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VOLUME 41‘‘Eddy Resolving’’ Observation of the North Pacific Subtropical Mode Water
EITAROU OKA,* TOSHIO SUGA,1,# CHIHO SUKIGARA,1,@ KATSUYA TOYAMA,1
KEISHI SHIMADA,&,** AND JIRO YOSHIDA&* Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Japan
Department of Geophysics, Graduate School of Science, Tohoku University, Sendai, Japan#
Research Institute for Global Change, Japan Agency for Marine-Earth Science, and Technology, Yokosuka, Japan@
Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, Japan&
Department of Ocean Sciences, Tokyo University of Marine Science and Technology, Tokyo, Japan** Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan
1(Manuscript received 12 May 2010, in final form 27 December 2010)
ABSTRACT
Hydrographic data obtained by high-resolution shipboard observations and Argo profiling floats have been
analyzed to study the mesoscale structure and circulation of the North Pacific Subtropical Mode Water(STMW). The float data show that in the late winter of 2008, STMW having a temperature of approximately
18.88, 17.78, and 16.68C formed west of 1408E, at 1408–1508E, and east of 1508E, respectively, in the recirculation gyre south of the Kuroshio Extension. After spring, the newly formed STMW gradually shift southward, decreasing in thickness. Simultaneously, the STMWs of 16.68 and 17.78C are gradually stirred and thenmixed in terms of properties. In late fall, they seem to be integrated to form a single group of STMWs having
a temperature centered at 17.28C. Such STMW circulation in 2008 is much more turbulent than that in 2006,
which was investigated in a previous study. The difference between the two years is attributed to the morevariable state of the Kuroshio Extension in 2008, associated with stronger eddy activities in the STMW
formation region, which enhance the eddy transport of STMW.
High-resolution shipboard observations were carried out southeast of Japan at 1418–1478E in the early fallof 2008. To the south of the Kuroshio Extension, STMW exists as a sequence of patches with a horizontal scaleof 100–200 km, whose thick portions correspond well to the mesoscale deepening of the permanent pycnocline. The western (eastern) hydrographic sections are occupied mostly by the 17.78C (16.68C) STMW, within
which the 16.68C (17.78C) STMW exists locally, mostly at locations where both the permanent pycnocline
depth and the STMW thickness are maximum. This structure implies that the STMW patches are transported
away from their respective formation sites, corresponding to a shift in the mesoscale anticyclonic circulations
south of the Kuroshio Extension. Furthermore, 20%–30% of the observed STMW pycnostads have two or
three potential vorticity minima, mostly near temperatures of 16.68 and 17.78C. The authors presume that sucha structure formed as a result of the interleaving of the 16.68 and 17.78C STMWs after they are stirred bymesoscale circulations, following which they are vertically mixed to form the 17.28C STMW observed in latefall. These results indicate the importance of horizontal processes in destroying the vertically uniform
structure of STMW after spring, particularly when the Kuroshio Extension is in a variable state.

1. Introduction
In the North Pacific Ocean, a deep mixed layer isformed on both the southern and the northern sides of
the Kuroshio Extension in late winter (e.g., Ladd andThompson 2000; Suga et al. 2004; Oka et al. 2007, 2011),
and remains in the subsurface in the following seasons as

Corresponding author address: Eitarou Oka, Atmosphere andOcean Research Institute, The University of Tokyo, 5-1-5,
Kashiwanoha, Kashiwa-shi, Chiba 277-8564, Japan.
E-mail: eoka@aori.u-tokyo.ac.jp
DOI: 10.1175/2011JPO4501.1
Ó 2011 American Meteorological Society

two prominent isopycnal layers known as the SubtropicalMode Water (STMW; Masuzawa 1969) and the Central
Mode Water (CMW; Nakamura 1996; Suga et al. 1997),
respectively. In the following winter, part of the formedmode waters is entrained into the mixed layer, affecting
the persistence of winter sea surface temperature anomalies (e.g., Alexander and Deser 1995; Hanawa andSugimoto 2004; Sugimoto and Hanawa 2005), whereas
the remaining part is subducted into the permanent
pycnocline and then advected away from its formation
regions. In association with the formation and circulation, the anomalies of temperature, salinity, and potential

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vorticity are transmitted downward to the subsurface
layers as well as horizontally as the volume and propertiesof the mode waters formed every winter change fromyear to year (e.g., Suga and Hanawa 1995a; Yasuda and
Hanawa 1997; Hanawa and Kamada 2001; Suga et al.
2003). Therefore, the two mode waters are believed tocause the fluctuations in the permanent pycnocline structure of the North Pacific subtropical gyre and to have acertain influence on the climate and its variability in the
Pacific region, through the basin-scale transport of temperature anomalies (e.g., Latif and Barnett 1994; Gu andPhilander 1997) and the generation of the SubtropicalCountercurrent (Kubokawa and Inui 1999; Kubokawa
1999; Aoki et al. 2002; Kobashi et al. 2006, 2008). Moreover, they possibly affect the long-term climate variability
by regulating the oceanic uptake of atmospheric CO2 (e.g.,
Bates et al. 2002).
The recent completion of a global array of temperature/salinity profiling floats under the international Argo
program (Roemmich et al. 2001) has enhanced our understanding of the formation and circulation of STMW
and CMW and their interannual variability, compared to
the last century, during which the observational studies
on water masses relied mainly on climatological data
and limited repeat section data. The formation and subduction of CMW were investigated using the float data
in 2003–08 by Oka et al. (2011). To the north of theKuroshio Extension, two regions of deep winter mixed
layers extend zonally at 338–398N and 398–438N fromthe east coast of Japan to ;1608W, corresponding to
the formation regions of the lighter and denser varieties of CMW (Tsujino and Yasuda 2004; Oka and Suga
2005), respectively. From the eastern part of each formation region east of 1708E, where the winter mixed
layer gradually becomes shallow eastward/downstream,
the respective varieties of CMW are subducted into thepermanent pycnocline and are then advected anticyclonically in the subtropical gyre toward the western boundary.
A greater number of studies have been performed
using the float data on the formation and circulation of
STMW and their interannual variation (Uehara et al.
2003; Oka and Suga 2003; Pan and Liu 2005; Qiu andChen 2006; Qiu et al. 2006, 2007; Oka 2009; Sugimotoand Hanawa 2010). Among these, Oka (2009) examined the formation and circulation of STMW over theentire distribution region in 2006. Thick STMW is formedin late winter in the recirculation gyre south of the Kuroshio
Extension, which zonally extends north of 288N, between
1358E and 1808. The recirculation gyre consists of several
anticyclonic circulations, in each of which STMW witha characteristic temperature is formed. By tracking thesetemperatures, the STMW circulation after spring can bevisualized. In 2006, a large part of the newly formed, thick

667

STMW is continually trapped in each anticyclonic circulation until late fall, remaining in the formation region.
From this stagnant pool of STMW, some portion gradually seeps into the southern region and is then transported southwestward by subsurface currents, as far as
the western boundary. Such STMW circulation differsfrom the traditional climatology-based picture that the
thick body of STMW is advected southwestward as
the season progresses (e.g., Suga and Hanawa 1995b).
At present, however, the degree to which the observed
STMW circulation in 2006 applies to the other years is
not clear.
The last decade also witnessed significant progress inthe modeling of currents and water masses in the North
Pacific (e.g., Hasumi et al. 2010; Masumoto 2010). Recently,
the high-resolution ocean general circulation models ofRainville et al. (2007) and Nishikawa et al. (2010) demonstrated the highly variable distribution of STMW causedby high mesoscale activity in the Kuroshio Extension region (e.g., Qiu 2002). In addition, Nishikawa et al. (2010)
quantitatively examined the role of mesoscale eddies inthe transport and subduction of STMW. At the southernboundary of the STMW formation region, anticycloniceddies transport STMW southward, at a right angle to
the westward mean flow. This occurs in two ways: 1) the
southward migration of an eddy that traps STMW and2) eddy mixing (i.e., a combination of the southwardtransport of STMW from the formation region in theeastern part of an eddy and the northward transport ofwater with a relatively high potential vorticity from thesouthern region in the western part of the eddy). Because of this southward transport, STMW crosses thefront of the winter mixed layer depth (Kubokawa andInui 1999; Kubokawa 1999; Nishikawa and Kubokawa
2007) and is thereby subducted into the permanent pycnocline. Such eddy subduction contributes to approximately
half of the total STMW subduction rate (Nishikawa et al.
2010).
Although the data from the Argo floats also imply
mesoscale STMW distribution (Uehara et al. 2003; Pan
and Liu 2005; Qiu et al. 2006; Oka 2009), the standard
float density of one per three degree square is too low to
obtain a synoptic STMW distribution with a resolution
comparable to the high-resolution models. Therefore,
we conducted a high-resolution hydrographic survey inthe western North Pacific southeast of Japan in fall 2008
during the KH-08-3 cruise leg 2 of R/V Hakuho-maru.
Two meridional lines at 1438 and 1468E and three shorter
zonal lines at 278309, 308, and 348N were occupied (Fig. 1)
using a conductivity–temperature–depth–oxygen profiler(CTDO2) at intervals of 309 or 18 in latitude/longitude andan expendable conductivity-temperature-depth profiler(XCTD) or an expendable bathythermograph (XBT) at

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r at a depth of j dbar at ith station, ri,j, was replaced by
0.25 3 ri21,j 1 0.5 3 ri,j 1 0.25 3 ri11,j). Then, using theavailable u or y, potential vorticity (Q) was calculated on
the basis of the definition:
Q5 

f 1 ›y/›x ›su
,
r
›z

(1)

for the zonal sections andQ5 

FIG. 1. Locations of CTDO2 (closed circles), XCTD (opencircles), and XBT (triangles) stations of the KH-08-3 cruise leg 2from 22 Oct to 4 Nov 2008, and Argo float observations (crosses)
during the cruise period. The background contours represent thealtimetric sea surface height on 29 Oct 2008. The contour is drawn
at an interval of 10 cm, with a thick one at 220 cm and a shaded
region for .250 cm.

109 intervals, along with other physical/biogeochemical
instruments. By analyzing the data from CTDO2/XCTD/
XBT as well as those from the Argo floats, we investigate
in this paper the mesoscale structure and circulation of
STMWs that have been inadequately investigated inprevious observational studies.

2. Data and processing
We used temperature T, salinity S, and dissolved oxygen data obtained by CTDO2 (Sea-Bird Electronics,
Inc.; Model 9-plus; 38 stations), XCTD (Tsurumi Seiki
Co. Ltd.; 108 stations), and XBT (Tsurumi Seiki Co. Ltd.;T-7 type; 10 stations) from 22 October to 4 November
2008, during the KH-08-3 cruise leg 2 of R/V Hakuhomaru (Fig. 1). The S data from particular XCTD probes
had a systematic bias, and were calibrated as is explainedin the appendix. An S profile at each XBT station along
308N was estimated using the observed T profile at thesame station and the T–S relations at nearby CTD stations. From these observed and estimated data, potential
temperature (u), in situ water density (r), potential density (su), and apparent oxygen utilization (AOU) were
calculated.
Zonal and meridional geostrophic velocities (u, y; positive eastward and northward) across each hydrographic
section were computed using a reference level of 1000 dbar
or the maximum observation pressure near 800 dbar (onlyat XBT stations along 308N) after r was horizontallysmoothed along the section using a three-point Hanningfilter with weights of 0.25, 0.5, and 0.25 (specifically,f  ›u/›y ›su
,
r
›z

(2)

for the meridional sections, where f is the Coriolis parameter, x and y are the cross- and along-stream coordinates (positive eastward and northward, respectively),and z is the vertical coordinate (positive upward). Here,the horizontal velocity shear (›y/›x, ›u/›y) was estimatedbetween adjacent hydrographic station pairs by letting Dx(Dy) be 109 in longitude (latitude). The calculated magnitude of the relative vorticity relative to the planetary
one (j›y/›x/fj, j›u/›y/fj) attains a maximum of 35% in the
Kuroshio Extension; however, it is much smaller (generally, less than 10%) and negligible south of the Kuroshio
Extension where STMW exists.
We also used T and S data obtained by Argo floats in
the North Pacific in 2003–08. These floats drift freely ata predetermined parking pressure (typically, 1000 dbar),and conduct T–S measurements from an intermediate
depth (2000 dbar) to near the sea surface (4 dbar) ata predetermined interval (10 days). The data collected at
approximately 70–120 sampling pressures, with a typical interval of 5 dbar at depths shallower than 200 dbar,
10–25-dbar intervals at 200–1000 dbar, and 50–100-dbar
intervals at deeper than 1000 dbar, are transmitted fromthe surfaced floats to satellites, and they are freely available within 24 h, after passing through the Argo real-time
quality control (Argo Data Management Team 2009).
We downloaded the real-time quality-controlled float
data from the ftp site of the Argo Global Data AssemblyCenter (see online at ftp://usgodae1.fnmoc.navy.mil/pub/outgoing/argo, ftp://ftp.ifremer.fr/ifremer/argo), and eliminated the defective T–S profiles such as those with measurements flagged as bad and those lacking intermediate
layers for certain depths, according to the procedures ofOka et al. (2007). We then interpolated each T–S profile
vertically on a 1-dbar grid using the Akima spline (Akima
1970), and calculated u, su, and Q. In the Q calculation for
the Argo data, the relative vorticity was disregarded.
We also used delayed-time sea surface height data
merged from Ocean Topography Experiment (TOPEX)/
Poseidon, Jason-1, Jason-2, European Remote Sensing
Satellite-1/2 (ERS-1/2), and Envisat altimeter observations,

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produced by Ssalto/Duacs and distributed by Archiving,
Validation, and Interpretation of Satellite Oceanographic data (AVISO; Ducet et al. 2000). The weekly
data in 2008, provided on a 0.258 3 0.258 grid, were
downloaded from the Web site of AVISO (see online at
http://www.aviso.oceanobs.com).
In this study, STMW is defined in each CTDO2/
XCTD/XBT/Argo profile as the layer that has a Q
value less than the critical value, with a minimum Q in
the u range of 168–19.58C, following the definition ofOka (2009). The low-Q layers with thickness less than
25 dbar and those existing only at depths less than
100 dbar are regarded as small-scale features and are
excluded. STMW is considered to outcrop (not to outcrop) at the sea surface if its top is located at a depth
less than (greater than) 10 dbar. Accordingly, such STMW
is hereafter referred to as either outcropping or nonoutcropping STMW.
Within the low-Q layer of STMW, the core, defined by
the Q minimum, is considered to be the least modified
since the low-Q portion was formed, thereby best preserving the water properties at the time of formation. In
the following part of the paper, a water property at thecore (e.g., u) is simply referred to as ‘‘core u.’’3. Large-scale STMW circulation in 2008
Before analyzing the one-time regional data from theKH-08-3 cruise leg 2, in this section, we use the Argo
float data from 2008 to examine the large-scale STMW
circulation over the entire distribution region, in comparison with that from 2006 presented in Oka (2009).
For several years prior to 2008, the STMW thickness inthe formation region decreases significantly, likely due
to its decadal variability (Qiu and Chen 2006). If wedefine STMW using the same critical Q value of 1.5 310210 m21 s21 as in Oka (2009), the thickness in May
and June, averaged in three longitudinal ranges (1408–
1508, 1508–1608E, and 1608–1708E) in the STMW formation region between the Kuroshio Extension and
288N (following Fig. 12 in Oka 2009), decreases steadily
from 2004 (167–192 dbar) to 2008 (83–131 dbar) by;70 dbar. A similar decreasing trend during 2004–08
was presented by Sugimoto and Hanawa (2010) for the
winter and summer STMW thicknesses in the northernpart of the formation region north of 318N. Because ofthis reduction, the STMW formed in the late winter of2008 is so thin that if we define it using the critical Q
value of 1.5 3 10210 m21 s21, only a small amount of it
remains in the fall, when the KH-08-3 cruise leg 2 wasconducted (not shown). Consequently, in the present
study, we define the STMW in 2008 using a less severe
critical value of 2 3 10210 m21 s21.

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In the late winter of 2008, thick STMW is formed
between the Kuroshio Extension and 288N (Fig. 2a),a region similar to that seen in 2006 (Fig. 2b) and thepreceding years (Oka 2009). Nonoutcropping STMW
south of 288N, presumably formed in previous winters, ismostly thinner than 150 dbar, forming a front of STMW
thickness at the southern boundary of the formation
region at ;288N (Fig. 2a). Exceptionally, relatively thick
STMW, thicker than 200 dbar, is found at 258–288N in
April. This STMW is likely to be formed in the late
winter of 2008 and then quickly transported southward.
After spring, the thick STMW gradually shifts southward
while gradually losing its thickness, and is located at 248–
338N in September–October. In November–December, it
almost flattens between 218 and 338N, except in the southof Japan where STMW thicker than 400 dbar remains ina strong anticyclonic recirculation (Figs. 2a,c). Such atemporal change in the STMW structure differs from thatduring 2003–06, when the newly formed, thick STMW does
not shift southward, staying in the formation region untillate fall (Figs. 2b,c; Oka 2009). It rather resembles theseasonal evolution of STMW in the climatological field(Suga and Hanawa 1995b).
The histogram of core u for relatively thick STMW in
May–June indicates that the STMW formed in the late
winter of 2008 can be classified into three modes (Fig. 3a,
top panel), in a similar manner to 2006 (Fig. 3b, top panel;Oka 2009) but much more clearly, thanks to the wider
range of core u. The three modes in 2008, namely, a warm
mode with a core u of 18.28–19.18C, a middle mode of
17.18–18.28C, and a cold mode of 16.48–17.18C, are mostly
located west of 1408E, between 1408 and 1508E, and east of
1508E in May–June, and they are probably formed in theirrespective regions in late winter (Fig. 4). The core u of thethree modes is still distinguishable in July–August, but theboundary between the two colder modes begins to blur in
September–October, and the two modes seem to become
integrated, forming a new mode in November–December(Fig. 3a). This differs from 2006, in which the core u of thethree modes changes little until late fall (Fig. 3b).
The STMW circulation after spring is also different
between 2006 and 2008. In 2008, the middle and coldmodes are gradually stirred as the season progresses,
and they finally coexist with each other in November–
December (Fig. 4). This suggests that the new mode is
produced in late fall owing to the mixing of the middle andcold modes. In 2006, on the other hand, the three modes
tend to be continually trapped in the respective anticyclonic circulations in the formation region, existing
separately until late fall (Fig. 7 of Oka 2009). Note thatthe difference in the STMW circulation between the twoyears is not due to that in the critical Q value used for the
STMW definition; the STMW circulation in 2006 (2008)

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FIG. 2. Plots of STMW thickness against latitude from January–February through November–December in (a)2008 and (b) 2006 (adapted from Oka 2009), based on Argo float data. The open (closed) circles denote outcropping(nonoutcropping) STMW. (c) Meridional variation of STMW thickness from January–February through November–
December in 2008 (thick curve) and 2006 (thin curve) averaged for each 18 bin of latitude.

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FIG. 3. Number of profiles with nonoutcropping STMW thicker than 100 dbar in each 0.18 binof core u from May–June through November–December in (a) 2008 and (b) 2006 (adaptedfrom Oka 2009), based on Argo float data.

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FIG. 4. (left) Distribution of nonoutcropping STMW with a core u of 16.48–17.18C (blue symbols), 17.18–18.28C(green symbols), and 18.28–19.18C (red symbols) and geopotential anomaly at 250 dbar relative to 950 dbar (m2 s22;
contours) from May–June through November–December in 2008, based on Argo float data. The symbol type denotes
STMW thickness: greater than 100 dbar (circles), 50–100 dbar (triangles), and less than 50 dbar (squares). The black
dots denote float observation points with STMW having the other core u, those with outcropping STMW, and those
without STMW. The geopotential anomaly at 250 dbar (average depth of the STMW core from spring through fall)
was calculated from the float data using an optimal interpolation technique (Bretherton et al. 1976; see Oka 2009, formore details), and its contour is drawn at an interval of 0.5 m2 s22, with thick ones at 10, 12, and 14 m2 s22. The thickblack lines denote the observation lines of the KH-08-3 cruise leg 2. (right) Number of profiles with nonoutcropping
STMW thicker than 100 dbar located north of 258N with a core u of 16.48–17.18C (blue curve), 17.18–18.28C (green
curve), and 18.28–19.18C (red curve) from May–June through November–December in 2008, counted for each 58 binof longitude.

does not change essentially, even if STMW is definedusing the critical value of 2.0 (1.5) 3 10210 m21 s21.
The observed difference in the STMW circulation
between 2006 and 2008 is expected to be due to that in
the flow field south of the Kuroshio Extension. On a
decadal time scale, the Kuroshio Extension is alternately in stable and variable states with weak (or strong)
eddy activity in the surrounding region during the former (or latter) period (Qiu and Chen 2005). It has beenin the variable state since 2006, and exhibits the mostvariable path in 2008 (Figs. 5 and 6). The associated eddy
activity in the STMW formation region, stronger in 2008
than in 2006, is believed to have caused the more turbulent STMW circulation during 2008.

4. Mesoscale structure of STMW southeast ofJapan in fall 2008
In 2008, the Kuroshio Extension, in a highly variablestate, meanders and fluctuates, occasionally detaching

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673

FIG. 5. Yearly paths of the Kuroshio and the Kuroshio Extension plotted every 14 days, as in Qiu and Chen (2005). (Figure courtesy of
Bo Qiu, University of Hawaii at Manoa.)or absorbing mesoscale eddies (Fig. 7). In association
with this fluctuation, anticyclonic circulations constituting the southern recirculation gyre shift, repeatedly
merging and separating, as described in Oka (2009). In
late October to early November, the hydrographic linesof the KH-08-3 cruise leg 2 cross the Kuroshio Extension
at five locations: near the western end of the 348N line,
358N of the 1438E line, and 338309, 348, and 368309N of
the 1468E line (Fig. 1). The southern portion of the hydrographic lines encompasses a cyclonic eddy centered at
298209N, 1448309E. This eddy is centered at 298N, 1548E
in April, and subsequently moves westward (Fig. 7).
The hydrographic sections of the cruise cross thestrong flow of the Kuroshio Extension at five locations
similar to those on the altimetric sea surface height map(Fig. 8). The isothermal, isopycnal, and low-Q (,2 310210 m21 s21) structure of STMW is seen at depthsof 100–500 dbar south of the Kuroshio Extension (eastof 1418309E in the 348E section, south of 348309N in the1438E section, south of 328309N in the 1468E section, andthroughout the 278309 and 308N sections), as well as in
the isolated anticyclonic recirculation near 358309N in

the 1468E section. The 278309N section also displays arelatively small but intense low-Q structure near 500 dbar,
at 1458E. This is a patch of CMW trapped in a subsurface
anticyclonic eddy, whose formation and migration werestudied by Oka et al. (2009).
The permanent pycnocline south of the Kuroshio
Extension exhibits undulations with a horizontal scale of
100–200 km, in addition to upward lifts at 288509N in the

FIG. 6. Time series of the Kuroshio Extension pathlength integrated from 1418 to 1538E, adapted from Qiu and Chen (2010). A
three-month box filter was applied to the original, weekly data.
Thick lines indicate the periods when the Kuroshio Extension is ina variable state.

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FIG. 7. Altimetric sea surface height maps southeast of Japan
from 30 Apr through 29 Oct 2008, at a time interval of approximately 1 month, drawn as in Fig. 1. Thick black lines indicate theobservation lines of the KH-08-3 cruise leg 2.

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1438E section, at 308109N in the 1468E section, and at
1458E in the 308N section associated with the cycloniceddy (Fig. 9a). Specifically, the permanent pycnocline
deepens at 1428409E in the 348N section; at 338309,
328109, 308309, and 298309N in the 1438E section; at
1438309 and 1468E in the 278309N section; and at 328209N
in the 1468E section, and becomes shallow in between.
These mesoscale structures, particularly those in the1438E section, are not fully resolved in the altimetric sea
surface height map constructed using temporal and spatial
interpolation (Fig. 1).
STMW also has mesoscale structure, existing as a sequence of patches with a horizontal scale of 100–200 km(Fig. 9c). Such mesoscale variation of STMW thickness
is mainly determined by the variation of STMW bottom
depth (Fig. 9b; correlation coefficient of 0.78), which inturn corresponds well to that of the permanent pycnocline depth (Fig. 9a; i.e., 0.83). In particular, the thickest
portion of each STMW patch corresponds well to where
the permanent pycnocline deepens (Fig. 9a). This strengthens
the conclusions of the previous observational and modeling studies that thicker (thinner) STMW is preferentiallyformed in winter and distributed after spring in anticyclonic (cyclonic) mesoscale circulations/eddies where thepermanent pycnocline is deeper (shallower), owing toweaker (stronger) background stratification and larger(smaller) oceanic heat loss in winter (Suga and Hanawa
1990; Uehara et al. 2003; Oka and Suga 2003; Pan and Liu
2005; Qiu et al. 2006; Rainville et al. 2007; Nishikawa et al.
2010; S. Kouketsu et al. 2011, manuscript submitted to J.
Oceanogr.). The STMW top depth tends to increase
northward (Fig. 9b), possibly because the upper portionof STMW at higher latitudes has been further eliminatedowing to the earlier onset of cooling and the resultant
deepening of the seasonal pycnocline (e.g., Qiu et al.
2006; Oka et al. 2011). This suggests that in order to examine the relation between the STMW structure and themesoscale circulations, hydrographic observations along
zonal lines are more preferable.
The core u exhibits an apparent bimodal distribution(Figs. 9d and 10a), which coincides with the middle andcold modes observed by the Argo floats in spring (Fig. 3a,
top panel). The western (eastern) hydrographic sections
along 348N, 1438E, and 278309N (1468E and 308N) are
occupied mainly by the middle (cold) mode, within which
the cold (middle) mode exists with a relatively small scale(Fig. 9). Such intrusions of the different modes tend tooccur at locations where the permanent pycnocline depth,
the STMW bottom depth, and the STMW thickness reach
their local maxima. This suggests that as the anticyclonic
circulations south of the Kuroshio Extension graduallyshift, repeatedly merging and separating (Fig. 7), theytransport STMW zonally from the formation region toAPRIL 2011

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FIG. 8. Distributions of (a) geostrophic velocity (cm s21), (b) u (8C), (c) S, (d) su (kg m23),and (e) Q (10210 m21 s21) in the hydrographic sections of the KH-08-3 cruise leg 2. Northward(eastward) velocity is exhibited for the zonal (meridional) sections, and ‘‘N,’’ ‘‘S,’’ ‘‘E,’’ and‘‘W’’ denote northward, southward, eastward, and westward flow, respectively, in (a). Contours
are drawn at 0.5, 1, 1.5, 2, 3, 4, and 5 3 10210 m21 s21 in (e). Long (short) ticks on the top ofeach indicate the locations of CTDO2 (XCTD/XBT) stations.

a remote region. The most visible examples are two thick
STMW patches observed near 348N, 1428309E and 278309N,
1438109E (Figs. 8 and 9), which are associated with
anticyclonic circulations centered at 338N, 1428309E and
288N, 1428E, respectively (Fig. 1). Both patches are thecold mode embedded in the middle mode (Fig. 9), and arethought to be formed east of 1508E in late winter andsubsequently transported westward (Fig. 4). On the basisof the monthly sea surface height maps (Fig. 7), we presume the following STMW circulation. The cold-mode
STMW associated with these two patches is originallyformed near 1508E, just south of the Kuroshio Extension
in the late winter of 2008, and is trapped in an anticyclonic
circulation centered at 318N, 1498E in April. This circulation is broken into two circulations centered at 318N,
1498E and 288N, 1488E in June. Then, the northern circulation propagates westward and is absorbed by anotheranticyclonic circulation centered at 338309N, 1438309E in
September. The other one also migrates westward, to288N, 1428E, in October. In association with these shifts,
the cold-mode STMW is transported westward. Similartransportation by anticyclonic circulations is expected to676

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FIG. 9. Plots of (a) depth of the su 5 26 kg m23 isopycnal representing the permanent pycnocline and (b) top (open circles) and bottom (closed circles) depths, (c) thickness, (d) core u, and(e) core AOU of STMW against longitude or latitude at the hydrographic stations of the KH-08-3
cruise leg 2. Long (short) ticks on the top of each panel indicate the locations of CTDO2 (XCTD/
XBT) stations. Note that the depth increases downward along the ordinate in (a) and (b).

occur in other locations in the STMW formation region as
well, stirring STMWs originated from different regions,as shown in Fig. 4.
The bimodal distribution of core u is also observed by
nearby Argo floats during the period of KH-08-3 cruiseleg 2 (Fig. 10b), but not after that. In November–December
the two modes are integrated to form one mode (Fig. 10c),
as mentioned earlier, based on the float data from the entire
STMW distribution region (Fig. 3a, bottom panel).
The core AOU is measured at the CTDO2 stations atintervals of 309 or 18 in latitude/longitude (Fig. 9e). It is
negatively correlated with the STMW thickness, witha coefficient of 20.56 (Fig. 11), which implies that thecore of a thicker STMW pycnostad associated with the
deeper permanent pycnocline did not tend to dissipate
since its formation. The lowest core AOU (0.44 ml l21)
is associated with the thickest STMW, which is expected
to have been the least dissipated. If we assume that thisSTMW pycnostad loses contact with the atmosphere at
the end of March having an AOU of 0.1–0.2 ml l21(Suga and Hanawa 1990), the AOU increase of approximately 0.3 ml l21 over 7 months yields an increase

rate of 0.5 ml l21 yr21 owing almost purely to the oxygen consumption through the remineralization of organic
matter. This value coincides with the rates obtained by
Suga et al. (1989) and Oka and Suga (2005) for STMW in
repeat hydrographic sections.
Our discussion so far assumes only one core (Q minimum) for each STMW pycnostad in a CTDO2/XCTD/
XBT/Argo profile. However, careful inspection reveals
that some profiles have two or even three Q minima inthe u range of STMW (Fig. 12), as previously observed
by Taneda et al. (2000). If we redefine the core as the
Q minimum being less than 2.0 3 10210 m21 s21 andhaving a magnitude of 0.3 3 10210 m21 s21 in the urange of 168–19.58C, permitting the existence of multiple
cores, 24 STMW pycnostads out of 107 (22%) observedin the KH-08-3 cruise leg 2 have a second core, and 2
also have a third core. Similarly, 9 pycnostads out of
27 (33%) observed by the nearby Argo floats during
the cruise period have a second core. These double or
triple cores, when observed in the CTDO2 profiles, areoften accompanied by corresponding AOU minima(Fig. 12). Such a structure cannot be formed by

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677

FIG. 10. Number of profiles with nonoutcropping STMW in each 0.18 bin of core u based on (a) hydrographic data of the KH-08-3 cruise leg
2 and Argo float data at 258–408N, 1408–1508E on (b) 22 Oct–4 Nov 2008 and (c) 5 Nov–31 Dec 2008.

vertical processes (e.g., vertical mixing, the verticalshift of isopycnals due to internal waves) alone. Instead, it is likely to be formed owing to the interleaving of STMW pycnostads with different densities
originating in different regions.
Actually, 17 out of 24 (71%) and 7 out of 9 (78%)
pycnostads with double/triple cores based on the cruiseand Argo data, respectively, have the first core of either
the cold or middle mode and the second core of theother mode (Table 1). This indicates that after the
middle and cold modes are stirred by mesoscale circulations (Fig. 4), they interleave with each other. Eventually, the two interleaving modes are expected to bevertically mixed with each other. This is a plausible explanation for the emergence of STMW with the intermediate core u between the cold and middle modes in
November–December (Figs. 3a and 10c).

gradually mixed in terms of their properties. In November–
December, they seem to be integrated to form a new mode
having a core u of approximately 17.28C.
The observed STMW circulation in 2008 is much
more turbulent than that in 2006 examined by Oka(2009). In 2006, STMW is formed in several anticyclonic
circulations in the southern recirculation in late winter,and tends to be continually trapped in the respective
circulations, remaining in the formation region until latefall. Such a difference between 2006 and 2008 is attributed to the Kuroshio Extension having a more variable
path in 2008, which is associated with stronger eddy
activities in the STMW formation region, which would
enhance eddy transport of STMW during that year.

5. Summary and discussion
Hydrographic data obtained by high-resolution shipboard observations and Argo floats have been analyzed
to study the mesoscale structure and circulation of
STMW. First, the STMW circulation over the entire distribution region in 2008 was examined by using the float
data. In the late winter of 2008, STMW is formed in thezonally elongated anticyclonic recirculation between the
Kuroshio Extension and 288N, a region similar to the preceding winters. The core u of the formed STMW clearly
differs among regions; warm, middle, and cold modes of
STMW with a core u of approximately 18.88, 17.78, and16.68C are formed west of 1408E, between 1408 and 1508E,
and east of 1508E, respectively. After spring, the newly
formed, thick STMW gradually shifts southward, decreasing in thickness. Simultaneously, the cold and middle modes of the STMW are gradually stirred, and finallycoexist with each other in late fall. The two modes are also

FIG. 11. Plots of core AOU against thickness for STMW at thehydrographic stations of the KH-08-3 cruise leg 2.

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FIG. 12. Vertical profile of Q (thick curve) and AOU (thin curve) with respect to u obtained at (a) 298N, 1468E; (b) 298309N, 1438E; and(c) 348N, 1428309E during the KH-08-3 cruise leg 2. These are examples of single, double, and triple cores, respectively.

High-resolution shipboard observations were carriedout in late October through early November 2008, along
five zonal/meridional lines in the region of 278309–378N,
1418–1478E, southeast of Japan. In the hydrographic
sections, STMW exists south of the Kuroshio Extension
as a sequence of patches with a horizontal scale of 100–
200 km, and its thick portions determined by the bottom
depth correspond well to the mesoscale deepening of thepermanent pycnocline. The core u of STMW in these
sections is classified into two groups that coincide withthe cold and middle modes observed by the Argo floats
in spring. The western (eastern) part of the hydrographic
sections are occupied mostly by the middle (cold) mode,
within which the cold (middle) mode with a small horizontal scale exists, mostly at locations where the permanent pycnocline deepens and the STMW thickness
attains a local maximum. Such a structure implies thatas the mesoscale anticyclonic circulations south of theKuroshio Extension gradually shift, repeatedly merging
and separating, they transport each STMW patch awayfrom its formation region, thereby stirring STMW patches
that originated in different regions.
Further inspection of the hydrographic data from thecruise and those from the nearby Argo floats during thecruise period reveals that 20%–30% of the observed
STMW pycnostads have 2 or 3 cores (Q minima), andmost of the double cores have the core u of the middle
and cold modes. Such a structure is probably formed
owing to the interleaving of the two modes after they arestirred by mesoscale circulations. We presume that the
double cores are then vertically mixed to form the single

core observed in November–December that has an intermediate core u between the two modes.
The present study has verified that mesoscale transport of STMW occurs as previously demonstrated by thehigh-resolution ocean general circulation models (Rainvilleet al. 2007; Nishikawa et al. 2010) and that this horizontallymixes STMW originating from different regions when the
Kuroshio Extension is in its highly variable state. It has
been further suggested that such horizontal processes
play an important role in destroying the vertically uniform structure of STMW after spring. The interleaving
TABLE 1. Number of hydrographic observations of the KH-08-3
cruise leg 2 and Argo float data at 2582408N, 140821508E from 22
Oct to 4 Nov 2008, classified by u of the first and second cores of
STMW pycnostads. 16.48217.18C and 17.18218.28C are the core uof the cold and middle modes, respectively. Core u higher than
18.28C were not observed in either data.
KH-08-3 cruise leg 2 data
Second coreFirst core

16.08216.48C

16.48217.18C

17.18218.28C

16.08216.48C
16.48217.18C
17.18218.28C

0
4
0

0
0
8

1
9
2

Argo float data
Second coreFirst core

16.08216.48C

16.48217.18C

17.18218.28C

16.08216.48C
16.48217.18C
17.18218.28C

0
0
2

0
0
4

0
3
0

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OKA ET AL.

of STMW with different densities might occur in association with the dissipation of mesoscale anticyclonic
circulations or eddies that have carried STMW away fromits formation region. Submesoscale-resolving models are
expected to be helpful in clarifying such smaller-scale
processes.
The results from Oka (2009) and the present study
suggest that not only the formation (Qiu and Chen 2006;
Qiu et al. 2007) but also the circulation and dissipationof STMW are closely related to the decadal variability of
the Kuroshio Extension system (Qiu and Chen 2005;
Qiu et al. 2007). Since the Kuroshio Extension has returned to its stable state in 2010 (Qiu and Chen 2011),
we expect to observe an STMW evolution distinct fromthat of 2008 in a few years by using the Argo float datathat are much richer than the data available for the lastperiod of stability (2002–04).

systematic negative bias of 20.05 to 20.1 comparedto the data from CTDO2, as previously reported by
Kashino et al. (2009). On the other hand, those from theprobes manufactured in 2008 did not appear to have abias. Therefore, we added the following values as a function of u to the S values from the problematic XCTD
probes:Acknowledgments. The authors are grateful to thecaptain, crew, and scientists participating in the KH-08-3
cruise leg 2 of R/V Hakuho-maru of the Japan Agency
for Marine-Earth Science and Technology for their efforts in conducting the CTDO2/XCTD/XBT measurements. They also thank Tsuyoshi Ohira for his assistancein preparing the Argo float data; Bo Qiu for providing
the figures and data on the Kuroshio Extension variability; and Keita Iga, Shin-ichi Ito, and Bo Qiu for theirinsightful comments. Detailed comments from two anonymous reviewers enabled us to improve an early versionof the manuscript. This work was partly supported by theJapan Society for Promotion of Science [KAKENHI,
Grant-in-Aid for Scientific Research (B) 21340133], theMinistry of Education, Culture, Sports, Science andTechnology, Japan [MEXT; Grant-in-Aid for Scientific
Research in Priority Areas: ‘‘Western Pacific Air-Sea
Interaction Study (W-PASS)’’ under Grant 21014004], theAgriculture, Forestry and Fisheries Research Council,
Japan [AFFRC; ‘‘Studies on Prediction and Applicationof Fish Species Alteration (SUPRFISH)’’], and the Cooperative Program (142 in 2009) provided by the Ocean
Research Institute, The University of Tokyo. The Argo
float data used in this study were collected and madefreely available by the International Argo Project and thenational programs that contribute to it (see online at
http://www.argo.ucsd.edu and http://argo.jcommops.org).

This procedure is believed to improve the S accuracy of
XCTD measurements to ;0.03, which is comparable tothe expected S accuracy of Argo profiling float measurements (less than 0.02; Oka and Ando 2004; Oka 2005).

APPENDIXXCTD Data Calibration
The S data of the KH-08-3 cruise leg 2 obtained by
XCTD probes manufactured in 2006 and 2007 had a

u2
,
6
u8
,
5 0.05 1 0.01 310
u  18
5 0.06 1 0.02 3,3
5 0.08,

DS 5 0.01 1 0.04 3

at 28 # u , 88C;
at 88 # u , 188C;
at

188 # u , 218C;

andat 218C # u(A1)

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