Journal of Oceanography, Vol. 65, pp. 151 to 164, 2009

Seasonal and Interannual Variation of North Pacific Subtropical Mode Water in 2003–2006
EITAROU OKA*
Ocean Research Institute, The University of Tokyo, Nakano-ku, Tokyo 164-8639, Japan(Received 16 October 2007; in revised form 1 May 2008; accepted 8 September 2008)

Temperature and salinity data from 2003 through 2006 from Argo profiling floats
have been analyzed to examine the formation and circulation of the North Pacific
Subtropical Mode Water (STMW) and the interannual variation of its properties overthe entire distribution region. STMW is formed in late winter in the zonally-elongated recirculation gyre south of the Kuroshio and its extension, which extends northof ~28°N, from 135°E to near the date line. The recirculation gyre consists of several
anticyclonic circulations, in each of which thick STMW with a characteristic temperature is formed. After spring, the thick STMW tends to be continually trapped in
the respective circulations, remaining in the formation region. From this stagnant
pool of thick STMW, some portion seeps little by little into the southern region, where
southwestward subsurface currents advect relatively thin STMW as far as 20°N tothe south and just east of Taiwan to the west. The STMW formed in the recirculation
gyre becomes colder, less saline, and denser to the east, with an abrupt change ofproperties across 140°E and a gradual change east of 140 °E. The STMW formed eastof 140°E exhibits coherent interannual variations, increasing its temperature by ~1°C
from 2003 through 2006 and also increasing its salinity by ~0.05 from 2003 through2005. These property changes are clearly detected in the southern region as far downstream as just east of Taiwan, with reasonable time lags.

1. Introduction
North Pacific Subtropical Mode Water (STMW;
Masuzawa, 1969) is identified as a thermostad of 16°–19°C within or near the top of the permanent thermocline
in the northwestern North Pacific subtropical gyre
(Hanawa and Talley, 2001). Its name derives from the factthat it produces a mode in the volume distribution on thetemperature–salinity diagram for the subtropical gyre.
The formation and circulation of STMW were examined intensively from the late 1980s through the 1990s,
during which a fundamental view was established. Specifically, a deep mixed layer is formed in late winter justsouth of the Kuroshio and the Kuroshio Extension (KE),where warm water supplied from the Kuroshio/KE andstrong winter monsoon from the Asian continent lead toa large heat loss from the ocean to the atmosphere(Hanawa, 1987; Suga and Hanawa, 1990; Bingham,
1992). The developed mixed layer, STMW, is capped by
the seasonal thermocline in spring, and then transportedby the southwestward Kuroshio recirculation to spread

Keywords:⋅ North Pacific
Subtropical Mode
Water,⋅ seasonal variation,⋅ interannual
variation,⋅ Argo.

over the northwestern subtropical gyre (Bingham, 1992;
Suga and Hanawa, 1995a).
Several studies have demonstrated that the amountand properties of STMW formed each winter are related
to the intensity of the winter monsoon, represented by
the sea level pressure difference between Nemuro, Japanand Irkutsk, Russia. That is, thicker and colder STMW
tends to be formed in winters with strong monsoon (Sugaand Hanawa, 1995b; Yasuda and Hanawa, 1999; Taneda
et al., 2000; Hanawa and Kamada, 2001; Hanawa andYoritaka, 2001), a relation that also applies to the subtropical mode water in the North Atlantic (Kwon andRiser, 2004). Therefore, through its formation process,
STMW is believed to transmit atmospheric conditions to
the ocean subsurface every winter, in the form of temperature anomalies. In subsequent winters, such temperature anomalies might reappear at the sea surface in thesame formation region or in a remote area, affecting sea
surface temperature there. It is thus expected that STMW
plays some role in regional climate variability, although
it was difficult to examine the role in the 1990s and earlier, when only climatological data and a limited numberof long time-series data were available.

* E-mail address: eoka@ori.u-tokyo.ac.jp
Copyright©The Oceanographic Society of Japan/TERRAPUB/Springer151

In 2000, profiling floats deployed in the international
Argo project (Argo Science Team, 2001) started measuring temperature and salinity profiles down to 2000 dbar
depth at 10 day intervals. These data, increasing in number
year by year, made it possible to take snapshots of STMW
at any time of a year, providing new perspectives on the
STMW circulation. For example, the float data in February and March 2003 demonstrated that the STMW formation region, i.e., the deep mixed layer just south of theKuroshio/KE, extends as far east as 175°E, which is much
farther than previous studies had recognized (Oka andSuga, 2003). This indicates that the previous studies were
only able to investigate the western portion of the STMW
circulation, due to data limitations.
Uehara et al. (2003) analyzed early float data obtained southeast of Japan in 2001, and suggested that
STMW is preferentially formed and transported in anticyclonic eddies prevailing in the Kuroshio recirculation
region. The effects of mesoscale eddies on STMW variation were also addressed by Qiu and Chen (2006), whoused available temperature data and altimetric sea surface height data during 1993–2004 to demonstrate thatthe formation rate of STMW south of Japan is controlled
primarily by the dynamic state of the KE rather than theatmospheric forcing. That is, the KE path adopts a relatively stable state and a variable state alternately oninterannual time scales, and less STMW is formed in thelatter state because the associated high regional eddy activity promotes input of high potential-vorticity KE water to the south of the KE, disturbing the development of
deep winter mixed layer there. Supporting this idea, Qiu
et al. (2007) used float data southeast of Japan to indicate that the low-potential vorticity signature of STMW
formed in late winter 2005 was weaker than in 2004, associated with a transition of the KE from the low eddy
activity mode to the high eddy activity one.
Another observed fact that might be related to the
STMW circulation is a significant positive correlation
between winter sea surface temperature anomaly southeast of Japan and that around 30°N east of the date line
one year later (Sugimoto and Hanawa, 2005). According
to Sugimoto and Hanawa (2005), this correlation arises
because part of STMW formed southeast of Japan isadvected eastward by the KE in one year, transporting
sea surface temperature anomaly. Such advection might
be possible because the northern limit of STMW lies
within the rapid eastward flow of the KE (Qiu et al., 2006),
although it would be opposite to the southwestward
STMW advection that has been commonly accepted.
These recent findings indicate the need for a detailed
examination of the circulation and interannual changesof STMW through the entire distribution region using
observed data obtained in situ. Fortunately, the Argo float
network over the northwestern North Pacific subtropical152

E. Okagyre has steadily improved since 2000, particularly thanks
to the intensive deployment of 20 floats in April–June
2004 and 28 floats in May–July 2005 southeast of Japan
under the Kuroshio Extension System Study (KESS) program (Qiu et al., 2006, 2007) contributing to Argo. In
2006, after the KESS floats scattered over a wide region,
we obtained an unprecedented float network performing
relatively uniform, high-density observations over thenorthwestern subtropical gyre (see Fig. 2 for the distribution of float observations). Using the float data in 2006,
we here re-examine the seasonal evolution of STMW and
its relation to the subsurface flow field south of theKuroshio/KE. We also examine the interannual variationof STMW properties using the 2003–06 data, and investigate how the advection of STMW transports temperature and salinity anomalies to the downstream region.
2. Data
We used temperature (T) and salinity (S) data obtained by Argo profiling floats in the North Pacific in
2003–2006. These floats drift freely at a predeterminedparking pressure (typically 1000 dbar), and ascend to thesea surface conducting T–S measurements at a predetermined interval (10 days) after descending to the maximum pressure (2000 dbar). Since the parking pressure ofmost floats is much deeper than the STMW depth range(typically shallower than 400 dbar), we assume here thatthe floats visit a different water parcel in the STMW depth
range in every ascent. In other words, we assume thateach float observation of STMW is independent.
The real-time quality controlled float data were
downloaded from the ftp site of Argo Global Data Assembly Center (ftp://usgodae1.fnmoc.navy.mil/pub/outgoing/argo, ftp://ftp.ifremer.fr/ifremer/argo). From these
data, defective T–S profiles, such as those with measurements flagged as bad and those lacking intermediate layers for certain depths, were eliminated, following the procedures of Oka et al. (2007). For 2003 and 2004, quarterly T–S data along the 137°E meridian collected by Japan Meteorological Agency (2005, 2006) using a shipboard conductivity-temperature-depth profiler were added
to fill data gaps south of Japan. After each T–S profile
was interpolated on a 1-dbar grid using the Akima spline(Akima, 1970), potential temperature (θ), potential density (σθ), and potential vorticity (Q) were calculated. Here,
Q is defined asQ=−f ∂σ θ
,ρ ∂z(1)where f is the Coriolis parameter, ρ in-situ water density,and z is the vertical coordinate (positive upward). In Eq.
(1), relative vorticity is neglected because it is much

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

4
3
2
1
0
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
θ (oC)(b) 500

Number

400
300
200
100
0
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
θ (oC)

Fig. 1. (a) Plots of Q against θ at the Q minimum in each profile at 20°–35°N, 130°–165°E in April–June of 2003–2006.
The Q minimum was chosen in a θ range of 10°–25°C.
(b) Number of plots in (a) with Q < 1.5 × 10–10 m –1s–1, classified in each 0.5° bin of θ.

smaller than planetary vorticity except in the vicinity of
the Kuroshio/KE axis (Qiu et al., 2006).
We also used delayed-time sea surface height data
merged from TOPEX/Poseidon, Jason, ERS-1/2, andEnvisat altimeter observations, produced by Ssalto/Duacs
and distributed by AVISO (Ducet et al., 2000). The weekly
data in 2003–2006, provided on a 0.25° × 0.25° grid, were
downloaded from the web site of AVISO (http://
www.aviso.oceanobs.com). The sea surface height maps
for one, two, or three months presented in the latter sections were constructed by averaging the original weekly
data.

fied since the low-Q portion was formed, thus best preserving the water properties at the time of formation. To
determine a θ range for the STMW core, Q and θ at thecore are plotted for all profiles in spring in the northwestern subtropical gyre (Fig. 1(a)). The core with Q <1.5 × 10–10 m–1s–1 exists over the whole θ range between
10° and 25°C, but the plots concentrate in a range between 16° and 19.5°C, showing an outstanding peak in
the histogram of number of the plots (Fig. 1(b)). Although
this peak is probably highly overestimated due to the uneven distribution of float observations, particularly associated with intense observations southeast of Japan by
the KESS floats during 2004 and 2005 (Qiu et al., 2006,2007), it implies that there is a greater volume of 16°–19.5°C water than colder or warmer water, producing amode in the volume distribution on the θ –S diagram, as
pointed out by Masuzawa (1969). We therefore define
STMW in each profile as a layer of Q < 1.5 × 10–10 m–1s–1
with the core θ between 16° and 19.5°C. The low-Q layers with thickness less than 25 dbar and those existing
only at depths less than 100 dbar are regarded as smallscale features and are excluded. STMW is considered tooutcrop at the sea surface if its top is located at a depth
less than 10 dbar.
4. Seasonal Variation of STMW Structure in 2006
Figure 2 shows distributions of STMW thickness in
two-month periods from January–February through November–December, 2006, superimposed on maps ofgeopotential anomaly at 250 dbar (average depth of the
STMW core from spring through fall) relative to 950 dbar.
Geopotential anomaly maps were drawn from the float
data using an optimal interpolation technique (Bretherton
et al., 1976), with the annual-mean geopotential anomaly
distribution based on the World Ocean Atlas 2001 climatology (Conkright et al., 2002) as an initial estimate anda correlation function of the form:

C = 0.5e − ( d / 500 ) + 0.5e − ( d / 150 ) ,2

3. Definition of STMW
Since STMW is characterized by vertical homogeneity, previous studies have defined it as a portion of avertical profile with Q lower than a certain value. This
critical value may be chosen arbitrarily, and values of~2 × 10 –10 m–1s–1 have been employed by many studies
using climatologies (e.g., Suga and Hanawa, 1995a; Sugaet al., 2004) or bottle data (e.g., Suga et al., 1989; Sugaand Hanawa, 1995b). In this study, we adopt a smaller
value of 1.5 × 10–10 m–1s–1, which is the same value asthat used by Uehara et al. (2003) to define STMW in Argo
profiles, taking advantage of float data with relatively highvertical resolution and without any averaging.
Within the low-Q portion of a profile, the core, defined by the Q minimum, is considered to be least modi-(2 )where d is distance in kilometers. This function is similar in form to the one used by Roemmich et al. (2007),
considering both large-scale variation and mesoscale variability in the subtropical North Pacific with a dominantscale length of ~150 km (Kuragano and Kamachi, 2000).
From January through April, STMW outcrops at thesea surface south of the Kuroshio/KE (the large southward gradient of geopotential anomaly at ~34°N). Thisoutcropping area is regarded as the formation region of
STMW in late winter 2006. The formation region extends
zonally from 136°E to near the date line, as in late winter2003 (Oka and Suga, 2003), and its southern end is located at 28°N west of 150°E and at 29°N east of 150°E,

North Pacific Subtropical Mode Water in 2003–2006

153

400

Thickness (dbar)

Jan-Feb

300
200
100
0
15

20

25

30

35

40

25

30

35

40

25

30

35

40

25

30

35

40

25

30

35

40

25 30 35
Latitude (oN)

40

600
Mar-AprThickness (dbar)

500
400
300
200
100
0
15

20

400

Thickness (dbar)

May-Jun

300
200
100
0
15

20

Thickness (dbar)

300
Jul-Aug

200
100
0
15

20

Thickness (dbar)

300
Sep-Oct

200
100
0
15

20

Thickness (dbar)

300
Nov-Dec

200
100
0
15

Fig. 2. Distributions of STMW thickness (dbar; stars and circles) and geopotential anomaly at 250 dbar relative to 950
dbar (m 2s–2; contours) from January–February through November–December in 2006. Stars (circles) denote STMW
outcropping (not outcropping) at the sea surface. Black dots
represent float observation points without STMW. Contour
interval for geopotential anomaly is 0.5 m2s–2, with thickcontours at 10, 12, and 14 m 2s–2.

shifting slightly northward toward the east. In the formation region, the STMW thickness exceeds 200 dbar westof 170°E, with the meridional maximum at 31°–32°N, a
few degrees south of the KE (Fig. 3).
STMW does not outcrop south of 28°N (Fig. 2). The
STMW observed there is supposed to be formed north of154

E. Oka20

Fig. 3. Plots of STMW thickness against latitude from January–February through November–December in 2006. Open(closed) circles denote outcropping (non-outcropping)
STMW.

28°N in previous winters and then advected southward
across that latitude. The STMW south of 28°N lies in thesouthwestward subsurface current, being transported as
far as 20°N to the south and just east of Taiwan to thewest. It is thinner than 150 dbar, becoming thinner toward the south (Fig. 3). Consequently, the STMW thickness decreases sharply southward at the southern end of
the formation region at 28°N. This latitude corresponds
to that at which the winter mixed layer depth changes
sharply (Fig. 4), a so-called mixed layer depth front thatis theoretically important for the subduction of mode
waters (Kubokawa and Inui, 1999; Kubokawa, 1999;

(a)

19.5

Core θ (oC)

19.0

300

18.5
18.0

200

17.5
17.0

100

16.5

0

16.0
130
34.90

20

25
30
Latitude (oN)35

40

Fig. 4. Plots of mixed layer depth against latitude in February
and March 2006. Plots were drawn only for profiles southof the Kuroshio Extension, judged by θ > 12°C at 300 dbar
(Mizuno and White, 1983). Mixed layer depth was definedas the depth at which σ θ increases by 0.1 kg m–3 from 10dbar depth.

(b)

140

150

160

170

180

140

150

160

170

180

140

150

160

170

180

150

160

170

180

34.85

Core S

15

34.80
34.75
34.70

(c)

34.65
130
25.5

Core σθ (kg m-3)

Mixed Layer Depth (dbar)

400

25.4
25.3
25.2
25.1
25.0

(d)

24.9
130
400

Thickness (dbar)

Nishikawa and Kubokawa, 2007).
In May, the STMW north of 28°N stops outcropping,
and thick STMW is left in the subsurface in almost thesame location as the formation region in January–April(Fig. 2). This thick part is gradually eroded as the season
progresses, while, in contrast to the traditional view, it
shifts little in the meridional direction, maintaining its
southern end at ~28°N until November–December (Fig.
3). The STMW south of 28°N remains relatively thin,
except that STMW with a thickness of 100–200 dbar is
temporarily advected from the formation region to 24°–26°N, 135°–140°E in July–August. As a result, the frontof STMW thickness exists at ~28°N throughout the year.
The background contours of geopotential anomaly
in Fig. 2 indicate that the region between 28°N and theKuroshio/KE corresponds to the zonally-elongated anticyclonic recirculation gyre of the Kuroshio/KE (e.g.,
Kawai, 1972; Qiu, 2002; Qiu and Chen, 2005). This implies that after thick STMW is formed in the recirculation
gyre in late winter, a large portion of it remains in thegyre until the end of the year, and a small portion leaves
the gyre, entering the region south of 28°N where the flow
is predominantly southwestward. The latter portion is socalled subducted, being expected to persist for years,
while the former portion is likely to be re-entrained into
the mixed layer developed in the recirculation gyre in thefollowing winter.
It is worth noting that while the thick STMW isformed and circulates in the recirculation gyre, relatively
thin SMTW with a thickness of several tens of decibars
also exists there in a considerable amount (Figs. 2 and
3). This suggests that the STMW distribution is highlyvariable due to mesoscale disturbances, as demonstrated
in a recent study using a high-resolution numerical model(Rainville et al., 2007).
How do the properties of the STMW formed in therecirculation gyre in late winter 2006 change zonally?

300
200
100
0
130

140

Longitude (oE)Fig. 5. Plots of (a) θ , (b) S, and (c) σ θ at the core and(d) thickness of non-outcropping STMW north of 28°N
against longitude in May and June 2006. Plots in (a)–(c)were drawn only for STMW thicker than 100 dbar, withclosed (open) circles for STMW thicker than 200 dbar (witha thickness of 100–200 dbar).

Since the properties of outcropping STMW in winter are
temporal ones during the formation process, properties
at the core of non-outcropping STMW in May–June justafter the formation period are presented in Fig. 5.
The core θ of STMW decreases toward the east, as
demonstrated in previous studies (Suga and Hanawa,
1990; Bingham, 1992; Oka and Suga, 2003). It decreases
sharply from 18.8°C to 17.8°C near 140°E, and more
gradually east of 145°E, being almost uniform at ~17.2°C
east of 160°E. The core S also decreases toward the east
from 34.85 at 135°E to 34.77 at 170°E, due to the southward Ekman transport of less saline surface water northof the KE (Hanawa and Talley, 2001). The core σ θ increases toward the east from 24.95 to 25.35 kg m–3, reflecting mainly the eastward decrease of the core θ aslarge as 1.5°C, rather than that of the core S. The STMW
thickness is at a maximum of 303 dbar at 140° and 142°E,
decreasing gradually toward the east.
North Pacific Subtropical Mode Water in 2003–2006

155

156

E. Oka40

May-JunNumber of Profiles

(a)
30

20

10

0
16.0 16.5 17.0 17.5 18.0 18.5 19.0 19.5
40

Jul-Aug

Number of Profiles

(b)
30

20

10

0
16.0 16.5 17.0 17.5 18.0 18.5 19.0 19.5
40

Sep-Oct

Number of Profiles

(c)
30

20

10

0
16.0 16.5 17.0 17.5 18.0 18.5 19.0 19.5
40

Nov-Dec(d)Number of Profiles

A further inspection of Fig. 5(a) reveals that the plots
tend to be concentrated at particular core θ values, showing a step-like distribution. Indeed, the histogram of
number of profiles having STMW in each core θ range
exhibits three large peaks at 17.1°–17.3°C, 17.5°–17.7°C,
and 17.8°–17.9°C and a smaller, broad peak at 17.9°–
18.6°C adjacent to the third large peak (Fig. 6(a)). The
thick STMW formed in late winter 2006 is thus classified into several “modes” according to the core θ .
Using these modes, we now examine the seasonalvariation of STMW distribution in more detail. Figure 7
shows monthly distributions of three modes (17.1°–
17.3°C, 17.5°–17.7°C, and 17.8°–18.6°C) of STMW from
May through December 2006, superimposed on maps ofgeopotential anomaly at 250 dbar and altimetric sea surface height. Compared to the sea surface height maps,
the geopotential anomaly ones constructed from the unevenly-distributed float data adequately capture the meandering path of the Kuroshio/KE and the southern
recirculation gyre, except in some data-sparse regions.
As demonstrated in previous studies (e.g., Kawai, 1972),
the recirculation gyre consists of several anticyclonic circulations, most of which are associated with crests of theKuroshio/KE meander.
These anticyclonic circulations are closely related
to the distribution of each mode of STMW. In May, a large
circulation exists at 134°–144°E, and four small circulations align to the east at 145°–149°E, 152°–155°E, 157°–
160°E, and 167°–171°E. The 17.8°–18.6°C STMW
thicker than 100 dbar mostly lies within the large circulation, the 17.5°–17.7°C one within the two western small
circulations, and the 17.1°–17.3°C one within the twoeastern small circulations. In the following months, the17.1°–17.3°C and 17.5°–17.7°C STMW thicker than 100
dbar tends to be continually trapped in the respective circulations while the circulations gradually shift, occasionally merging/separating. Note that these anticyclonic circulations are relatively stagnant, compared to mesoscaleeddies to the south which are propagating westward at aconsiderable speed (Ebuchi and Hanawa, 2001; also see,e.g., an anticyclonic eddy propagating along 25°N from
157°E in May to 142°E in December in the sea surface
height maps in Fig. 7). Consequently, each mode of thick
STMW trapped in the circulations does not migrate significantly in the zonal direction nor in the meridional direction. From these stagnant pools of thick STMW, some
STMW parcels seep southward little by little, being subsequently transported southwestward.
In contrast with the 17.1°–17.3°C and 17.5°–17.7°C
STMW, the 17.8°–18.6°C one tends to leave the western,large circulation. After spring, a large portion of the 17.8°–
18.6°C STMW thicker than 100 dbar is discharged southward from the southeastern part of the large circulationnear 135°–140°E, reaching 27°N in May and 24°N in July.

30

20

10

0
16.0 16.5 17.0 17.5 18.0 18.5 19.0 19.5

Core θ (oC)Fig. 6. Number of profiles with non-outcropping STMW thicker
than 100 dbar in each 0.1° bin of core θ in (a) May–June,(b) July–August, (c) September–October, and (d) November–December of 2006.

The discharged portion then appears to lose its thickness
quickly in the downstream region. These changes are reflected in the number of STMW thicker than 100 dbar
with each core θ (Fig. 6). While the 17.1°–17.3°C and
17.5°–17.7°C STMW continues to be a mode until November–December, the 17.8°–18.6°C one rapidly decreases in number, almost disappearing in late fall.
It should be mentioned that some parcels of STMW,
such as the 17.5°–17.7°C one at 34°N, 166°E in May, the17.8°–18.6°C one at 32°N, 163°E in June, and the 17.5°–
17.7°C one at 31°N, 171°E in July, are found in a much
more easterly location than the other STMW parcels having the same core θ (Fig. 7). Since these remote parcels
are located in the vicinity of the KE axis, they are likely
to be transported eastward by the rapid flow of the KE.

Fig. 7. Monthly distributions of SMTW with a core θ of 17.1°–17.3°C (blue symbols), 17.5°–17.7°C (green symbols), and 17.8°–
18.6°C (red symbols) from May through December in 2006. The symbol type denotes STMW thickness: greater than 100 dbar
(circles), 50–100 dbar (triangles), and less than 50 dbar (squares). Black dots denote float observation points with STMW
having the other core θ and those without STMW. Background contours represent geopotential anomaly at 250 dbar relative
to 950 dbar (left panels) and altimetric sea surface height (right panels). Contour for geopotential anomaly is drawn at aninterval of 0.5 m2s–2, with thick ones at 10, 12, and 14 m2s–2. Contour for sea surface height is drawn at an interval of 10 cm,
with a thick one at 220 cm and a shaded region for higher than 250 cm.

These transports, though low in number, might be related
to the close correlation between winter sea surface temperature anomaly in the STMW formation region and thateast of the date line one year later, indicated by Sugimotoand Hanawa (2005).
The anticyclonic circulations in the recirculation gyreare important not only for the circulation of each modeof STMW after spring but probably also for its formaNorth Pacific Subtropical Mode Water in 2003–2006

157

tion. That is, each mode of STMW is thought to be formed
in a different anticyclonic circulation in late winter. In
May, most of the 17.1°–17.3°C (17.5°–17.7°C) STMW
is observed east (west) of 157°E, at which longitude thetrough of the KE exists in January–April (Fig. 2). Such
STMW distributions probably result from the formationof the 17.1°–17.3°C (17.5°–17.7°C) STMW in a circulation located east (west) of the trough.
The observed/inferred formation and circulation patterns of STMW seem to be well reproduced in the highresolution numerical model of Rainville et al. (2007). In
their model, STMW is formed in several anticyclonic circulations just under the crests of the Kuroshio/KE meander, follows each circulation, and gradually escapes fromthe circulations, mainly toward the south. Complementary use of the Argo float data and such numerical models in the future would lead to the clarification of moredetailed variations of STMW and their mechanisms.
5.

Fig. 8. Distributions of thickness of non-outcropping STMW
(dbar; circles) in April–June and October–December of
2003–2005. Black dots represent float observation points
without STMW. Background contours denote altimetric sea
surface height (for 2003) and geopotential anomaly at 250
dbar relative to 950 dbar (for 2004 and 2005), drawn as inFig. 7. (In 2003 there were not enough float observations todraw geopotential anomaly maps.) Three red rectangles in
the lowermost panel indicate the regions of averaging in
Fig. 13.

158

E. OkaInterannual Variation of STMW Properties in
2003–2006
In 2005 and the preceding years, there are fewer float
data, which are distributed more unevenly in space withdecreasing time, especially before April 2004 when the
KESS project started deploying floats intensively in the
STMW distribution region (Fig. 8). These insufficient datamean that it is somewhat difficult to examine detailed
distributions of STMW properties, particularly that of the
thickness showing small-scale structure (Fig. 5). The other
STMW properties, such as the core θ and S, have large
spatial scales and are easier to investigate.
In January–March of 2003–2005, thick STMW isformed between the Kuroshio/KE and ~28°N, as in thesame months of 2006 (Fig. 9). After April in each year
this thick part stays in a similar latitude range to the winter months, gradually losing its thickness as the season
progresses. (The thick part seems to shift southward by
three degrees of latitude from July–September to October–December in 2005, but this is probably due to theobservation gaps near 31°N, 144° and 152°E in the lattermonths (Fig. 8).) In the non-outcropping region south of
28°N, STMW is mostly thinner than 150 dbar throughout
2003–2005, as in 2006, producing the stationary front of
STMW thickness at ~28°N. Thus, the STMW circulation
observed in 2006, characterized by the stagnant pool ofthick STMW formed in late winter in the recirculation
gyre and the southwestward flow of relatively thin STMW
seeping from the thick pool, seems to apply to 2003–2005
on a large scale.
While the southern end of the STMW formation region is located at ~28°N every year, the latitude at which
the thickest STMW is formed is different between years.
It is at 34°N in 2003 and 2004, while it is located farther
south at 31°–32°N in 2005 and 2006. This change isThickness (dbar)

500

Jan-Mar2003

Apr-Jun
2003

Jul-Sep
2003

Oct-Dec
2003

400
300
200
100
0
15 20 25 30 35 40 15 20 25 30 35 40 15 20 25 30 35 40 15 20 25 30 35 40

Thickness (dbar)

500

Jan-Mar2004

Apr-Jun
2004

Jul-Sep
2004

Oct-Dec
2004

400
300
200
100
0
15 20 25 30 35 40 15 20 25 30 35 40 15 20 25 30 35 40 15 20 25 30 35 40

Thickness (dbar)

500

Jan-Mar2005

Apr-Jun
2005

Jul-Sep
2005

Oct-Dec
2005

400
300
200
100
0
15 20 25 30 35 40 15 20 25 30 35 40 15 20 25 30 35 40 15 20 25 30 35 40

Thickness (dbar)

500

Jan-Mar2006

Apr-Jun
2006

Jul-Sep
2006

Oct-Dec
2006

400
300
200
100
0
15 20 25 30 35 40 15 20 25 30 35 40 15 20 25 30 35 40 15 20 25 30 35 40
Latitude (oN)
Latitude (oN)
Latitude (oN)
Latitude (oN)

Fig. 9. Plots of STMW thickness against latitude from January–March through October–December of 2003–2006. Open (closed)
circles denote outcropping (non-outcropping) STMW.

mainly due to the change in position of the KE
recirculation gyre southeast of Japan between about 140°
and 155°E (Fig. 10). In 2003 and 2004, the KE is in astable state, showing two quasi-stationary crests at 144°E
and 150°E (Qiu and Chen, 2006), and is accompanied by
anticyclonic circulations just south of the crests. At 34°N
in these circulations where the permanent thermocline isdeep, the thickest STMW is considered to be formed. In
2005, the KE transits to a variable state (Qiu et al., 2007),and cyclonic eddies detached from the KE occupy theregion just south of the KE, pushing the anticyclonic circulations southward. In 2006, the KE takes a southerly
path, likely blocked by cyclonic eddies near 146°E, resulting in the southerly positions of anticyclonic circulations. Accordingly, the thickest STMW is formed at thesoutherly locations in the latter two years.
Zonal distribution of the core θ of thick STMW
formed in late winter 2003–2005 exhibits a feature thatis not observed in 2006 (Fig. 11). In each of the three

years, the core θ drops eastward by 1.5°C across 140°E,
while it decreases much more gradually east of 140°E.
This drop is also likely to be related to the distributionsof the anticyclonic circulations in the recirculation gyre.
The anticyclonic circulations just south of the Kuroshio/
KE clearly separate across 140°E, not only in winter 2005
when the Kuroshio takes a large-meander path south ofJapan but also in winter 2003 and 2004 when it takes a
non-large-meander path, implying that the warmer(colder) STMW is formed in the circulation west (east)of that longitude (Fig. 10). On the other hand, the circulation is continuous across 140°E in 2006, leading to the
STMW formation near 140°E and the continuous decreaseof the core θ there. The zonal jump across 140°E in 2003–
2005 is also seen in the core S and σ θ distributions (Fig.
11). The STMW formed south of Japan in these years iswarmer by 1.5°C, more saline by 0.05–0.1, and less dense
by 0.3 kg m–3 than that formed east of 140°E.
The core properties east of 140°E exhibit coherent
North Pacific Subtropical Mode Water in 2003–2006

159

Thickness / Depth (dbar)(a) 40°N

0
100
200
300
400
500
600
700
800

35°N

30°N

25°N
135°E

140°E

145°E

150°E

155°E

30°N

140°E

145°E

150°E

155°E

40

25

30

35

40

25

30

35

40

30

35

40

0
100
200
300
400
500
600
700
800

35°N

30°N

140°E

145°E

150°E

155°E

Thickness / Depth (dbar)(d) 40°N

0
100
200
300
400
500
600
700
800

35°N

30°N

25°N
135°E

35

Thickness / Depth (dbar)(c) 40°N

25°N
135°E

30

0
100
200
300
400
500
600
700
800

35°N

25°N
135°E

25

Thickness / Depth (dbar)(b) 40°N

140°E

145°E

150°E

155°E

25

Latitude (oN)

Fig. 10. Altimetric sea surface height maps southeast of Japanin February and March, drawn as in Fig. 7 (left panels) andplots of non-outcropping STMW thickness (closed circles)
and the 12°C isotherm depth representing the permanent
thermocline depth (open circles) against latitude between
141° and 147°E in April–June (right panels) in (a) 2003,(b) 2004, (c) 2005, and (d) 2006.

interannual variations. The core θ increases from 2003
(16.5°–17.0°C) to 2006 (17.1°–18.2°C) by about 1°C, if
we neglect local high/low values. The core S also increases
from 2003 (34.71–34.79) to 2005 (34.77–34.87) by about
0.05, remaining high in 2006. This increase is possibly
related to a recent salinity increase in the North PacificTropical Water (Cannon, 1966), which is the main sourceof salt in the North Pacific subtropical gyre. Hayashi etal. (2005), using T–S data from the repeat section along
137°E, reported that the maximum salinity of the Tropical Water observed at about 10°–16°N increases from1999 (~34.9) to 2004 (~35.1) by about 0.2. The core σθdecreases from 2003 (25.30–25.45 kg m –3 ) to 2006
(25.10–25.35 kg m–3) by about 0.15 kg m–3, reflecting
mainly the increase of the core θ.
160

E. OkaUnlike the cases of the core properties, it is difficult
to assess the interannual variation of STMW thickness.
The maximum thickness in spring is observed between
140° and 150°E every year, being largest in 2004 (442dbar), second largest in 2005 (406 dbar), third largest in
2003 (379 dbar), and smallest in 2006 (336 dbar). However, Fig. 10 strongly suggests that the thickest STMW isformed locally where the permanent thermocline is particularly deep, and therefore the maximum thickness ineach year may not reflect the thickness over a large area.
In addition, as mentioned earlier, the float observations
before 2006 are neither dense nor uniform enough to resolve such small-scale structure of STMW thickness. For
example, the paucity of STMW thinner than 100 dbar
between 140° and 150°E in spring 2004 (Fig. 11) is likely
because the KESS floats are concentrated within the anticyclonic circulation south of the semi-permanent crest of
the KE at 144°E (see figure 2 of Qiu et al., 2006).
With an understanding of these limitations, we averaged STMW thickness in the recirculation gyre in eachlongitudinal range (Fig. 12). At 140°–150°E, the thickness is large in 2003 and 2004 and small in 2005 and2006, with a significant decrease from 2004 to 2005,
which agrees with the result of Qiu at al. (2007). The
thickness shows a similar change at 160°–170°E, but not
at 150°–160°E in between. The thickness at 150°–160°E
increases conversely from 2004 to 2005, being thickest
in 2005. Thus, as far as the float data during 2003–2006
are concerned, interannual variation of STMW thickness
is different among regions. Again, this is a crude estimate; we need at least several more years of data fromthe profiling float array comparable to that in 2006, tomore properly assess the interannual variation of STMW
thickness.
To what degree is the year-to-year change of the coreproperties detected in the southern, downstream region?
Figure 13 shows the time series of core θ during 2003–
2006 in three boxes south of the formation region, where
STMW never outcrops at the sea surface in winter (seethe lowermost panel of Fig. 8 for the box locations). The
eastern box at 23°–26°N, 155°–165°E is expected to capture STMW originating in the eastern part of the formation region east of ~160°E, according to the geopotential
anomaly maps (e.g., Fig. 8). In this box the core θ steadily increases from the end of 2003 through the end of
2006 (Fig. 13(a)), clearly reflecting the core θ increasein the formation region presented in Fig. 11. Note thatthe core θ of 17.1°–17.5°C, which characterizes STMW
formed at 160°–170°E in winter 2005 (but not that formed
in winter 2003 and 2004, which has lower core θ ), starts
to become apparent in the box in August 2005. This implies that the STMW with the core θ of 17.1°–17.5°C istransported from the formation region to the box in about
a half year. The southern end of the formation region at

2003

2005

2004

Core θ (oC)

19.5
19.0
18.5
18.0
17.5
17.0
16.5
16.0
130 140 150 160 170
34.95
34.90
34.85
34.80
34.75
34.70
34.65
34.60
130 140 150 160 170
25.6
25.5
25.4
25.3
25.2
25.1
25.0
24.9
24.8
130 140 150 160 170500

130 140 150 160 170

130 140 150 160 170

130 140 150 160 170

130 140 150 160 170

130 140 150 160 170

Thickness (dbar)

Core σθ (kg m-3)

Core S130 140 150 160 170

2006
19.5
19.0
18.5
18.0
17.5
17.0
16.5
16.0
130 140 150 160 170 180
34.95
34.90
34.85
34.80
34.75
34.70
34.65
34.60
130 140 150 160 170 180
25.6
25.5
25.4
25.3
25.2
25.1
25.0
24.9
24.8
130 140 150 160 170 180
500

400

400

300

300

200

200

100

100

0
130 140 150 160 170
Longitude (oE)0
130 140 150 160 170
Longitude (oE)

130 140 150 160 170
Longitude (oE)

130 140 150 160 170 180
Longitude (oE)Fig. 11. Plots of θ (top panels), S (second top panels), σθ (third top panels) at the core and thickness (bottom panels) against
longitude for non-outcropping STMW north of 28°N in April–June from 2003 through 2006. Plots of the core properties weredrawn only for STMW thicker than 100 dbar, with closed (open) circles for STMW thicker than 200 dbar (with thickness 100–
200 dbar).

~28°N and the northern boundary of the box at 26°N are
separated by 2° in latitude. If we roughly estimate thesouthward component of subsurface flow between the tworegions by dividing the zonal gradient of geopotential
anomaly shown in Fig. 8 by the Coriolis parameter at
27°N, we obtain1 m 2 s −2
= 0.01 m s −1 .
1.5 × 10 6 m × 6.6 × 10 −5 s −1

(3)

Dividing the distance between the two regions by this flow
speed yields an advection time of eight months, which isconsistent with the observed advection time of a half year.
The middle box at 23°–26°N, 135°–140°E is expected to be fed by various parts of the zonally-elongated
STMW formation region. In this box, relatively high core
θ is observed from late winter to early summer each year:
17.8°–18.4°C in April–August 2003, 18.4°–18.7°C in
April–July 2004, 19.4°C in May 2005, and 18.5°–19.0°C
in February–March 2006 (Fig. 13(b)). Each of these coreθ values coincides with that of STMW originating in thewesternmost part of the formation region west of 140°E
in the same year (Fig. 11), implying that warm STMW

formed south of Japan is quickly transported southward
every year. Except for these high values, the core θ in thebox increases gradually from the end of 2003 through theend of 2006, presumably reflecting the core θ increase in
the formation region east of 140°E with various time lags
longer than about a half year.
The core θ increase is observable even in the western box at 22°–26°N, 122°–127°E just east of Taiwan,
although the data represent only the last 2.5 years there(Fig. 13(c)). The box exhibits high core θ of 18.5°–18.8°C
from September 2004 through January 2005 and the othercore θ increasing for 2.5 years, probably associated with
the STMW originating west and east of 140°E in the formation region, respectively. A comparison of this core θvariation with that in the middle box suggests that the
STMW observed in the western box is advected from near
the middle box in about 3–5 months.
The increase of the core S in the formation region
from 2003 through 2005 presented in Fig. 11 is also clearly
detected in the three boxes (not shown). Thus, the float
data for four years clearly demonstrate that STMW transports the property anomalies from the formation region
to the western boundary. In the present study, however,
we could not examine in detail how the property anomaNorth Pacific Subtropical Mode Water in 2003–2006

161

200

(a)

19.5

18.5
18.0

150

100

23-26N, 155-165E

core θ (oC)

Thickness (dbar)

19.0

17.5
17.0
16.5

140-150E
150-160E
160-170E

2003

16.0

2004

2005

2006

(b)

Year

J F M AM J J A S ON D J F MA M J J A S O N D J F M AM J J A S ON D J F MAM J J A S O N D

2003

2004

2005

2006

19.5
19.0

23-26N, 135-140E

core θ (oC)

18.5
18.0
17.5
17.0
16.5
16.0

(c)

J F M AM J J A S ON D J F MA M J J A S O N D J F M AM J J A S ON D J F MAM J J A S O N D

2003

2004

2005

2006

19.5
19.0

22-26N, 122-127E

18.5

core θ (oC)Fig. 12. Time series of STMW thickness in May and June,averaged north of 28°N, between 140° and 150°E (solid
curve with circles), 150° and 160°E (dashed curve withsquares), and 160° and 170°E (dashed curve with triangles).
The thickness only at float observation points where the
12°C isotherm is deeper than 475, 450, and 425 dbar, respectively, was averaged, first in each 1° square and then ineach area spanning 10° in longitude.

18.0
17.5

lies produced in various parts of the formation region
reach a given area in the downstream region with different time lags, due to brevity of the data and the inadequate data density in the southern region. In several
years, using longer time-series float data and complementary high-resolution numerical models, we shall be able
to examine quantitatively how these anomalies accumulate in the downstream region, changing the oceanic structure and flow field there.
6. Summary and Discussion
Temperature and salinity data during 2003–2006
from Argo profiling floats have been analyzed to examine the formation and circulation of STMW andinterannual variation of its properties. In 2006, thick
STMW is formed in late winter in the zonally-elongatedrecirculation gyre south of the Kuroshio/KE, which extends north of ~28°N, from 135°E to near the date line.
The recirculation gyre consists of several anticyclonic
circulations, in each of which STMW is formed with acharacteristic θ . After spring, the thick STMW, while
gradually decreasing its thickness, tends to be continually trapped in the respective circulations, remaining in
the formation region. From this stagnant pool of thick
STMW, some portion seeps little by little into the southern region, where the southwestward subsurface current
advects relatively thin STMW as far as 20°N to the south
and just east of Taiwan to the west. Such STMW circulation seems to apply to the other years, 2003–2005.
The thick STMW just after the formation in each year
becomes colder, less saline, and denser to the east, with ajump of the core properties across 140°E in 2003–2005.
In these three years, the STMW formed in the anticyclonic
circulation south of Japan near 135°E is warmer by 1.5°C,
162

E. Oka

17.0
16.5
16.0

J F M AM J J A S ON D J F MA M J J A S O N D J F M AM J J A S ON D J F MAM J J A S O N D

2003

2004

2005

2006

Fig. 13. Time series of the core θ of STMW observed in(a) 23°–26°N, 155°–165°E, (b) 23°–26°N, 135°–140°E, and(c) 22°–26°N, 122°–127°E.

more saline by 0.05–0.1, and less dense by 0.3 kg m–3
than that formed in the anticyclonic circulations east of
140°E, regardless of whether the Kuroshio south of Japan takes a large-meander path or a non-large-meander
path. This jump is not observed in 2006, when the KE
takes a southerly path southeast of Japan and the anticyclonic circulation is continuous across 140°E. East of
140°E, the core properties of the thick STMW graduallychange zonally every year, and show coherent interannual
variations. The core θ increases from 2003 through 2006
by about 1°C, and the core S also increases from 2003
through 2005 by about 0.05, remaining high in 2006.
These changes are clearly detected in the downstream
region south of the recirculation gyre where STMW isnever ventilated in winter, as far downstream as just eastof Taiwan, with reasonable time lags.
Since the interannual variation of core θ is expected
to be related to that of the winter cooling in the STMW
formation region, we averaged net surface heat flux at
29°–36°N, 142°–160°E from October through March eachyear, using daily mean reanalysis data of the National
Centers for Environment Prediction and the National
Center for Atmospheric Research (NCEP/NCAR; Kistler
et al., 2001). The average heat flux from the ocean to theatmosphere is largest in 2002/03 (280 W m–2), second

largest in 2005/06 (256 W m–2), third in 2003/04 (252
W m–2), and smallest in 2004/05 (247 W m–2), showing a
weakly decreasing trend. There is also a suggestion thatthe interannual variation of core θ is related to that of thetransport of the Kuroshio that brings warm surface water
to the STMW formation region (Yasuda and Hanawa,
1997; Hanawa and Kamada, 2001). The one-year moving average of the Kuroshio transport in the East China
Sea, estimated using sea-level data and the regression
equation of Kawabe (1995), steadily increases from 24 ×
10 6 m3s –1 in winter 2002/03 to 27 × 106 m3s–1 in winter2005/06 (M. Kawabe, personal communication). Thus, thetrends in both the surface heat flux and the Kuroshio transport during the four years, particularly the latter, are consistent with the increasing trend of the core θ.
The present study has demonstrated that the existence and state of the recirculation gyre of the Kuroshio/
KE are essential in the formation and circulation of
STMW and in determining the distribution of its properties. The state of the recirculation gyre also affects theinterannual variation of STMW thickness, through theassociated cyclonic eddies carrying high potentialvorticity KE water into the gyre (Qiu and Chen, 2006).
The relation of the STMW variation to the state of therecirculation gyre, as well as the other factors, needs further comprehensive examination, using more data fromthe Argo float array that has just been completed.
Acknowledgements
The author thanks Tsuyoshi Ohira for help in preparing the Argo float data, Shigeki Hosoda for advice on
the optimal interpolation technique, and Toshio Suga,
Masami Nonaka, Bo Qiu, and an anonymous reviewer for
helpful comments. The Argo float data used in this study
were collected and made freely available by the International Argo Project and the national programs that contribute to it (http://www.argo.ucsd.edu, http://
argo.jcommops.org). Argo is a pilot program of the Global Ocean Observing System.
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