J Oceanogr (2014) 70:521–534
DOI 10.1007/s10872-014-0248-2

ORIGINAL ARTICLE

Synoptic observation of Central Mode Water in its formation
region in spring 2003
Eitarou Oka • Kazuyuki Uehara • Toshiya Nakano •Toshio Suga • Daigo Yanagimoto • Shinya Kouketsu •Sachihiko Itoh • Shota Katsura • Lynne D. Talley

Received: 30 January 2014 / Revised: 5 August 2014 / Accepted: 22 September 2014 / Published online: 10 October 2014
Ó The Oceanographic Society of Japan and Springer Japan 2014

Abstract Hydrographic data east of Japan from fiveresearch cruises and Argo profiling floats in spring 2003
have been analyzed to examine the relationship of theformation of Central Mode Water (CMW) and Transition
Region Mode Water (TRMW) in late winter 2003 tothermohaline fronts and mesoscale eddies. TRMW and thedenser variety of CMW (D-CMW) were formed continuously just south of the subarctic frontal zone between
155°E and 165°W with little relation to eddies, suggestingthat the absence of the permanent thermocline and halocline in this area is essential for the formation. The lighter
variety of CMW (L-CMW) was formed south of the Kuroshio bifurcation front and east of 165°E, partly in an
anticyclonic eddy associated with the Kuroshio Extension.
E. Oka (&)  D. Yanagimoto  S. Itoh  S. Katsura
Atmosphere and Ocean Research Institute, The University ofTokyo, Kashiwa 277-8564, Japane-mail: eoka@aori.u-tokyo.ac.jp
K. Uehara
Department of Marine Science, School of Marine Science andTechnology, Tokai University, Shizuoka 424-8610, Japan
T. NakanoGlobal Environment and Marine Department, Japan
Meteorological Agency, Tokyo 100-8122, Japan
T. Suga
Department of Geophysics, Graduate School of Science,
Tohoku University, Aoba-ku, Sendai 980-8578, Japan
T. Suga  S. Kouketsu
Research Institute for Global Change, Japan Agency for MarineEarth Science and Technology, Yokosuka 237-0061, Japan
L. D. Talley
Scripps Institution of Oceanography, University of California,
San Diego, La Jolla, CA 92093-0230, USA

Some portion of D-CMW and L-CMW likely had been
subducted to the permanent pycnocline by crossing
southward the Kuroshio bifurcation front and the Kuroshio
Extension front, respectively. In contrast, the formation ofthese waters in the western regions was inactive and wassignificantly different from that described previously using
multiyear Argo float data. West of 155°E, TRMW and
D-CMW were formed only in two anticyclonic eddies that
had been detached from the Kuroshio Extension 1–2 years
ago. L-CMW was hardly formed west of 165°E, whichmight be related to the upstream Kuroshio Extension being
in its stable state characterized by low regional eddy
activity.
Keywords Central Mode Water  Formation  Fronts 
Mesoscale eddies  Decadal variability

1 Introduction
Central Mode Water (CMW; Nakamura 1996; Suga et al.
1997) is a low-potential vorticity (Q) water formed as thedeep winter mixed layer north of the Kuroshio Extension. It
is subducted to the permanent pycnocline around thenorthern edge of the North Pacific subtropical gyre and isthen transported anticyclonically in the gyre. Formation,
subduction, and circulation of CMW and their temporalvariability are important not only physically for the threedimensional structure of the subtropical gyre (Yasuda and
Hanawa 1997), reemergence of winter sea surface temperature anomalies (Hanawa and Sugimoto 2004; Sugimoto and Hanawa 2005), and the maintenance of the
Subtropical Countercurrents as well as its impact on theatmospheric circulation (Kobashi and Kubokawa 2012),
but also biogeochemically for oceanic uptake of

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atmospheric CO2 and the subsequent CO2 transportation(Ono et al. 1998; Ishii et al. 2001; Midorikawa et al. 2006),
timing and magnitude of spring algal blooms (Shiozaki
et al. 2013), and circulation of dissolved organic matter(e.g., Yamashita and Tanoue 2009) and radioisotopes(Aoyama et al. 2008; Rossi et al. 2013).
Investigation of such CMW’s roles using observations
and/or numerical models requires precise knowledge on the
CMW formation and subduction, which has been pursued
during the past decade. Oka and Suga (2005) examined therepeat hydrographic section along 165°E to demonstrate
that, as suggested by an earlier observation (Mecking andWarner 2001) and an ocean general circulation model(Tsujino and Yasuda 2004), CMW has two varieties: thelighter variety (L-CMW) having h = 11-14° C,
S = 34.3-34.6, and rh = 25.8-26.2 kg m-3 (h and rh arepotential temperature and density; S is salinity) formed
between the Kuroshio Extension front (KEF) and Kuroshio
bifurcation front (KBF) and the denser variety (D-CMW)
with 8-10° C, 34.0-34.2, and 26.3-26.4 kg m-3 formed
between the KBF and the subarctic frontal zone (SAFZ).
Saito et al. (2007) analyzed a hydrographic survey conducted northeast of Japan in July 2002 to reveal the existence of a colder, fresher low-Q water with 5-7° C, 33.533.9, and 26.5-26.6 kg m-3 around 43°N, 160°E south of
SAFZ, and named it Transition Region Mode Water(TRMW). More recently, Oka et al. (2011a), using Argo
profiling float data during 2003-2008, presented that two
zonally elongated bands of deep winter mixed layer corresponding to the formation regions of L-CMW and
D-CMW/TRMW extend at 33°-39° and 39°-43°N,
respectively, from the east coast of Japan to 160°W. The
winter mixed layer depth in both bands decreases eastward
east of 170°E, implying large-scale eastward CMW subduction by the eastward mean flow between 170°E and160°W. This agrees with the CMW signature in the subsurface, that is, low-Q distribution on various isopycnals inthe subtropical gyre (Oka et al. 2011a).
In spite of such recent progress, the picture of the CMW
formation still needs to be updated. The first reason is an
unclarified relationship between the CMW formation andthermohaline fronts. While the CMW and TRMW
observed in the hydrographic sections were closely associated with the fronts (Oka and Suga 2005; Saito et al.
2007), the overall relationship between the CMW/TRMW
formation and the fronts is hard to obtain by statisticalanalyses of Argo float data (e.g., Oka et al. 2011a). As themost noticeable example, SAFZ is located at relatively low
latitudes in the area east of Japan (e.g., Yuan and Talley
1996; Yasuda 2003), thus crossing the western part of the
D-CMW/TRMW formation region detected by Argo float
observations (fig. 2 of Oka and Qiu 2012). The second
reason is the role of mesoscale eddies in the CMW

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formation. A recent composite analysis of Argo float andaltimetric sea surface height anomaly data indicated thatthick mode waters are preferentially formed in anticycloniceddies, particularly in a latitude band between 35° and
40°N corresponding mainly to the L-CMW formation
region (Kouketsu et al. 2012). This relationship needs to befurther investigated because (1) eddies cannot be distinguished from meanders of fronts in the sea surface heightanomaly distributions (Stabeno et al. 2009; Kouketsu et al.
2012), and (2) the eddy field in the CMW formation region
exhibits large year-to-year variability, particularly inassociation with the decadal variability of the Kuroshio
Extension (Qiu and Chen 2005, 2011; Qiu et al. 2007). We
need to clarify in which particular area the CMW formation
occurs (or does not occur) in eddies and how such a formation contributes to the large-scale CMW subduction andits temporal variation.
To examine the relationship of the CMW formation to
the fronts and eddies, synoptic observation data withenough zonal/meridional resolution over the CMW formation region are indispensable. Unfortunately, the Argo
profiling float data in a particular month alone are insufficient for such purpose, even today when we have more than
3,000 floats in the world oceans with an average float
density of 1 per 3° square (Roemmich et al. 2009; Freeland
et al. 2010). Alternatively, in this paper, we use shipboardhydrographic data that were intensively collected in the
CMW formation region in spring 2003, rather by chance,
during five research cruises. The data include those along
several meridional sections, which are suitable to reveal the
relationship between the CMW formation and the fronts.
By analyzing the shipboard hydrographic data supplemented by Argo float data, we examine the relationship of
the CMW formation in late winter 2003 to the fronts andmesoscale eddies.
The data are explained in Sect. 2. After determining thelocation of each front in Sect. 3 and detecting CMW and
TRMW in Sect. 4, we described the relationship of thesewaters to the fronts and eddies in Sect. 5. Discussion is inSect. 6, and summary is provided in Sect. 7.

2 DataWe used shipboard temperature and salinity data measured
by a conductivity-temperature-depth profiler (CTD) and an
expendable CTD (XCTD) from five research cruises inspring 2003 (Fig. 1a): the R/V Kofu-maru 0304 cruise, theR/V Seifu-maru 0304 cruise, and the R/V Ryofu-maru 0306
cruise by the Japan Meteorological Agency (2005), the R/V
Hakuho-maru KH-03-1 cruise by the Ocean ResearchInstitute, The University of Tokyo (Kawabe 2005; Komaki
and Kawabe 2009), and the R/V Shoyo-maru FY2003 1stSynoptic observation of central mode water
Fig. 1 a Distributions ofobservations conducted by
shipboard CTD (circles) and
XCTD (triangles) and Argo
floats (black dots) in spring
2003. Color of symbols denotes
the research cruises. b Date ofobservations shown in (a),plotted against longitude

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Seifu

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research cruise by the Fisheries Research Agency (Uehara
et al. 2004). In the latter two cruises, dissolved oxygen was
also measured by the sensor during the CTD casts. The fivecruises were conducted between April 22 and July 3,
mostly in May and June (Fig. 1b). To supplement the
shipboard observations, we added Argo profiling float data
in a 1-month period from May 15 through June 15, whichwere downloaded from the ftp site of the Argo Global Data
Assembly Center (ftp://usgodae.org/pub/outgoing/argo,
ftp://ftp.ifremer.fr/ifremer/argo) and edited as outlined in
Oka et al. (2007). Note that the Argo float density in 2003
was much lower than that in recent years.
We also used delayed-time sea surface height (SSH) and
SSH anomaly (SSHA) data merged from TOPEX/Poseidon, Jason, ERS-1/2, and Envisat altimeter observations,produced by Ssalto/Duacs and distributed by AVISO(Ducet et al. 2000). The weekly data, provided on a
0.25° 9 0.25° grid, were downloaded from the AVISO
web site (http://www.aviso.oceanobs.com).

3 Frontal structure
We first determined the structure of thermohaline fronts inspring 2003. The zonal section at 358N and meridional

sections between 147° and 167.5°E crossed a deep andstrong h and S front near 35°-36°N (Fig. 2). Since thisfront was associated with the 12° C isotherm at 300 dbar, it
was judged to be KEF described in Mizuno and White(1983) and Levine and White (1983). The location of KEF
in each section, which was defined as the horizontal hgradient maximum at 300 dbar, corresponded well to thezonal or meridional SSH gradient maximum (Fig. 3).
Accordingly, the KEF distribution on the map was determined so as to correspond to the large horizontal SSH
gradient between sections (Fig. 4).
All the meridional sections also exhibited a near-surface
density-compensating h and S front, characterized by theoutcrop of 33.2-33.8 isohalines constituting the subarctic
permanent halocline (Fig. 2). Based on this structure, thefront was judged to be SAFZ studied by Roden (1970,1972), Zhang and Hanawa (1993), and Yuan and Talley(1996). The location of SAFZ was determined first in eachsection as the horizontal S maximum at 50 dbar, and thenon the map (Fig. 4) using S distribution at that depth (notshown) constructed from our hydrographic observations, inorder that the front separates the subtropical water with
S [ 33.8 and the subarctic water with S \ 33.2.
The meridional sections between 155 and 175°E included another weaker h and S front between KEF and SAFZ

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a 35°N and the meridional sections at b 147°E, c 155°E, d 160°E,
e 165°E, f 167.5°E, g 175°E, and h 165°W. Gray shading denotes
regions of Q \ 1.0 9 10-10 m-1 s-1 (left) and S \ 33.8 (right).
Inverted triangles on the top of each panel indicated locations ofhydrographic stations. E, B, and S denote the central locations of
KEF, KBF, and SAFZ, respectively

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Fig. 3 SSH distributions along the hydrographic sections on the day
closest to the hydrographic observations: a 35°N on May 21, b 147°E
on May 7, c 155°E on May 28, d 160°E on June 4, e 165°E on July 2,
f 167.5°E on June 18, g 175°E on June 11, and h 165°W on June 11.
E, B, and S denote the central locations of KEF, KBF, and SAFZ,
respectively

Synoptic observation of central mode water

525

50˚N
25.0 25.6 25.8 26.0 26.2 26.4 26.6 26.7

45˚N

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Fig. 4 Distributions of core rh (kg m-3; color dots) of pycnostads inspring 2003. The observation points without a pycnostad are indicatedby black dots. Red, orange, and green curves denote KEF, KBF, andSAFZ, respectively. The background contours represent SSH averaged in May–June. The contours are drawn at an interval of 10 cm,
with darker shading for larger values(Fig. 2). This front was associated with 6-8° C isotherms
at 300 dbar, and was therefore judged to be KBF describedin Mizuno and White (1983) and Levine and White (1983).
The location of KBF in each section, which was defined as
the horizontal h gradient maximum at 300 dbar, also corresponded to the meridional SSH gradient maximum(Fig. 3). Accordingly, the KBF distribution on the map wasdetermined so as to be aligned with SSH contours (Fig. 4).
In spring 2003, KEF meandered eastward from 1408E,
where it left the southern coast of Japan, to a little beyond
180°. The upstream KEF was during its stable period (Qiu
and Chen 2005), showing two quasi-stationary meanders
with crests at 144° and 150°E, as demonstrated in previous
studies (e.g., Kawai 1972; Mizuno and White 1983). In
contrast, KEF east of 160°E was surrounded by mesoscaleeddies, particularly anticyclonic eddies to its north andcyclonic eddies to its south. This is consistent with Qiu andChen (2011), who clarified that the downstream KEF
fluctuates decadally in the opposite phase with the
upstream KEF, being unstable when the upstream KEF isin a stable state.
KBF extended between 155° and 175°E along thenorthern rim of the anticyclonic eddies. It was supposed tobe bifurcated from KEF somewhere between 150° and
155°E. SAFZ left the east coast of Japan at *36°N andmeandered northeastward. It crossed the 160°E section at
44.375°N and extended eastward farther east. In eachhydrographic section, it appears as a single, sharp h/S front(Fig. 2), as previously observed in the western North
Pacific (Yuan and Talley 1996).

4 Classification of pycnostads
As the mixed layer in the mode water formation region in
the North Pacific becomes deepest in February-April (Okaet al. 2007), we expect that the data analyzed in this study
were obtained within 3 months after the mode water formation in late winter 2003 and capture the structure of
newly formed mode waters relatively well. To extract such
newly formed mode waters from individual CTD/XCTD/
Argo profiles, we define a pycnostad in each profile as alayer with Q \ 1.0 9 10-10 m-1 s-1, a more severe criterion than previous mode water studies (e.g., Oka 2009; Oka
et al. 2011a; Saito et al. 2011). Here, Q is defined neglecting
relative vorticity (e.g., Talley 1988; Oka et al. 2011b) as
Q ¼ gf

orh
;
op

ð1Þ

where f is the Coriolis parameter, g the gravity acceleration, and p is pressure. After detecting pycnostads, we
exclude ones with thickness \50 dbar, bottom depth \100dbar, or core rh [ 26.7 kg m-3. The thickness and bottom
depth conditions are imposed to neglect small-scale features and spring mixed layers, respectively. Core propertiesare defined as properties at the Q minimum, which isconsidered to be the least modified portion since the pycnostad was formed, thus best preserving the water properties at the time of formation; the core rh condition is
imposed because rh = 26.6 kg m-3 is the densest mixed
layer density class in the open North Pacific (e.g., Talley
1993; Oka et al. 2011a).

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45˚N
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10
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0
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Fig. 5 The h–S relationship (dots) at the core of pycnostads in spring
2003. Red, yellow, green, and blue shading indicates properties of
STMW, L-CMW, D-CMW, and TRMW, respectively, based on Argo
float data in February–April of 2003–2008 (Oka et al. 2011a). Solidand dashed contours denote rh (kg m-3)

Our observations in spring 2003 captured pycnostads
with a wide range of core properties: rh = 25.026.7 kg m-3, h = 1-19° C, and S = 33.2-34.9 (Figs. 4,5). Relatively warm and light pycnostads south of KEF andwest of 170°E with core rh \ 25.6 kg m-3 were the Subtropical Mode Water (STMW; Masuzawa 1969; Fig. 2b),which was formed in late winter 2003 (Oka and Suga
2003). Most of them had nearly zonally uniform coreproperties of rh = 25.35-25.45 kg m-3, h = 16.517.0° C, and S = 34.7-34.8 (fig. 11 of Oka 2009; Fig. 5).
Pycnostads at 35°-36°N, 167°-169°E trapped in an
anticyclonic recirculation had core rh of 25.6825.69 kg m-3, core h of 15.0-15.1° C, and core S of
34.64-34.65, which were located in a gap between the
STMW and L-CMW properties on the h-S diagram.
Our observations also captured pycnostads colder than
5° C, that is, colder than TRMW (Fig. 5). Most of themwere located in the area east of Japan, particularly to thenorthwest of SAFZ (Fig. 6). These pycnostads were the
Oyashio Water, whose low-Q characteristics are attained in
the Okhotsk Sea (Ohtani 1989; Kono and Kawasaki 1997;
Yasuda 1997). In the Oyashio Water pycnostads including
those at 37°N, 142.5°E, 37°N, 143.5°E, and 41.5°N,
143.7°E whose core h is slightly higher than 5° C, h andS were stratified, as exemplified in Fig. 7a, indicating thatthe pycnostads were not formed in the local winter mixed
layer. On the onshore side of the Oyashio Water, relatively

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150˚E

Fig. 6 Distribution of core h (°C; color dots) of pycnostads,
constructed mostly from the Kofu-maru and Seifu-maru cruises in
April 22–May 25, 2003. The observation points without a pycnostad
are indicated by black dots. Red and green curves denote KEF and
SAFZ, respectively. The background contours represent SSH on May
7. The contours are drawn at an interval of 10 cm, with darker
shading for larger values. Dashed lines indicate CTD stations used forFig. 7

warm pycnostads with h of 6-8° C existed near the east
coast of Japan at 39°-41°N (Fig. 6). These pycnostads
having h and S similar to TRMW were the Tsugaru Warm
Water originating from the Japan Sea (Sugiura 1958; Ohtani 1971).
By excluding the above waters (STMW, Oyashio Water,
and Tsugaru Warm Water) and pycnostads whose coreproperties deviate from CMW/TRMW, a synoptic distribution of CMW/TRMW pycnostads in spring 2003 is
obtained (Fig. 8a). Compared to the former distribution(Fig. 4), pycnostads have disappeared in the area east ofJapan, except for those associated with either of two anticyclonic eddies centered at 40.5°N, 146°E and 38.5°N,
149°E (Fig. 6). In these pycnostads, h and S were almost
vertically uniform (Fig. 7b, c), in contrast with the Oyashio
Water (Fig. 7a), indicating that they originated in themixed layer in late winter 2003.

5 Distribution of CMW and TRMW in relation
to fronts and eddies
In spring 2003, CMW/TRMW pycnostads were distributed
between 33° and 44°N, from 145°E to 163°W (Fig. 8a), a
geographical range similar to that of the formation regionof these waters detected using the 2003-2008 late winterArgo float data (Fig. 4 of Oka et al. 2011a). However, thedetailed structure of mode waters is quite different between
the two datasets, particularly in the western part of theSynoptic observation of central mode water

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Q (10-10 m-1 s-1)

Fig. 7 Vertical profiles of (left) S, (middle) h, and (right) Q obtainedby the Kofu-maru at a four stations between 40.0° and 41.0°N along
144°E, b seven stations between 144.8° and 146.8°E along 40.25°N,

and c four stations between 37.7° and 38.7°N, 148.5° and 149.2°E, in
April 23–25 (b, c) and May 24 (a) of 2003. The locations of thesestations are indicated by dashed lines in Fig. 6

region. West of 165°E, a group of CMW/TRMW pycnostads were found in only three areas: two anticycloniceddies east of Japan (Sect. 4; Fig. 6) and an area south of
SAFZ between 155° and 163°E. All three areas containedmode waters thicker than 200 dbar, which were not seen
east of 165°E (Fig. 8b).
The southeastern eddy east of Japan contained pycnostads with uniform properties of rh = 26.39 kg m-3,

h = 9.6° C, and S = 34.20, which are characteristic of
D-CMW (Figs. 7c, 8a, 9). The core Q of these D-CMW
pycnostads was low (0.02-0.18 9 10-10 m-1 s-1),
implying that they were formed in late winter 2003, that is,a few months earlier than our observations. SSH data (notshown) indicate that the southeastern eddy was detachedfrom the Kuroshio Extension in October 2002 and spent
one winter before our observations in spring 2003. Past

123

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45˚N

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(a)

25.8 25.9 26.0 26.1 26.2 26.3 26.4 26.5 26.6 26.7

30˚N
140˚E

145˚E

150˚E

155˚E

160˚E

165˚E

170˚E

175˚E

180˚

175˚W

170˚W

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160˚W

45˚N

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50

30˚N
140˚E

(b)

75 100 150 200 250 300

145˚E

150˚E

155˚E

160˚E

165˚E

170˚E

175˚E

180˚

175˚W

170˚W

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160˚W

45˚N

40˚N

35˚N

0

30˚N
140˚E

(c)

100 150 200 250 300 400 500

145˚E

150˚E

155˚E

160˚E

165˚E

170˚E

175˚E

180˚

175˚W

170˚W

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160˚W

45˚N

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(d)

-0.1 0.1 0.2 0.3 0.4 0.5 0.6 0.7

30˚N
140˚E

123

145˚E

150˚E

155˚E

160˚E

165˚E

170˚E

175˚E

180˚

175˚W

170˚W

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Synoptic observation of central mode water529

b Fig. 8 Distributions of a core rh (kg m-3), b thickness (dbar), c top

depth (dbar), and d core AOU (ml l-1) of L-CMW/D-CMW/TRMW
pycnostads in spring 2003 (color dots). The observation points
without a pycnostad are indicated by black dots. Plots in (d) are madeonly for CTD stations in the Hakuho-maru and Shoyo-maru cruises atwhich dissolved oxygen data were available. Color curves denote thefronts, as in Fig. 4

35.0

25.0

15

34.5

25.5

10

σθ (kg m-3)

20

S

Fig. 9 Plots of a h, b S, andc rh against longitude (blackdots) at the core of L-CMW/DCMW/TRMW pycnostads.
Background dots in yellow,
green, and blue denote theproperties of L-CMW, D-CMW,
and TRMW, respectively, basedon Argo float data in February–
April of 2003–2008 (Oka et al.
2011a)

θ (oC)

shipboard observations demonstrated that, inside an anticyclonic eddy pinched off from the Kuroshio Extension,
STMW was modified to CMW or TRMW due to wintercooling and interaction with the ambient water (Tomosada
1986; Yasuda et al. 1992). The D-CMW held in thesoutheastern eddy might have been formed in a similar
way. A pycnostad at 37°N, 146.5°E (Figs. 6, 8a) had
exactly the same core properties as those in the southeastern eddy, which suggests that the pycnostad wasformed in the eddy and then ejected from it.
The northwestern eddy held uniform TRMW pycnostads
with rh = 26.57 kg m-3, h = 5.5° C, and S = 33.68(Figs. 7b, 8a, 9). The core Q of these TRMW pycnostads
was also low (0.00-0.14 9 10-10 m-1 s-1), indicatingthat they were just formed in late winter 2003. The
northwestern eddy lived for a long period from September
2001 to June 2005, drifting southeast of Hokkaido until
summer 2004 and then propagating northeastward along
the Kuril-Kamchatka trench, as described by Itoh andYasuda (2010a) and Itoh et al. (2011). It was therefore
supposed to have overwintered twice when observed inspring 2003, which probably made the contained mode
water even cooler and fresher than that held in the southeastern eddy. According to the previous analysis of SSHA
data (Itoh and Yasuda 2010b), anticyclonic eddies southeast of Hokkaido, including the northwestern one in ourstudy, tend to propagate northeastward along the KurilKamchatka trench, while those in the further offshore,
including the southeastern one in our study, have a weaktendency to propagate northeastward along SAFZ. Hence,
eddies such as the southeastern one might supply low Q to

the broad TRMW formation region south of SAFZ mentioned below.
TRMW pycnostads with rh = 26.49-26.61 kg m-3,
h = 5.7-7.3° C, and S = 33.71-33.90, which were similar to the properties of TRMW in the northwestern eddy,
spread south of SAFZ at 40°-44°N, 155°-163°E(Figs. 2c, d, 8a, 9). They had relatively low coreQ (0.44 9 10-10 m-1 s-1 on average), and their top isrelatively shallow at 80-130 dbar (Fig. 8c). Furthermore,
apparent oxygen utilization (AOU) at their core is mostly
lower than 0.1 ml l-1 (Fig. 8d), which is characteristic ofmode waters just after formation (Suga et al. 1989; Oka
and Suga 2005). Based on these properties, these TRMW
pycnostads were also judged to be formed in late winter2003. Their locations correspond to where TRMW was
discovered during the cruise in July 2002 (Saito et al. 2007)
and to the location of the deepest winter mixed layer in the
North Pacific appearing in the previous climatologies(Huang and Qiu 1994; Ladd and Thompson 2000; Sugaet al. 2004). The reason why this area south of SAFZ isfavorable for the TRMW formation is explained as follows.
First, this area lies in a zonal band of lateral minimum inthe vertical stability south of SAFZ, called the stability gap(Roden 1970, 1972; Yuan and Talley 1996), where neither
the permanent thermocline characterizing the subtropicalstratification nor the permanent halocline characterizing thesubarctic stratification exists (Ladd and Thompson 2000;
Suga et al. 2004; Fig. 2c). Secondly, a quasi-stationary jet
associated with SAFZ (Isoguchi et al. 2006) brings warmerand saltier water from the south to this area, enhancing theoceanic heat loss and making the surface water denser inwinter (Saito et al. 2007; Tomita et al. 2011). Actually,
TRMW observed in spring 2003 was thickest about
100 km southeast of SAFZ (Fig. 8b), at which maximum
oceanic heat loss in winter was observed (Tomita et al.
2011).
A TRMW pycnostad was also observed solely at 348N,
1628E within KEF (Figs. 8a, 10). It had core rh of

34.0

33.5

5

(a)

0
140 150 160 170 180 190 200

Longitude (oE)

26.0

26.5

(b)

33.0
140 150 160 170 180 190 200

Longitude (oE)

(c)

27.0
140 150 160 170 180 190 200

Longitude (oE)123Fig. 10 Vertical profiles of(left) S, (middle) h, and (right)
Q obtained on May 21, 2003 by
an Argo float at 33.7°N, 162.1°E

E. Oka et al.

Pressure (dbar)

530
0

0

0

100

100

100

200

200

200

300

300

300

400

400

400

500

500

500

600

600

600

700

700

700

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900
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900
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S

26.49 kg m-3, core h of 7.2 °C, and core S of 33.85,
similar to those of TRMW pycnostads in the above formation area, and was relatively thick (=100 dbar), but was
located more than 500 km south of the formation area andat depths of 480-580 dbar in the lower part of the main
thermocline. Its low core Q (0.08 9 10-10 m-1 s-1) suggests that it was formed in late winter 2003 and thensubducted isopycnally to these depths across KBF and
KEF. Such southward CMW/TRMW subduction across thefronts was pointed out by previous studies analyzing
meridional hydrographic sections (Mecking and Warner
2001; Oka and Suga 2005). It might be related to thefrontal instability (Spall 1995), and the subducted CMW asa form of subsurface mesoscale eddy can reach 1000 km
south of KEF within 1 year from the formation (Oka et al.
2009). It should also be mentioned that the pycnostad at
34°N, 162°E corresponded to a vertical S minimum(Fig. 10), although its core rh was lower than that of theNorth Pacific Intermediate Water (26.7-26.8 kg m-3;
Reid 1965; Talley 1993; Yasuda 1997). This might mean apossible contribution of the cross-frontal TRMW subduction to the NPIW formation around KEF (e.g., Yasuda et al.
1996).
East of 165°E, D-CMW and TRMW pycnostads werealigned zonally, with core rh decreasing eastward andabsence of TRMW east of 180° (Fig. 9), resembling thestructure of these waters based on the 2003-2008 late
winter Argo float data (Oka et al. 2011a). Just south of
SAFZ, TRMW/D-CMW pycnostads with core rh of 26.426.5 kg m-3 were located at 169°E-179°W, and D-CMW
pycnostads with 26.3-26.4 and 26.2-26.3 kg m-3 were at
178°-171°W and 168°-165°W, respectively (Fig. 8a).
Most of these pycnostads had a top depth of about 100 dbar
(Fig. 8c), a low core AOU of 0.02-0.06 ml l-1 (Fig. 8d),
and relatively low core Q (0.46 9 10-10 m-1 s-1 onaverage), and were judged to be formed in late winter 2003.
These newly-formed TRMW/D-CMW pycnostads south of
SAFZ, including those at 155°-163°E, were not particularly associated with anticyclonic eddies, as the observation
points of these pycnostads were not related to strong

123

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900
0

5

10

θ (oC)

15

20

0

1

2

3

4

5

Q (10-10 m-1 s-1)

maxima or minima of SSHA (Fig. 11). This suggests thatthe existence of stability gap, rather than eddies, is essential
for the formation of these waters south of SAFZ. The zonal
connection of these waters, particularly the influence ofbroad TRMW formation near 155°-163°E to the TRMW/
D-CMW formation in the eastern (downstream) areas is not
fully understood. Saito et al. (2011) analyzed Argo float
data to indicate that the core h and S of TRMW increase
after spring due to double-diffusive salt-finger convection,
which leads to the termination of TRMW at *180°.
Between 175° and 178°E, relatively thin TRMW pycnostads with core rh = 26.4-26.5 kg m-3 were foundnorth of SAFZ (Figs. 2g, 8a, b). They were shallow andhad a low core AOU of 0.05-0.14 ml l-1 (Fig. 8c, d), andwere considered to be locally formed in late winter 2003.
Similar pycnostads north of SAFZ were observed in therepeat hydrographic section at 165°E (Oka and Suga 2005),
although not observed at 165°E in spring 2003.
D-CMW pycnostads were also observed several degrees
south of SAFZ. Those with core rh = 26.3-26.4 kg m-3
located at 38°-40°N, 173°-175°E south of KBF had a low
core AOU of 0.13-0.22 ml l-1, but had a top depth of
200-210 dbar, which was larger than that (90-160 dbar) ofmajority of pycnostads (Figs. 2g, 8a, c, d). Since D-CMW is
expected to be formed between KBF and SAF (Tsujino and
Yasuda 2004; Oka and Suga 2005), these D-CMW pycnostads were believed to be formed north of KBF in late
winter 2003 and then quickly subducted to the permanent
pycnocline by crossing KBF southward. In the further
downstream, D-CMW pycnostads with core rh = 26.326.4 kg m-3 at 35°-37°N, 178°-175°W and those with
26.2-26.3 kg m-3 at 34°-36°N, 172-171°W were also
deep, having a top depth of 230-340 dbar. One of these
pycnostads at 37°N, 175°W had a core AOU of 0.64 ml l-1,
a typical value for mode waters formed more than a year
ago (Oka and Suga 2005), which suggests that some of these
pycnostads were formed in late winter 2002, unlike theother, newly formed pycnostads observed in spring 2003.
L-CMW is expected to be formed between KEF andKBF (Tsujino and Yasuda 2004; Oka and Suga 2005), butSynoptic observation of central mode water

531

45˚N

40˚N

35˚N

-40 -30 -20 -10 +10 +20 +30 +40

30˚N
140˚E

145˚E

150˚E

155˚E

160˚E

165˚E

170˚E

175˚E

180˚

175˚W

170˚W

165˚W

160˚W

Fig. 11 Distributions of SSHA (cm; color dots) at the observation
points of L-CMW/D-CMW/TRMW pycnostads in spring 2003. From
weekly SSHA data, those closest in time to each observation werechosen and interpolated horizontally to obtain the SSHA value at theobservation point. The background contours represent SSHA averaged in May–June. The contours are drawn at an interval of 10 cm,
with thick contours at 0 cm and darker shading for larger values. Red,
orange, and green curves denote KEF, KBF, and SAFZ, respectively

was hardly seen west of 165°E in spring 2003; the only
L-CMW pycnostad with core rh of 26.27 kg m-3, core h of
11.0° C, core S of 34.35, and relatively low core Q of
0.53 9 10-10 m-1 s-1 was observed at 38°N, 153°E(Figs. 8, 9). Between 165° and 175°E, L-CMW pycnostads, or pycnostads with properties between L-CMW and
D-CMW, exhibited a patchy distribution, compared to thedense hydrographic observations. Those with corerh = 26.2-26.4 kg m-3 at 38°-39°N, 165°-167°E andthose with core rh = 26.0-26.3 kg m-3 at 358-38°N,
1748-175°E, located between KEF and KBF, were relatively shallow with a top depth of 90-210 dbar, while
those with core rh = 25.9-26.2 kg m-3 at 34°-36°N,
169°-171°E in the recirculation south of KEF were deep
with a top depth of 260-380 dbar, implying that they were
subducted to the permanent pycnocline by crossing KEF
southward. East of 180°, L-CMW pycnostads were shallow, had lower core rh, and exhibited a more continuous
distribution. Among them, those with core rh of
25.82 kg m-3, core h of 14.1° C, core S of 34.55, and low
core AOU of 0.23 ml l-1 at 36°-37°N, 180°-179°W wereclosely associated with an anticyclonic recirculation of theKuroshio Extension, or an anticyclonic eddy that repeateddetachment from and absorption by the Kuroshio Extension(Fig. 11). These pycnostads might be also converted fromSTMW through the winter of 2002/03, like D-CMW pycnostads held in the southeastern eddy east of Japan.

area east of Japan except in two anticyclonic eddies andthat L-CMW hardly existed west of 165°E. The former factindicates that, in the region north of SAFZ occupied by thecold and fresh Oyashio Water, mode water is formed
mainly in anticyclonic eddies, which are detached from theKuroshio Extension and carry the subtropical water. In the
2003-2008 winter mixed layer climatology, the TRMW/
D-CMW formation region extends as far west as the east
coast of Japan (Oka et al. 2011a). The western part of thisformation region might be a composite of mode waters
contained in eddies.
The absence of L-CMW west of 165°E might berelated to the upstream KEF being in its stable state,
which is characterized by low regional eddy activity (Qiu
and Chen 2005). Since L-CMW, among the mode waters
in the western North Pacific, exhibits the strongest relationship between its formation and eddies (Kouketsu
et al. 2012), less frequent detachment of anticycloniceddies from the stable upstream KEF, within which
STMW can be modified to L-CMW (Tomosada 1986;
Yasuda et al. 1992; Sect. 5), possibly resulted in theabsence of L-CMW. Consistently, a recent analysis ofArgo float data demonstrated that the winter mixed layer
just north of the upstream KEF corresponding to the
L-CMW formation region was significantly shallower
during the stable KEF period of 2002-2005 and 20102011 than the unstable KEF period of 2006-2009 (Okaet al. 2012). This relationship was re-examined using
more recent data from the fully developed Argo float
network (Fig. 12). The winter mixed layer depth during
the stable KEF period of 2010-2013 was shallower than
2006-2009 at 36°-40°N, 145°-155°E corresponding to
the L-CMW formation region, although less noticeable
than the difference in the STMW formation region southof KEF where the winter mixed layer is deeper during the

6 Discussion
The L-CMW/D-CMW/TRMW distributions in spring 2003
(Fig. 8a) were greatly different from those in the Argobased 2003-2008 winter mixed layer climatology (fig. 4 ofOka et al. 2011a) in that D-CMW/TRMW was absent in the

123

532

E. Oka et al.
50˚N
45˚N
40˚N
35˚N
30˚N
25˚N
130˚E

140˚E

150˚E

160˚E

170˚E

Fig. 12 Difference of the averaged mixed layer depth in February
and March between 2010–2013 and 2006–2009 (former minus latter),
calculated using Argo float data as in Oka et al. (2012; Fig. 4). The
contour interval is 25 m, and negative values are lightly shaded.
Heavy shading denotes grid boxes without data. Mixed layer depth foreach Argo profile was calculated as the shallower value of the depth atwhich rh increases by 0.03 kg m-3 from 10-dbar depth and that at
which h changes by 0.2° C from 10-dbar depth, following thedefinition of de Boyer Montégut et al. (2004) and Oka et al. (2007).
Then, the mixed layer depth for each 4-year period was averaged foreach 1° 9 1° grid box using the Argo-based values from a 3° 9 3°
grid box centered by the 1° 9 1° grid box and weight function of d-2(d is a distance in degrees from the center of the 1° 9 1° grid box) for
observation points where d [ 1°stable KEF period, as demonstrated by Qiu and Chen
(2006) and Qiu et al. (2007).

7 SummaryTemperature, salinity, and dissolved oxygen data from fiveresearch cruises and Argo profiling floats in spring 2003
were analyzed to examine the relationship of the CMW and
TRMW formation in late winter 2003 to the thermohaline
fronts and mesoscale eddies. The CMW/TRMW formation
region east of Japan is delimited by three fronts. In spring
2003, KEF meandered at *35°N from 140°E to a little
beyond 180° (Fig. 4). KBF at *40°N was identified
between 155° and 175°E. SAFZ appearing as a single front
extended northeastward from 36°N, 141°E to 44°N, 160°E
and eastward farther east.
TRMW pycnostads with core rh = 26.5-26.6 kg m-3
were broadly formed just south of SAFZ between 155° and
163°E, as demonstrated by previous studies. TRMW and
D-CMW pycnostads were also formed just south of SAFZ
in the farther east up to 165°W, with core rh decreasing
eastward to 26.2 kg m-3 and absence of TRMW east of
180°. Such a TRMW/D-CMW formation was similar tothat delineated using 2003-2008 late winter Argo float
data (Oka et al. 2011a), and was not particularly associatedwith eddies, suggesting that the existence of stability gap isessential for the formation. D-CMW pycnostads were also
observed several degrees south of SAFZ, which were much123

deeper than that just south of SAFZ, existing at depths
more than 200 dbar. Most of these pycnostads were probably formed north of KBF in late winter 2003 and thenquickly subducted to the permanent pycnocline by crossing
KBF southward. This suggests that cross-frontal subduciontransports a significant portion of D-CMW to the permanent pycnocline.
West of 155°E, TRMW and D-CMW were formed onlyin two anticyclonic eddies east of Japan, which differedsignificantly from the Argo-based picture, in which theformation region of CMW/TRMW extended as far west as
the east coast of Japan (Oka et al. 2011a). TRMW withcore rh = 26.6 kg m-3 was formed in a northwestern eddythat had spent at least two winters in this area, and D-CMW
of 26.4 kg m-3 was formed in a southeastern eddy that was
detached from the Kuroshio Extension in the previous fall.
Within these eddies detached from the Kuroshio Extension,
the contained subtropical water might have been modified
to CMW and TRMW due to winter cooling and interaction
with the ambient water, as demonstrated by previous
studies.
L-CMW was hardly formed west of 1658E, which also
differed significantly from the Argo-based picture. Thismight be related to the upstream Kuroshio Extension being
in its stable state characterized by low regional eddy
activity. Newly-formed L-CMW exhibited a patchy distribution between KEF and KBF at 165°-175°E and amore continuous distribution east of 180°. Some L-CMW
pycnostads were subducted to the permanent pycnocline bycrossing KEF southward, and some were formed in an
anticyclonic eddy associated with the Kuroshio Extension.
The present analysis has demonstrated a clearer relationship of CMW/TRMW formation to the fronts andeddies than the previous studies. A re-analysis of accumulated Argo float data with SSH and SSHA data based on
the present results will lead to further understanding offormation and subduction of these waters and their temporal variability. Additional high-resolution hydrographic
sections (e.g., Oka et al. 2011b), particularly zonal ones,
are also desired to clarify the modification of these waters
within the formation region.
Acknowledgments The authors are grateful to the captain, crew,
and scientists participating in the R/V Kofu-maru 0304 cruise, theR/V Seifu-maru 0304 cruise, and the R/V Ryofu-maru 0306 cruise of
the Japan Meteorological Agency, the R/V Hakuho-maru KH-03-1
cruise of the Ocean Research Institute, The University of Tokyo, andthe R/V Shoyo-maru FY2003 1st research cruise of the Fisheries
Research Agency for their efforts in conducting the CTD and XCTD
measurements. They also thank Tsuyoshi Ohira for his assistance inpreparing the Argo float data and two anonymous reviewers for theirhelpful comments. The Argo float data used in this study were collected and made freely available by the International Argo Project andthe national programs that contribute to it (http://www.argo.ucsd.edu,
http://argo.jcommops.org). This study was initiated in 2005 when

Synoptic observation of central mode water
E.O. was sent from Institute of Observational Research for Global
Change, Japan Agency for Marine-Earth Science and Technology to
Scripps Institution of Oceanography (SIO) as a visiting scholar. He
thanks the late Nobuo Suginohara and Nobuyuki Shikama for giving
him an opportunity to visit SIO. This study is supported by the Japan
Society for Promotion of Science (KAKENHI, Grant-in-Aid for
Scientific Research (B), No. 21340133 and 25287118) and the Ministry of Education, Culture, Sports, Science and Technology, Japan(MEXT; Grant-in-Aid for Scientific Research on Innovative Areas
under Grant No. 22106007 and 25121502). Comments from participants at the ‘‘Research Meetings on Air-Sea Interaction’’ in 20112013 held as a part of the Collaborative Research Program of
Hydrospheric Atmospheric Research Center, Nagoya University andat the research meeting ‘‘North Pacific Ocean Circulation and theChanges’’ in 2013 supported by the Atmosphere and Ocean ResearchInstitute, The University of Tokyo and the Marine MeteorologicalSociety of Japan were helpful.

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