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GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L08607, doi:10.1029/2009GL037540, 2009

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ArticleSubduction of North Pacific central mode water associated with
subsurface mesoscale eddy
Eitarou Oka,1 Katsuya Toyama,2 and Toshio Suga2,3Received 30 January 2009; revised 20 March 2009; accepted 30 March 2009; published 29 April 2009.

[1] During a shipboard high-density hydrographic survey
carried out in the western North Pacific in fall 2008, we
observed an anticyclonic eddy with a thickness of 150 dbar
and a diameter of 40 km near 500 dbar depth at 27.5°N,
145°E. This subsurface mesoscale eddy contains the North
Pacific central mode water (CMW), which has anomalously
low potential vorticity and high dissolved oxygen comparedto the climatological CMW properties in the same region.
Profiling float measurements detect similar CMW patches
near and south of the Kuroshio Extension as well as
southward CMW migration within the CMW formation
region north of the Kuroshio Extension. These observed
facts suggest that CMW is subducted into the permanent
pycnocline not only through large-scale eastward advection
near the northern edge of the subtropical gyre but also
through southward cross-frontal advection associated withthe formation and migration of subsurface mesoscaleeddies. Citation: Oka, E., K. Toyama, and T. Suga (2009),
Subduction of North Pacific central mode water associated with
subsurface mesoscale eddy, Geophys. Res. Lett., 36, L08607,
doi:10.1029/2009GL037540.

1. Introduction[2] In October – November 2008, a high-density hydrographic survey was carried out in the western North Pacificsoutheast of Japan during the KH-08-3 cruise leg 2 of R/V
Hakuho-maru. Two meridional lines at 143° and 146°E andtwo shorter zonal lines at 27.5° and 34°N were occupied(thick black lines in Figures 3 and 4) using a conductivitytemperature-depth-oxygen profiler (CTDO2) at intervals of
300 or 1° in latitude/longitude and an expendable conductivity-temperature-depth profiler (XCTD) at 100 intervals,
along with other physical/biogeochemical instruments.
These observations primarily aim to clarify mesoscale
structure of the North Pacific subtropical mode water(STMW) [Masuzawa, 1969], which is comparable to that
presented recently by high-resolution numerical models[e.g., Rainville et al., 2007], and physical/biogeochemical
processes associated with STMW; these aspects will bepresented in separate papers.
[3] In the 27.5°N section of the cruise, we observed awater parcel with a thickness of 150 dbar and a horizontal
width of a few tens of kilometers near 500 dbar depth,
which is characterized by anomalously low potential vor1

Ocean Research Institute, University of Tokyo, Tokyo, Japan.
Department of Geophysics, Graduate School of Science, Tohoku
University, Sendai, Japan.
3
Institute of Observational Research for Global Change, Japan Agency
for Marine-Earth Science and Technology, Yokosuka, Japan.
2

Copyright 2009 by the American Geophysical Union.
0094-8276/09/2009GL037540$05.00

ticity (Q) and high dissolved oxygen (DO). The water parcel
is classified as the North Pacific central mode water (CMW)[Nakamura, 1996; Suga et al., 1997], but its location andproperties differ significantly from those expected from thelarge-scale CMW circulation that has been assumed. Using
data from the cruise, the World Ocean Atlas 2001 climatology (WOA01) [Conkright et al., 2002], and Argo profiling
floats [Argo Science Team, 2001], we here discuss thesignificance of this CMW parcel, and present anotherpossible CMW formation mechanism. Specifically, CMW
is subducted into the permanent pycnocline not only
through large-scale eastward advection near the northernedge of the subtropical gyre but likely also through southward cross-frontal advection associated with the formationand migration of subsurface mesoscale eddies, which has
hardly been observed due to its small spatial scale.

2. Observed CMW Pycnostad[4] The 27.5°N sections of potential temperature (q),salinity (S), potential density (sq), and Q exhibit theseasonal pycnocline with q of 20°– 26°C at depths of 50–
100 dbar and the permanent pycnocline with q of 6°– 20°C
at 100– 750 dbar (Figure 1). The seasonal pycnocline hashigh Q exceeding 5  1010 m1 s1, while the permanent
pycnocline, characterized by moderately high Q of 2 – 4 1010 m1 s1, contains two significant low-Q areas(pycnostads) with Q < 1.5  1010 m1 s1; one is locatedat 150– 400 dbar west of 144°400E and east of 145°100E,
and the other exists at 450– 600 dbar at 144°500 – 145°E.
The former pycnostad having q of 16° – 18°C is considered
to be STMW, formed in the recirculation gyre south of theKuroshio Extension and subsequently transported southward to this latitude [Suga and Hanawa, 1990, 1995; Oka
and Suga, 2003; Oka, 2009]. The latter pycnostad having a
distinct Q minimum of 0.35  1010 m1 s1 at 516 dbar at
145°E is classified as CMW because it has q of 10.2°C, S of
34.30, and sq of 26.37 kg m3 at the Q minimum (called
core henceforth), at which the water properties at the time of
pycnostad formation is expected to be best preserved.
[5] The biconvex shape of isopycnals implies that the
CMW pycnostad is trapped in a subsurface anticycloniceddy. Actually, if we assume gradient wind balance with acurvature radius of 16 km (the CTDO2/XCTD station
spacing at 27.5°N), we obtain anticyclonic circulation
flowing to the north (south) at 144°400 –144°500E (145°–
145°100E) at depths of 400– 650 dbar, with a maximum
speed of 17 cm s1 (not shown). Current velocity dataobtained by a shipboard acoustic Doppler current profiler
also clearly detect the eddy structure with a diameter of
40 km and a somewhat larger maximum speed of 31 cm s1
at almost the same location and depths (not shown).

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Figure 1. Zonal sections along 27.5°N of (a) q (°C), (b) S,(c) sq (kg m3), and (d) Q (1010 m1 s1) obtained by
CTDO2 and XCTD on 27–28 October 2008 in the KH-08-3
cruise leg 2. Black (white) inverted triangles on the top ofeach plot indicate the locations of CTDO2 (XCTD) stations.
Values less than 34.2 is shaded in Figure 1b. Contours are
drawn at 0.5, 1, 1.5, 2, 3, 4, and 5  1010 m1 s1, andvalues less than 1  1010 m1 s1 (of 1 – 2  1010 m1s1) are heavily (lightly) shaded in Figure 1d.
[6] The CMW pycnostad is characterized not only by low
Q but also by high DO with a value of 5.74 ml l1 at thecore at sq = 26.37 kg m3, which is higher than the wholeoverlying layers at the same location (Figure 2). On theother hand, apparent oxygen utilization (AOU), which is thedifference between the saturated and observed DO concentrations, is quite low in the pycnostad, being 0.59 ml l1 atthe core. Such an intense AOU minimum is not observed at
the other stations in the KH-08-3 cruise. The anomalous
existence of the CMW pycnostad is further explained in thenext section, by comparing its properties with the climatological CMW properties.

L08607front, located approximately at 35° and 42°N, respectively[Nakamura, 1996; Suga et al., 1997]. The formation
region is further divided by the northern bifurcation of
the Kuroshio Extension at 38°N into two regions, in thenorthern (southern) one of which a lighter (denser) typeof CMW with the core sq = 25.8– 26.2 (26.3 – 26.4) kg m3 isformed [Oka and Suga, 2005]. According to this classification, the CMW pycnostad observed in the KH-08-3 cruise(CMWP08) having the core sq of 26.37 kg m3 is the denser
type formed north of the Kuroshio bifurcation. Both types of
CMW have been believed to be subducted into the permanent
pycnocline in the central North Pacific, and then advected by
the anticyclonic subsurface circulation to spread in thesubtropical gyre.
[8] In a climatological Q map on the sq = 26.37 kgm3 isopycnal constructed from WOA01, the thickest
CMW with Q < 1.5  1010 m1 s1 is found at
32° –42°N, 175°E– 145°W (Figure 3a). Its northwestern
end at 40°N, 175°E reasonably corresponds to the late
winter outcrop of the isopycnal, at which subduction
presumably occurs. From this subduction point, a tongueof low Q extends southeastward, and then southwestward
to 27°N, 171°E, and to 21°N, 125°E. This anticyclonic
CMW advection all the way to the western boundary
takes roughly 15 years, according to geostrophic calculation. During the journey, the AOU of CMW increases
from 0.5– 1 ml l1 to 3 ml l1 due to the oxygen
consumption through remineralization of organic matterand to mixing (Figure 3b).
[9] The hydrographic sections of the KH-08-3 cruise
lie in the region of relatively high Q exceeding 2.25 
1010 m1 s1, located northwest of the low-Q tongue of
CMW (Figure 3a). The observed Q at sq = 26.37 kg m3 inour 27.5°N section (2.06– 3.57  1010 m1 s1; Figures 1cand 1d) is comparable to the climatological Q, except that
there are a few extremely low values near 145°E, repre-

3. Relation to Large-Scale CMW Circulation[7] CMW originates in the deep winter mixed layer
formed between the Kuroshio Extension and the subarctic

Figure 2. Vertical profiles of (a) Q, (b) DO, and (c) AOU
with respect to sq, obtained by CTDO2 and XCTD on 22
October –4 November 2008 in the KH-08-3 cruise leg 2.
Black curve (grey curves) indicates the properties at 27.5°N,
145°E (the other stations).

Figure 3. Annual-mean distributions of (a) Q (1010 m1s1) and (b) AOU (ml l1) on the isopycnal of sq = 26.37 kgm3 in the North Pacific from WOA01. Thick grey curvesindicate the outcrop of the isopycnal in March from WOA01.
Thick black lines denote CTDO2/XCTD sections of the KH08-3 cruise leg 2.

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CMW formation region to 27.5°N, 145°E that does notappear on climatological maps.

4. CMW Circulation in 2008 From Argo Float
Data

Figure 4. Distributions of Q on the isopycnal of sq =26.37 kg m3 in the North Pacific from March – April
through September – October 2008 from Argo float data.
Thick grey curves indicate the positions of the Kuroshio
Extension and the subarctic front between 140°E and 180°.
Thick black lines denote the CTDO2/XCTD sections of theKH-08-3 cruise leg 2.

sented by the core Q of CMWP08 (0.35  1010 m1 s1).
The core AOU of CMWP08 (0.59 ml l1; Figure 2c) is alsomuch lower than the AOU at sq = 26.37 kg m3 at the
nearby CTDO2 stations (2.5 ml l1), which is comparable
to the climatological AOU at the same location (2.25 mll1; Figure 3b). Rather, the core AOU is comparable to theclimatological AOU at 40° – 42°N, east of 175°E justdownstream of the subduction point.
[10] The core AOU of CMWP08 is comparable to thelowest core AOUs of the STMW pycnostads observed in
the KH-08-3 cruise, which occur in the sq range of 25.1 –25.4 kg m3 (Figures 2a and 2c). These STMW pycnocstads are believed to be formed in late winter 2008, based on
a comparison of STMW properties between 2007 and 2008
using Argo profiling float data (not shown). It is therefore
highly possible that CMWP08 is also formed in late winter2008, being about a half year old when it is observed in the
27.5°N section. It is thus unlikely that CMWP08 is advectedfrom the central North Pacific to 27.5°N, 145°E by thelarge-scale anticyclonic circulation, which would take
10 years or longer. There should be a shortcut from the[11] In climatological Q maps similar to Figure 3a,
mesoscale and smaller-scale features such as CMWP08
are smoothed out and do not appear. To trace CMWP08
and investigate the existence of similar CMW pycnostads,
we now examine temperature and salinity data from Argo
profiling floats in the North Pacific [Oka et al., 2007]. In
Figure 4, Q distributions on the sq = 26.37 kg m3
isopycnal from late winter to fall 2008 constructed fromthe float data are presented, along with the positions of theKuroshio Extension and the subarctic front (the northernand southern boundaries of the CMW formation region)
determined using optimally interpolated q and S and thedefinitions of q300 dbar = 12°C [Mizuno and White, 1983]
and S100 dbar = 33.8 [Yuan and Talley, 1996], respectively.
[12] In March –April, two groups of the thickest CMWwith Q < 0.5  1010 m1 s1, probably just after theformation, are seen at 143°– 154°E and 171° –175°E in anorthern part of the CMW formation region, presumably
north of the Kuroshio bifurcation. As the season progresses,
the eastern group is advected eastward while gradually
increasing its Q values, passing through the subduction
point indicated by WOA01 (Figure 3a). Therefore, thisgroup is thought to be subducted into the permanent
pycnocline through large-scale eastward advection, as inferred by previous observational and numerical studies[Kubokawa and Inui, 1999; Xie et al., 2000; Suga et al.,
2004; Tsujino and Yasuda, 2004]. On the other hand, thewestern group migrates southeastward, and is located at
35° – 40°N, 155° – 162°E in fall. Part of this group is
expected to cross the Kuroshio bifurcation to enter thesouthern formation region, as seen in the repeat hydrographic section across 165°E [Oka and Suga, 2005], while
the remaining part in the northern formation region is likely
to be re-entrained into the mixed layer developed in thefollowing winter.
[13] The low-Q signature of CMW extends anticyclonically from the formation region to the eastern North Pacific,
then to the lower-latitude western North Pacific centered atabout 20°– 25°N, although the boundary between the southern part of CMW and the low Q in the tropical region ismuch less evident compared to the climatological Q map(Figure 3a). Inside this anticyclonic CMW pathway, there isa region where Q mostly exceeds 1.5  1010 m1 s1 westof 170°E, between the Kuroshio Extension and 27°N, as
seen in the climatological map.
[14] In this high-Q pool we find several CMW patcheswith Q < 1.5  1010 m1 s1, such as one at 33°N, 146°E
in April (with the core Q of 0.91  1010 m1 s1) and one
at 29°N, 142°E in July (1.32  1010 m1 s1). Their
isolated distributions suggest that they have relatively small
horizontal scales, possibly comparable to that of CMWP08.
Although some of them are observed near the hydrographic
sections of the KH-08-3 cruise, none of them are considered
to be identical to CMWP08 because they have higher core
Q and slightly different core q or S. It is reasonable that
CMWP08 is not detected by the Argo float observations,

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because the spatial resolution of Argo array (300 km) is
one order larger than the horizontal scale of CMWP08.
[15] In addition to the CMW patches in the high-Q pool,
we find several of them in the vicinity of the Kuroshio
Extension, such as one at 34°N, 150°E in March and one at
33°N, 146°E in September. The existence of these CMW
patches near and south of the Kuroshio Extension, as well as
the southward migration of the western group in the CMW
formation region, suggests that a certain amount of CMW istransported southward from the northern formation region
across the Kuroshio bifurcation and Extension in the formof subsurface mesoscale eddies, thereby subducting into thepermanent pycnocline.
[16] Such an idea of CMW subduction in the form of
mesoscale eddies seems to be well supported by a theoretical study of Spall [1995]. He used analytic and numericalmodels to demonstrate that baroclinic instability and theresulting frontogenesis occurring at upper ocean fronts
produce downward velocity and deep ageostrophic crossfrontal flow, which carry water in the mixed layer on one
side of the front below and across the front. If thissubduction occurs from a deep mixed layer, the subductedwater parcels have thickness larger than the surrounding
water of the same density, and quickly develop anticyclonic
circulation with a typical diameter of 25 km throughgeostrophic adjustment [Spall, 1995]. Thus, the modeled
cross-frontal subduction occurs in the form of mesoscale
anticyclonic eddies, which corresponds well to the observed
structure of CMWP08. The remaining question is by whatmechanism CMWP08 reaches as far south as 27.5°N soquickly after crossing the Kuroshio Extension. If we assume
that CMWP08 crosses it at 35°N at the end of April 2008,
the mean southward propagation speed will be as high as
5.4 cm s1.
[17] The CMW patches near and south of the Kuroshio
Extension are also seen in the float data-based PV maps on
the isopycnals between 25.8 and 26.4 kg m3 in the CMW
sq range (not shown). Therefore, the cross-frontal CMW
subduction due to subsurface mesoscale eddies might be aconsiderable component of the total CMW subduction, andcould be important for the Q redistribution within thesubtropical gyre as well as the meridional transport ofbiogeochemical substances such as carbon and nutrients.
However, it seems not easy to clarify the rate, mechanism,and effects of this cross-frontal subduction based on observations, due to difficulty in repeating high-density hydrographic surveys comparable to ours and to the relatively
sparse distribution of Argo profiling floats. For such purposes, recent super-high resolution ocean general circulation models having a grid spacing of a few kilometers are
expected to be useful.

5. Conclusion[18] In the KH-08-3 cruise conducted in October –November 2008, we observed a subsurface mesoscale
eddy containing anomalously oxygen-rich CMW at
27.5°N, 145°E. The existence of this eddy and otherisolated CMW patches observed by the profiling floats
suggests that CMW is subducted into the permanent pycnocline not only through the classical eastward advection near
the northern edge of the subtropical gyre but also through theL08607

southward cross-frontal advection associated with the formation and migration of subsurface mesoscale eddies.
[19] Acknowledgments. The authors are grateful to the captain, crew,
and scientists participating in the KH-08-3 cruise leg 2 of R/V Hakuhomaru of Japan Agency for Marine-Earth Science and Technology for theirefforts in conducting the CTDO2/XCTD measurements. They also thankTsuyoshi Ohira for help in preparing the Argo float data and Shinya
Kouketsu and four anonymous reviewers for helpful comments. The Argo
float data used in this study were collected and made freely available by theInternational Argo Project and the national programs that contribute to it(htt://www.argo.ucsd.edu, http://argo.jcommops.org). EO was partly supported by Ocean Research Institute Cooperative Research Fund. TS waspartly supported by funds from the Grant-in-Aid for Scientific Research inPriority Areas ‘‘Western Pacific Air-Sea Interaction Study (W-PASS)’’
under grant 19030004 from Ministry of Education, Culture, Sports, Scienceand Technology (MEXT), Japan, and from the ‘‘Studies on Prediction andApplication of Fish Species Alteration (SUPRFISH)’’ sponsored by the
Agriculture, Forestry and Fisheries Research Council (AFFRC), Japan.

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E. Oka, Ocean Research Institute, University of Tokyo, Nakano-ku,
Tokyo 164-8639, Japan. (eoka@ori.u-tokyo.ac.jp)
T. Suga and K. Toyama, Department of Geophysics, Graduate School of
Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan.

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