attributed to downfront
wind stress that occurred during the cruise (Nagai et al.,2012). Previous studies have shown that downfront winds
can change the sign of Ertel’s potential vorticity andinduce symmetric instability (Thomas & Lee, 2005) andthree‐dimensional turbulence in the Kuroshio Extension
front (D’Asaro et al., 2011). The downfront wind induced
mixing is known to generate a lens of homogeneous low
PV water, which is subducted along isopycnals. This subduction process could play a role in forming the observed
nitrate tongue; however, because the nitrate interleaving

O

120

Figure 6.15 (a) Ship track observations made by the R/V Natsushima during October 2009 is shown as dashed line.
The thick black lines in (a) indicate the meridional ranges shown in (b) and (c). Black contours are sea surface height
every 0.1 m in (a). (b) and (c) are the vertical section of the nitrate concentration (m mole N m−3) in Leg E and D,
respectively. The surface mixed layer depth is shown by the thick dashed black line. Black contours in (b–c) are σθ.

0004279817.INDD 120

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Multiscale Routes to Supply Nutrients Through the Kuroshio Nutrient Stream  121

u
z

ug
z

(6.5)

Results from recent observations and numerical simulations of the Kuroshio nutrient stream and related­diapycnal mixing processes, important for the lateral andvertical nutrient supply in the Kuroshio region, are summarized here.
We have employed a basin‐scale eddy permitting
numerical model of the Kuroshio region to reproduce theKuroshio nutrient stream. We found our model results tobe consistent with World Ocean Atlas and regional observations of nitrate distributions in the Kuroshio. The
major source of nitrate transported by the Kuroshio
nutrient stream is found to be at the tropical regions ofwestern North Pacific on the south side of the North
Equatorial Current at ~15°N.
We found that the Kuroshio nutrient stream transports alarge amount of nitrate to Japanese coastal waters, forminga mostly positive meridional gradient of nitrate along
density surfaces off Honshu Island. After the Kuroshio
flows away from the coast to the Kuroshio Extension
region, the meridional along‐isopycnal nitrate gradientbecomes negative on the north of the Kuroshio nutrient
stream. These regional differences in the along‐isopycnal
nitrate gradient result in different patterns of eddy‐induced
nitrate fluxes. Off Honshu Island, the positive along‐isopycnal meridional nitrate gradient means that eddy‐induced
fluxes tend to remove the nitrate‐rich waters from between
the Kuroshio and the coast, transporting them to the south
and into deep subsurface layers. On the other hand, in the
Kuroshio Extension region where the along‐isopycnal
meridional nitrate gradient has both signs, positive on thesouth side, and negative on the north, the eddy‐induced
nitrate flux is directed upward and northward on the northside, and downward and southward on the south side of theKuroshio Extension. These results are consistent with theoretical and numerical studies which show that eddies are
responsible for homogenizing existing along isopycnal
tracer anomalies (Rhines & Young, 1982; Lee & Williams,
2000; Gruber et al., 2011; Nagai et al., 2015c).
Recent in situ microstructure observations, for example
Tsutsumi et al. (2017) and Nagai et al. (2017), show thatthe upstream Kuroshio regions, the Okinawa Trough andthe Tokara Strait, are “mixing hotspots”. The observed
banded coherent layers of strong turbulence are attributed to the breaking of propagating high vertical wavenumber near‐inertial internal waves. The Kuroshio
appears to be a catalyst for energy dissipation and diapycnal mixing by trapping the near‐inertial internal waves in
the regions of anticyclonic vorticity (Nagai et al., 2017).

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These residual shear and lateral nitrate gradients canbe further split into subinertial ageostrophic and internal
wave fluctuation components along the Lagrangian trajectories of the water parcels. Specifically, the 30‐hour
average of residual shear and that of the lateral nitrategradient along the Lagrangian particle trajectories are
subtracted from the instantaneous residual shear and thelateral nitrate gradients, respectively, to obtain internal
wave shear and the lateral nitrate gradient fluctuations
due to internal waves. The resultant Lagrangian rate ofchange in the nitrate vertical gradient becomes:

6.7. ­SUMMARY AND CONCLUSION

FS

ur
z

include the double‐diffusive convection parameterization and so may underestimate the contributions fromthe near‐inertial internal wave shear.

O

where NO3− is the nitrate concentration, x, y, z are zonal,
meridional, and vertical coordinates, and u, v, w are zonal,
meridional, and vertical velocities, respectively, M is the mixing term, and B is the biological sink/source. The first two
terms on the right‐ hand side, the product of vertical shears
and lateral nitrate gradients, can be split into three parts: geostrophic, ageostrophic, and internal wave fluctuations. The
residual shear, ∂ur/∂z, which includes ageostrophic subinertial shear and internal wave shear, can be obtained by subtracting the 30‐hour averaged geostrophic shear ∂ug/∂z fromthe total shear ∂u/∂z in the Eulerian frame, to give:

ug
ua
h NO3
z
z
w NO3
M B.
z
z

h NO3

u IW
z

h NO3

EV
IS
ED

D NO3
Dt z

(6.6)

R

where the overbar represents the 30‐hour average, ua
ageostrophic velocity, uIW internal wave velocity. Notethat the sum of the resultant three components of shear,lateral nitrate gradients, and their covariance recovereach original instantaneous term. To evaluate each termof equation (6.6) along the simulated Lagrangian trajectories, 25,000 passive Lagrangian particles were releasedin the model three‐dimensional flow field. The results
show that subinertial ageostrophic shear is the dominantterm in modifying the vertical nitrate gradient comparedto other shear terms. While the near‐inertial internal
wave shear generated by a six‐hourly wind can induce
large amplitude fluctuations of the vertical nitrate gradient, they only strengthen and weaken the gradientback and forth at near‐inertial frequencies and do not
influence the formation of the nitrate interleaving structures (Nagai & Clayton, 2017). However, as discussed in
section 6.5.3, the near‐inertial shear is effective in acting
as a catalyst to produce active double‐diffusive
convection. The model used in this study does not

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KUROSHIO CURRENT

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are found to be effective in forming nitrate interleaving
structures. Observations suggest that these nitrate“tongues” can extend from the Kuroshio Extension
nutrient stream to the south below the mixed layer withlateral scales of O(10 km) and vertical scales of O(10 m).
The interleaving nitrate layers can then be entrained into
the surface mixed layer on the southern side of the front
by strong upper layer turbulence driven by downfront
wind stress. The diapycnal turbulent nitrate flux is estimated to be O(1 mmol N m−2 day−1).
The results of this study indicate that the Kuroshio islikely to play multiple roles in supplying nutrients laterally and vertically through multiscale processes(Figure 6.16, Table 6.1). At the largest scale, the Kuroshio
acts as a basin‐scale subsurface nutrient conduit thattransports large amounts of nutrients from the tropicalocean to the Kuroshio Extension and the Kuroshio–
Oyashio confluence. There should be little loss by
biological uptake of nutrients transported in the dark
subsurface of the Kuroshiro, and there should be somegain through the smallest process of diapycnal turbulent
mixing, especially when the Kuroshio passes over shallow
topographic features, such as the Okinawa Trough and

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The banded near‐inertial shear and associated enhanced
turbulent mixing are frequently observed not only in the
upstream Kuroshio, but are also seen with moderate amplitudes in the downstream Kuroshio Extension thermocline.
These banded shears and elevated turbulence in the
Kuroshio thermocline could be intrinsic characteristics,
which might promote the upward flux of nutrients to form
and/or maintain streaks of relatively higher nutrient concentrations along the Kuroshio nutrient stream. In addition,
under the Kuroshio Extension thermocline there are widespread thermohaline interleaving structures that are likely
generated by vertical shear of subinertial flows and near‐inertial internal waves. The observed large microscale
thermal variance dissipation rates of O(10−7‐10−6 K2s−1) andthermal diffusivity O(10−3 m2s−1) with weak turbulence anddouble‐diffusion favorable thermohaline interleaving structures suggest active double‐diffusive convection. Since thesesubsurface layers (σθ = 25‐27 kg m−3) include the core of thenutrient stream, double‐diffusion could act as an important
agent for ­diffusing nutrients up into the euphotic zone.
During the fall, when the mixed layer deepens to forma mixed layer front on top of the Kuroshio Extension,
subinertial ageostrophic and geostrophic vertical shears

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122

15° N
30° N

R

35° N

Figure 6.16 Schematic of the multiscale routes for nutrient supply. The basin‐scale Kuroshio nutrient stream flows
over shallow topography in the Okinawa Trough and the Tokara Strait, where strong turbulent mixing can mix
nutrient upward. The warm core eddies and warm filaments on the northern side of the Kuroshio Extension may
transport nutrient within the Kuroshio nutrient stream upward and northward. On the southern side, meso andsubmesoscale shears produce nutrient interleaving structures, which can be entrained into the surface mixed
layer. In and below the thermocline of the Kuroshio Extension, thermohaline interleaving structures formed bymeso, submesoscale and near‐inertial wave shears, enhance the double‐diffusion, which could diffuse nutrient
upward. (See insert for color representation of this figure.)

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Multiscale Routes to Supply Nutrients Through the Kuroshio Nutrient Stream  123Table 6.1 Summary Table for the Expected Nutrient Flux Along and Across the Kuroshio Path*.

Dense (coastal/north) side

Upstream Okinawa
TokaraSouth of Honshu

Light (offshore/south) side

Very strong ↑ upward
diapycnal flux by
microscale turbulence
Moderately strong ↑upward
diapycnal flux by
microscale turbulence↓ Downward and → offshore flux
by mesoscale eddies andfilaments

Strong ↑ upward flux bydouble‐diffusive
convection.
Moderately strong ↑upward diapycnal flux by
microscale turbulence↓ Downward and → southward
flux by mesoscale eddies andfilaments (may induce nutrient
tongues below the mixed layer
front)O

Kuroshio Extension↑ Upward and ← onshore flux by
Kuroshio induced bottom cold
water intrusion events to thecoastal seas
↑ Large upward and ← northward
flux by mesoscale eddies andfilaments

Kuroshio axis

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Locations along
and across theKuroshioPRO* Suggested by the results from the present study. Blank indicates that it is yet to be known. The arrow ↑ represents upward flux,↓ downward flux, ← onshore/northward flux, → offshore/southward flux.

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the Tokara Strait. Another microscale process, double‐
diffusive convection in and below the thermocline of theKuroshio Extension is found to be an important diapycnal mixing process. These double‐diffusion favorable
thermohaline interleaving structures in the Kuroshio
Extension are actively produced by the next two largest‐
scale flows, mesoscale, submesoscale and near‐inertial
internal wave shears. These mesoscale and submesocale
shears also stir the positive nutrient anomalies on thedensity surfaces associated with the Kuroshio nutrient
stream to form nutrient interleaving structures on thesouthern side of the Kuroshio Extension front, which arethen entrained into the surface mixed layer by enhanced
turbulence by downfront winds (Figure 6.16, Table 6.1).
Although the observations and the numerical simulations
described in this study provide evidence for these multiscale
nutrient supply process, many of these nutrient fluxes are notwell quantified, especially for the diapycnal nutrient fluxes.
This is primarily due to a lack of sufficient simultaneous
observations of nutrients and microstructures. Although the
Kuroshio nutrient stream is a basin‐scale phenomenon, theinterannual variations in the nutrient stream have yet to beinvestigated sufficiently. More intensive in situ observationsof simultaneous microstructures and nutrients, and basin‐
scale high resolution numerical simulations using proper
mixing parameterizations for internal waves and double‐diffusion are necessary in the near future to address these issues.
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