How the Kuroshio enriches thesouthern coast of Japan and its
downstream regions
Takeyoshi Nagai, Gloria Silvana Duran Gomez, Diego Andre Otero Huaman,
Yoshie Naoki, Kazuki Ohgi, Daisuke Hasegawa, Sophie Clayton, Yusuke UchiyamaO

海物質

T. Takahashi et al. / Deep-Sea Research II 56 (2009) 554–577

The Kuroshio Currentzone and especially in the zone north of 141N (Fig. 12C). The highwind speeds induce not only intense upwelling in the western
Arabian Sea, but also cause high rates of sea–air gas exchange. Thesouthern temperate Indian Ocean also has higher wind speeds
than the southern Pacific and Atlantic, and hence higher Tr values
ranging between 0.05 in summer and 0.08 in winter. The mean
monthly values (area weighted) for the global ocean are nearly
constant around 0.05 g-C month!1 m!2 matm!1 (Fig. 12D).

6.3. Global distribution of the net sea–air CO2 flux
The climatological mean sea–air CO2 flux in each box area foreach month is computed by multiplying monthly mean values for
Tr (Eq. (8)) and DpCO2 (Eq. (4)) in each box. Since no significant
correlation of individual DpCO2 values with 6-h mean wind speed
data is found in a single month, the product of monthly meanvalues is considered as a valid approximation for the CO2 flux.
Over the polar regions, where sea-ice fields form seasonally, thefollowing assumptions are made for computing the sea–air CO2flux. When the ice cover is less than 10%, a box area is assumed to
be all water. For ice covers between 10% and 90%, the flux isassumed to be proportional to open water area. Since ice fields
have leads and polynyas due to dynamic motions of sea ice asevidenced by ‘‘sea smoke’’, we accept the estimates by Saundersand Ackley (personal communication) that 10% of fields is open
water at any given time even in areas where satellite observations
indicate 100% ice cover. The ice cover values used in this study arebased on the NCEP/DOE 2 Reanalysis data (2005) (provided by theNOAA/OAR/ESRL PSD). The original data given in a Gaussian gridfor 1970–2005 are re-gridded to our 41 " 51 grid, and the ice covervalues in each pixel are averaged for each month (30.5 days).

569

6.3.1. Mean annual distribution
Fig. 13 shows the climatological mean annual sea–air CO2 flux
(g-C m!2 y!1) for the reference year 2000, and Fig. 14 shows 41
zonal aggregates for the flux in the major ocean basins and theglobal oceans. The annual fluxes in six climatic zones in eachocean basin are summarized in Table 6. The equatorial Pacific is
the most prominent source area for atmospheric CO2 with aseasonally persistent sea-to-air flux of 0.48 Pg-C y!1. Togetherwith the tropical Atlantic and Indian Oceans, the global tropicaloceans emit 0.69 Pg-C y!1 to the atmosphere. A belt of CO2 sinkzone is located in 20–501 latitudes in the both hemispheres(Fig. 14). This is attributed primarily to strong winds between 401
and 501S, and the low pCO2 values produced along the subtropicalconvergence zone, where the cooled subtropical gyre waters
with low pCO2 meet the subpolar waters with biologicallylowered pCO2. Together, the mid-high latitude northern(22–501N) and southern (22–501S) oceans constitute, respectively
a sink of !0.70 and !1.05 Pg-C y!1. A sink area (!0.27 Pg-C y!1) isseen also in the North Atlantic north of 501N, including the Nordic
Seas and portion of the Arctic. This is attributed to strong
phytoplankton blooms in spring and strong cooling in winter.
The annual CO2 flux over the Southern Ocean seasonal ice
zone is small due to the ice cover that reduces sea–air gas
transfer in winter and by the cancellation of the seasonal sourceand sink fluxes.
The mean annual sea–air CO2 flux values for the four major
ocean basins are compared in Table 6. While the Atlantic has only
23% of the global ocean area, it takes up 41% of the annual global
ocean flux of !1.42 Pg-C y!1. On the other hand, while the Pacifichas the largest area (47%), twice as large as the Atlantic, it takes up
only 33% of the global flux. This is due to the large CO2 source

Sassa 2019

Many fish species including
Lanternfish use the Kuroshio
region for their spawning andnursery sites.

Takahashi et al. 2009!2!1The Kuroshio Extension
region is one of the major net
CO2 sink in the ocean

40°S

The Kuroshio Current-6060°S-80∆pCO2 (µatm)

80°S

-100

Sarmient and Gruber 2006

etal. 2019
WOANitrate
on=25 100°W Nagai
60°E
100°E
140°E
180° σθ140°W
60°W
20°W
20°E

20°E

80°N

3432Surface Nitrate

60°N

23080°N

28

1.5

26

40°N

60°N

1242220°N

40°N

0.5

2018Eq0.4

1620°N

0.3

1420°S

12Eq

0.2

1040°S

820°S

0.1

660°S

0.08

440°S

2Sea-surface nitrate (mmol m-3)

80°S

0.06

10.04

60°S
20°E60°E

100°E

140°E

180°

140°W

100°W

60°W

20°W

20°E

0.02

Sea-surface chlorophyll (mg Chl m-3)

80°S

However, surface layers of theKuroshio is nutrient poor.

80°N
60°N

Eq

60°S

60°E

100°E

140°E

180°

140°W

100°W

60°W

20°W

20°E

Sarmient and Gruber 2006

2.2
2.0

80°N

20°N

40°S

20°E 2.4

200

40°N

20°S

2.6

On the other hand, subsurface layersof the Kuroshio carries a large
amount of nutrients (Guo et al. 2012),
with elevated concentrations on thedensity surface (Nagai et al. 2019).

1001.860°N

1.6Surface NPP

4030

1.440°N

201.2

1820°N

1.0

160.8Eq

140.6

120.420°S

1080°S

Sea-surface phosphate (mmol m )Thoughit is caused by other reasons
as well, bands of high production in
the Kuroshio Extension could be dueto this nutrient transport by theKuroshio.
-320°E

80°N
60°N
40°N

60°E

100°E

140°E

180°

140°W

100°W

60°W

20°W

0.2

840°S

0.0

620°E

60°S
80°S

42NPP (annual) (mol m-2 yr-1)

8075

20°E

7065
60060°E

100°E

140°E

180°140°W

100°W

60°W

20°W

20°E

Gulf Stream: Nutrient Stream
The North Atlantic Nutrient Stream281(d)

Figure 4

6 of 18

Fig. 1. (continued).

Pelegri et al. (1996) found that theGulf Stream transport a large
amount of nutrients, and nutrient
concentration is higher along thestream on the density surface.

2.2 Estimates of nutrient irrigation
In order to discuss the location of the nutrient stream of the North Atlantic, or “Nutrient GulfStream”, we have used four summer hydrographic sections from Pelegrí and Csanady (1991).
These will be identified by the latitude or longitude at which they approximately crossed thecurrent: 36°N (June 1981), 64°W (April 1985), 53°W (May 1983), and 35°W (August 1983);
their approximate location is shown in Fig. 1 of Pelegrí and Csanady (1991). We have also
analyzed a short hydrographic section north of Cape Hatteras, but about 100 km southwest ofsection 36°N, taken between September 6 and 9, 1985.
In Fig. 2 we present the combined σt and nitrate flux distributions for all four Pelegrí andCsanady’s (1991) hydrographic sections (the nitrate flux distributions were already shown intheir Figs. 3 and 6; in this and the following figures the small crosses indicate the location ofnutrient data). The (alongstream) nitrate flux, or nitrate transport per unit area, is simply

This is also confirmed by more recent
Figure 10. Properties from hydrographic section 1 from CLIMODE J
fielddata4b).
in (a)
CLIMODE(PalterLozier concentration asee Figure
Potential densityand (b)&phosphatestream distance. (c) Phosphate concentration (colored contours) with is2008)!1!1
white (0– 1 m swith a contour interval of 0.25 m s ) and (d) tem

Induction
The subsurface nutrients are transported by adiabatic flows along isopycnal.
GB1016WILLIAMS ET AL.: NUTRIENT STREAMS AND INDUCTION
They encounters deeper mixed layer while they travel northward in the winter season.
The nutrients can be obducted through inclined mixed layer base by along isopycnal flows
WILLIAMS ET AL.: NUTRIENT STREAMS AND INDUCTION
Williams et al. 2006

GB1016

GB1016

sonal boundary layer, while the subsequent seasonal cycleof the mixed layer and convection dictates the timing of anypeaks in productivity, such as the spring or autumn blooms.

2.1. Climatological View of the End of Winter
Conditions-1
m molmixed
m-2yrInduction
m mol m-2yr-1
6] The instantaneous
layer distribution
is highly[Ekmanvariable in time through the diurnal and seasonal cycles insurface forcing, and the passage of synoptic-scale weather
systems, as well as highly variable in space throughstratification changes linked with ocean mesoscale eddy
and frontal features. In order to provide a large-scale context
for the nutrient transfer process, it is useful to define aninterface marking the maximum extent of the convection
process, or the seasonal boundary layer, given by thethickness of the mixed layer at the end of winter, H. This
interface is diagnosed over the North Atlantic from climatological density profiles, defined by the depth where thepotential density has increased by 0.125 kg m!3 from thesurface density. The end of winter mixed layer is characterized by a thickness of 50 m over the tropics, increasing
poleward
to 200 m overdiagnostics
the northernflank meanof thevolume
subtropicalFigure
4. Climatologicalof annualand nitrate fluxes in the upper ocean pgyre,then increasing
to upwelling,
morethanm volume
followinga the thermocline into the base, and et(b)flux fromunit
horizontal
area: (a) Ekman
wek300Williamsal. 2006
!1(m yr gyre), referredto herean induction
flux; (c) Ekman upwellingthewinter mixed!Sanncyclonic
circuitlayer,of thesubpolar(Figures3aasand3d).
!1(d) theinduction
nitrate thickening
flux, !SannNofmM). Positive represents a flux direct
nitrate,wekNH, andH (mConcomitantwith
poleward
theyr mixedfromthethermocline
intothemixedlayer.
layer, there is an increase in the mixed-layer density from
Figure 2. A schematic figure depicting the induction less than sq = 24.0 in the tropics, reaching between sq =The
ultimate
supplytheoverStream extension and recirculation, and over induction flux of nitrate into the mixed layer rea
process nutrient
where fluid andnutrients aretransferredin
fromthethe Gulf
26.75
27.0 overboundary
the subpolar gyre.and
In addition,
the thezerosubtropical/subpolarline of Sann is much than
1000 mand
yr!1mM over the Gulf Stream extensi
thermocline
intothemixed
layer:(a)a
meridional
sectionsubpolar regions is achieved farther
by south
27.5
in the northwest
cornerof theincreasing
to sqto=thecontribution
from thethan that furtherof wek owing
subpolargyre (Figure 4d), which is to be exp
where a particle moves with a vertical velocity, w, anda subpolar
gyre (Figure3b).layer
Thisandpolewardof oftheNH and !Sann (Figures 3c and 4b)
the lateraltransfer between
the mixed
the ther-thickening
the patterns
inductionhorizontal velocity, u, leading to a vertical and lateral
flux is much larger than the correspondmocline. mixed
A recent
eddy-resolving
model withstudyanofincrease
the induction
layer
is also associatedin the nitratetransfer into a poleward thickening mixed layer; (b) a threesubduction process is broadly supportive of these end of upwelling, w N , which only reaches 500 m yr

Ekman upwelling < Induction

Scientiﬁc questions• • IsIsthe
the induction
induction important
important inin thethe Kuroshio
Kuroshionutrient
nutrient

stream?
stream?
•Any interannual modulations in the subsurface
nutrients in the Kuroshio? If so, how?

•Whether and how the subsurface nutrients in theKuroshio are supplied to the southern coast of Japan?

ROMS N2PZD2 and PN-Line and 137oE nitrate data
ROMS N2PZD2 Model [Gruber et al.
2006] forced 10 years withclimatological forcing and 35 year
NCEP(1980-2015) forcing twice.

Eddy permittingresolving simulation can
reproduce elevatednitrate along theKuroshio with rightmagnitude.

MLDNitrate flux through mixed layer base in 35 years
MLD
Model mixed layer
depth show goodagreement with theclimatological mean.

NO3u ⋅ ∇MLD−Kz∂NO3∂zz=MLD

Daily induction isstrongest in late winter,
affected by mesoscale
mixed layer variations

36 year meansuggests more than
30% of total nitrate isprovided by inductionin this region.

NO3u ⋅ ∇MLD

Total

Scientiﬁc questions•

Any interannual modulations in the subsurface
nutrients in the Kuroshio? If so, how?
Long et al. 2018, and Hu et al. 2018 both suggested
influence from PDO related Kuroshio modulations onnutrient transport in the nutrient stream, but it is not clear
how.
PDO related Kuroshio Extension path modulations occurs
mostly in the downstream.

Scientiﬁc questions•

Is the induction important in the Kuroshio nutrient
stream?

•Any interannual modulations in the subsurface
nutrients in the Kuroshio? If so, how?

•Whether and how the subsurface nutrients in theKuroshio are supplied to the southern coast of Japan?

Tow-yo Turbulence observations in November 2017

We tow-yo UVMP along theKuroshio axis across the Tokara
StraitUnderway-CTD winch

R.T.V. Kagoshima-maru

Underway VMP5gLe4g
Le
Leg2
Leg1

Leg

3

Five transects

VMP250 (Rockland ScientificInternational, Victoria BC,Canada)ADCP Shear November 2018 southeast of Kyushu: Hyuganada sea
Ray tracingfeffωmin2Nl − bx mx
cg =m 2ω

2 2bml−Nlxz
cg =m 3ωDk= − ∇ωDt

The 1.1f near-inertial ray-path inquiescent condition agrees wellwith the angle of the shear awayfrom the front.
M2 ray-path agrees better for thesteep shear at the Kuroshio axis
than the near-inertial ray-path.
With the group velocities
considering a 2-D front, thenear-inertial ray-path becomessteeper at the front, and agrees
The near-inertial ray-paths reflect back and better with the observed shear
forth in the regions of lower feff and ωmin .
angle.
Nagai et al. 2019

Turbulence and nitrate ﬂux in southeast of Kyushu: Hyuganada sea

TKE Dissipation Rate
30km

Strong turbulence isfound over 20-30 km
lateral and 200 m
vertical scales.

Using water samples and SUNA data,
estimated nitrate diffusive flux is 1-10 m
mol m-2day-1, which is 10-100 times
larger than Kaneko et al. (2012).
When the Kuroshio approaches to the coast, the large amount ofnutrients can be injected between the Kuroshio and the continental slope

Nagai et al. 2019Conclusions
Induction:~5 m mol N m-2day-1•

Is the induction important in the Kuroshio nutrient stream?

•Yes it is. The induction supplies ~5 m mol m-2day-1of nitrate on
average, which is about 30% of the total nitrate supply in theKuroshio Extension and the Kuroshio-Oyashio confluence
region. The maximum induction occurs late winter, and they arestrongly affected by the mesoscale mixed layer structures.

•

Any inter annual modulations in the subsurface nutrients in theKuroshio? If so, how?

•Whether and how the subsurface nutrients in the Kuroshio
supplied to the southern coast of Japan?

•A large amount of subsurface nutrients in the Kuroshio is
injected to the region between continental slope and theKuroshio when the Kuroshio approaches to the continentalslopes at 1-10 m mol m-2day-1 caused by the turbulence
probably due to trapped near-inertial internal waves.

-1 )-2 dNol0maymm-1(1OO(1m mol N m-2day-1)

AGU-Wiley
Geophysical Monograph Vol 243