GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 23, 2205, doi:10.1029/2003GL018581, 2003

Formation region of North Pacific subtropical mode water in the late
winter of 2003
Eitarou Oka1 and Toshio Suga1,2Received 9 September 2003; accepted 21 October 2003; published 6 December 2003.

[1] The mixed layer structure and properties in theformation region of the North Pacific Subtropical Mode
Water (STMW) east of Japan in the late winter of 2003 were
examined using temperature and salinity data primarily
from Argo profiling floats. The formation region extends
south of the Kuroshio Extension, between 30°N and 35°N
and as far east as 175°E. It is characterized by a mixed layer
with depths greater than 200db and meridionally uniform
temperature between 16.5°C and 18.2°C, associated with amixed layer front at its southern edge. The mixed layer inthe formation region becomes colder, fresher, and denser
toward the east, leading to the formation of warmer, saltier,and lighter STMW in the western part of the region. The
spatial variation of the mixed layer depth in the formation
region corresponds well with the underlying permanent
INDEX TERMS: 4223 Oceanography:
thermocline depth.
General: Descriptive and regional oceanography; 4283
Oceanography: General: Water masses; 4572 Oceanography:
Physical: Upper ocean processes. Citation: Oka, E., andT. Suga, Formation region of North Pacific subtropical mode
water in the late winter of 2003, Geophys. Res. Lett., 30(23), 2205,
doi:10.1029/2003GL018581, 2003.

1. Introduction[2] North Pacific Subtropical Mode Water (STMW) is
identified by a thermostad of 16° –19°C lying just above
and within the permanent thermocline in the western North
Pacific subtropical gyre [Masuzawa, 1969, 1972]. It is
formed in late winter in the deep mixed layer immediately
south of the Kuroshio and the Kuroshio Extension (KE)[Hanawa, 1987; Suga and Hanawa, 1990; Bingham, 1992]
and spreads through advection over the Kuroshio recirculation region [Bingham, 1992; Suga and Hanawa, 1995a].
[3] As temporal and spatial coverage of hydrographic
data in the region has been sparse, previous studies
concerning the formation and spreading of STMW reliedon climatologies, and those concerning its year-to-year
property variations depended on time-series data at selected
sections or locations [e.g., Suga and Hanawa, 1995b]. Thissituation, however, has been changing due to the Argo
project, initiated in 2000 [Argo Science Team, 2001]. Argoaims to deploy roughly 3000 profiling floats over the global
oceans at three-degree intervals, to obtain vertical profiles of1
Frontier Observational Research System for Global Change, Yokosuka,
Japan.
2
Department of Geophysics, Graduate School of Science, Tohoku
University, Sendai, Japan.

Copyright 2003 by the American Geophysical Union.
0094-8276/03/2003GL018581$05.00

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temperature and salinity from 2000db to the sea surface
over a ten-day cycle.
[4] In the western North Pacific, Argo floats have been
deployed intensively by Japan. The addition of more than
40 floats in January and February 2003 resulted in good
float coverage over much of the STMW distribution region.
It is now possible to monitor the distribution and properties
of STMW continuously, which should dramatically improve
our understanding of the water. In this paper, the mixed
layer structure and properties in the STMW formation
region east of Japan in the late winter of 2003 are presented,
using the data from the floats and an expendable conductivity-temperature-depth profiler (XCTD) section. Almost
the entire part of the STMW formation region is describedfor a specific winter, an accomplishment which would
hardly be possible without the Argo array.
[5] The STMW formation region is located immediately
south of the Kuroshio/KE and north of about 30°N with ameridional extent of several degrees [Hanawa, 1987; Sugaand Hanawa, 1990; Bingham, 1992]. At its southern end,
the mixed layer shallows sharply southward, forming a socalled mixed layer front [Kubokawa and Inui, 1999]. The
formation region extends from 132°E to at least 160°E,
although the location of its eastern end has not beenclarified due to insufficient data. The deep mixed layer inthe formation region becomes colder and denser toward theeast because the surface water in the Kuroshio/KE, which isthe source of the deep mixed layer water, is cooled throughthe air-sea heat exchange as it flows downstream.
[6] Recently, Uehara et al. [2003] demonstrated using
Argo float data from the southeast of Japan during 2001 thatthe formation of STMW is affected by mesoscale eddies;
specifically, the deeper mixed layers are formed preferentially in anticyclonic eddies where the thermocline is deep.
Their results will be tested in this study, using the float dataover a much larger area.

2. Data[7] Two hundred and fifteen temperature and salinity
profiles obtained by 39 profiling floats in the region of
25°– 40°N, 140°E – 170°W during February and March
2003 were used. Thirty-seven floats were deployed as partof Argo, while the other two were part of the Subarctic Gyre
Experiment (SAGE). After the salinities of five profiles
from three floats were corrected using the method established by Wong et al. [2003] and the climatological datasetof the JAMSTEC/FORSGC Selected Hydrographic Dataset(Kobayashi and Minato, Performance of Argo automaticdelayed-mode salinity calibrations and suggestions for historical database improvements, submitted to the Journal of
Atmospheric and Oceanic Technology, 2003), the salinities

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OKA AND SUGA: NPSTMW FORMATION REGION IN WINTER 2003

Figure 1. Distributions of (a) SST (°C) and (b) MLD (db) in February and March 2003. Circles (triangles) denoteobservation points located south (north) of the KE. The blue-solid curve indicates the approximate path of the KE. The reddashed curve in (b) roughly encloses a region of MLD greater than 200db. The background contours in (b) show the MLD
distribution in the climatology of Suga et al. [2003].
of all profiles are believed to be accurate to 0.01 on the 1978
practical salinity scale, except for those of one profile at
31.7°N, 154.2°E, which still have an error of about 0.05
(resulting in a density error of 0.04 kg m 3). The
temperature and salinity data at 60– 70 sampling depths
between the sea surface and 2000db were interpolated toevery 1db resolution using the Akima spline [Akima, 1970].
Forty-six temperature and salinity profiles, obtained by
XCTD in the same region on 4 – 11 February during the
R/V Mirai MR02-K06 cruise, were also used. Potential
temperature and density (called temperature and density
henceforth) referred to the surface were calculated fromthese temperature and salinity data. It should be mentionedthat during the period under investigation no Argo float was
operating south of Japan (130° – 140°E), so the westernmost
part of the STMW formation region is not examined in thisstudy.
[8] Values of temperature, salinity, and density at ornearest to 10db are chosen as characteristic of the sea
surface, except that 40-db values are used for an observation
point at 30.5°N, 162.3°E on 26 March because we areinterested in the winter deep mixed layer properties. At thispoint, the winter mixed layer, deeper than 300db, wascapped by a warm surface water shallower than 30db.
[9] The mixed layer depth (MLD) is defined as the depth
at which temperature decreases by 0.5°C from the surface
values, as in many previous studies. An alternative definition using a density increase of 0.125 kg m 3 from thesurface values yields similar MLD values with a constantsalinity of about 35.0 [Levitus, 1982]. Our data, however,showed that each MLD value defined by density tends to beslightly larger than that defined by temperature. This occurs
because the downward decrease in salinity from the bottomof the mixed layer partially compensates the increase indensity due to the decrease in temperature. We therefore

adopted the temperature definition that detects the mixed
layer bottom more accurately.
[10] Observations at which temperature at 300db ishigher (lower) than 12°C are considered to be located south(north) of the KE because the KE is represented by the 12°C
isotherm at 300-m depth [Mizuno and White, 1983]. The
observation at 36.8°N, 164.8°E on 1 March has a temperature slightly higher than 12°C at 300db, but is judged to benorth of the KE, because the mixed layer temperature(12.8°C) and salinity (34.47) are more similar to those atother points north of the KE than those south.

3. Results[11] During the late winter of 2003, the KE shifts
gradually southward as it meanders eastward (Figure 1).
Sea surface temperature (SST) depends primarily on latitude
and increases southward (Figure 1a), particularly across the
KE located around 36°– 37°N at 145°– 155°E, 34°– 35°N at
160° – 170°E, and 32°– 33°N at 175°E– 175°W (Figures 2a –2c). Immediately south of the KE, however, SST is nearly
uniform for several degrees in latitude, with values of 16°–18°C. The southern end of this uniform-SST region is at
30°N in every longitudinal range. These features were seen
as meridional widening of the SST 16° – 19°C zone south of
the KE in late winter in Hanawa [1987] but are captured
more clearly here.
[12] The MLD south of the KE exceeds 200db in a region
extending eastward between 30°N and 35°N up to 175°E(Figure 1b; red-dashed curve). A maximum MLD of 430db
is observed at 33.6°N, 147.9°E and 33.9°N, 152.9°E in thenorthwestern part of the region. The MLD south of the KE
at 30°– 35°N tends to decrease toward the east; it mostly
exceeds 250db at 145°– 155°E and 200db at 160°– 170°E inthe region, while it is mostly less than 200db at 175°E–

OKA AND SUGA: NPSTMW FORMATION REGION IN WINTER 2003

Figure 2. Plots of SST (a– c) and MLD (d – f ) against
latitude at 145° – 155°E (a,d), 160° – 170°E (b,e), and175°E– 175°W (c,f ). Dots (crosses) denote observations
located south (north) of the KE.
175°W east of the region (Figures 2d– 2f ). In contrast, theMLD south of 30°N exhibits a small zonal variation withvalues less than 200db in every longitudinal range. As aresult, a sharp meridional gradient of MLD exists around
30°N west of 170°E at the southern end of the region.
[13] The large-MLD region is part of the uniform-SST
region south of the KE, and the southern ends of the tworegions coincide at 30°N (Figures 1 and 2). Thus, the largeMLD region has meridionally uniform SST of 16° – 18°C
and mixed layers deeper than 200db accompanied by amixed layer front at its southern edge. This region isregarded as the STMW formation region east of Japan in
the late winter of 2003. The formation region extends
easterly much farther than suggested by previous studies[Suga and Hanawa, 1990; Suga et al., 1997; Suga et al.,2003] and approaches the date line, as inferred by Hanawa[1987].
[14] Compared with the STMW formation region in thepresent study, that in Suga et al.’s [2003] climatology,
represented roughly by the 150-db contour south of the
KE in Figure 1b, not only extends less eastward, but also
has smaller MLD and a much less sharp mixed layer front at
the southern edge. The mixed layer front for each winter isbelieved to be smoothed out considerably in the climatology
through the averaging process.
[15] MLDs greater than 200db are also found at several
observation points north of the KE around 37°N, between
151°E and 165°E. Judging from the SST (11.6°– 13.3°C)
and sea surface density (25.90 – 26.19 kg m 3), these points
are presumably the formation sites of North Pacific Central
Mode Water [Nakamura, 1996; Suga et al., 1997]. This
supports the likely formation region of the water presented
by Suga et al. [2003], which extends as far west as 143°E incontrast with that previously described as located east of
175°E.
[16] In the STMW formation region, the mixed layer
properties exhibit significant zonal variations (Figure 3).
The SST (from 16.5° to 18.2°C) and sea surface salinity(34.65 to 34.80) decrease eastward across the region by
1°C and 0.1, respectively. As a result, the sea surface
density (25.05 to 25.45 kg m 3) increases across the

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Figure 3. Plots of (a) SST, (b) sea surface salinity (SSS),(c) sea surface density (SSD), and (d) MLD against
longitude in the STMW formation region. The formation
region is defined as the group of observations south of theKE with MLD greater than 200db, excluding several points
apart from the region shown by the red-dashed curve in
Figure 1b.
region by 0.2 kg m 3. These variations imply the formation of warmer, saltier, and lighter (colder, fresher, anddenser) STMW in the western (eastern) part of the region.
The zonal change of both temperature and density is steep
west of 148°E and much less east of it. This result is
consistent with Bingham [1992], who showed that theeastward decrease of climatological SST in the STMW
formation region is abrupt in the west (130°– 145°E) andgradual in the east (145°– 175°E), although the SST dropfrom 145°E to 175°E in the present study (about 0.5°C) ismuch smaller than in Bingham’s study (>1°C). MLD is
primarily between 200db and 300db, but is particularly
large around 148°E and 153°E with the maximum of
430db and 168°E with 380db.
[17] The eastward decrease of SST in the formation
region is due to the cooling of the KE surface water, whichis the source of the deep mixed layer water [Bingham,
1992]. The eastward decrease of the sea surface salinity ispossibly due to the southward Ekman transport, which

Figure 4. MLD plotted against (a) 12°C-isotherm depthand (b) SST, otherwise following Figure 3. The regression
line (solid line) and its 95% confidence interval band(dashed curve) determined by the least square method areshown in (a).

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OKA AND SUGA: NPSTMW FORMATION REGION IN WINTER 2003

brings fresher surface water north of the KE to this region[Hanawa and Talley, 2001]. How, then, is the MLD
distribution in the formation region determined? The relation of the permanent thermocline depth, represented by the
12°C-isotherm depth as in Uehara et al. [2003], and SST to
MLD in the formation region is examined (Figure 4). MLD
is highly correlated with the 12°C-isotherm depth (coefficient 0.82) but is little correlated with SST ( 0.18). The
slope of the regression line between the 12°C-isotherm
depth and MLD is near unity, and MLD is approximately
equivalent to the 12°C-isotherm depth, minus 200db. Thus,
the distribution of MLD corresponds well with the underlying thermocline depth.
[18] The 12°C-isotherm depth in the formation region(not shown) significantly changes on a scale of a few
degrees, as MLD in the region changes. Therefore, thethermocline depth in the region is most likely governedby mesoscale eddies, which show high variability in the
Kuroshio recirculation region [Qiu, 2002]. It is thus suggested that the particularly deep mixed layer, around 148°,
153°, and 168°E, is associated with anticyclonic eddies,supporting the results of Uehara et al. [2003].

4. Summary and Discussion[19] Almost the entire STMW formation region was
sampled for a specific winter. The STMW formation region
east of Japan in the late winter of 2003 extends south of the
KE between 30°N and 35°N and as far east as 175°E, much
farther than recognized by previous studies based on climatology. It is characterized by a mixed layer with depths
greater than 200db and meridionally uniform temperature
between 16.5°C and 18.2°C, associated with a mixed layer
front at its southern edge. The mixed layer front is effectivein the subduction of a vertically homogeneous structure bylateral induction, according to subduction theory [e.g.,Marshall et al., 1993]. The meridional uniformity of themixed layer temperature helps to form a large amount ofsuch structure, specifically mode water.
[20] The eastern end of the formation region is also
marked by a mixed layer front at 175°E (Figure 1b), west
(east) of which MLD is greater (less) than 200db. Thisfeature has appeared only in the numerical models [e.g.,
Figures 7 and 8 in Xie et al., 2000] and has not been
captured by observation to date. This result demonstrates
the potential usefulness of the Argo float array, even for
detecting relatively small-scale structures. Although thefloats are to be distributed at an average spacing of threedegrees, they drift, sometimes making it possible to examine the detailed spatial variation of properties.
[21] The mixed layer in the STMW formation region
becomes colder, fresher, and denser toward the east, leading
to the formation of warmer, saltier, and lighter (colder,fresher, and denser) STMW in the western (eastern) partof the region. The spatial variation of MLD in the region
corresponds well with the underlying permanent thermocline depth. The particularly deep mixed layer, formed
locally in the region, is probably associated with anticyclonic eddies, supporting the results of Uehara et al. [2003].
As the distribution of mesoscale eddies changes from year
to year [e.g., Qiu, 2002], it may significantly affect the

MLD distribution in the formation region of each winterand, therefore, the volume of STMW formed each year.
[22] Acknowledgments. The authors are grateful to the captain, crew,
and technical staff of the R/V Mirai MR02-K06 cruise for their support for
the XCTD observations and float deployments. They also thank N. Shikama
for kindly supplying the SAGE float data. They also thank their colleagues
at the Frontier Observational Research System for Global Change and theJapan Marine Science and Technology Center for helpful discussions.
Finally, they thank constructive comments from the two anonymous
reviewers and English editing kindly made by ‘‘Reviewer #2’’. This work
was supported by ‘‘The ARGO Project—Advanced Ocean Observing
System’’ as one of the Millennium Projects of the Japanese Government.

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(okae@jamstec.go.jp)