AUGUST 2002
2299
AOKI ET AL.
Subsurface Subtropical Fronts of the North Pacific as Inherent Boundaries in the
Ventilated Thermocline
YOSHIKAZU AOKI, TOSHIO SUGA,
AND
KIMIO HANAWA
Department of Geophysics, Graduate School of Science, Tohoku University, Sendai, Japan
(Manuscript received 18 January 2001, in final form 28 December 2001)
ABSTRACT
The two subsurface temperature–density fronts and the accompanying eastward currents in the central and
southern part of the North Pacific subtropical gyre are identified using the repeat hydrographic sections in the
western part of the gyre and the sections over the whole gyre from the World Ocean Circulation Experiment
(WOCE) Hydrographic Program (WHP). The northern eastward current corresponds to what has been known
as the subtropical countercurrent (STCC). The countercurrent/front is located typically near 248N and extends
from the western boundary approximately to the international date line. The previous suggestion that the STCC
is located at the southern edge of North Pacific Subtropical Mode Water (STMW) is confirmed; the front appears
as the southern boundary of the lower potential vorticity (PV)/apparent oxygen utilization (AOU) waters on the
isopycnals within the STMW layer. The southern eastward current corresponds to what was recognized earlier
but has not been documented in detail. The southern countercurrent/front is located near 188N west of the date
line, but shifts to the north east of the date line. Its eastern limit is around 1658W at 258N. The southern front
corresponds to the southern boundary of the lower PV/AOU waters on the isopycnal surfaces of the wide s u
range including those of North Pacific Central Mode Water (CMW). While CMW seems to contribute largely
to this low PV/AOU layer, the contribution from other waters is still significant. Judging from the associated
PV/AOU features in the thermocline, both fronts can be regarded as inherent boundaries in the ventilated
thermocline, dividing the regions ventilated to distinctive degrees. Correspondence between the observational
features of the countercurrents/fronts and some of the previously proposed theories concerning the STCC are
discussed.
1. Introduction
A shallow eastward current accompanied by a subsurface temperature/density front has been recognized
in the central to southern latitudes of the North Pacific
subtropical gyre as the subtropical countercurrent
(STCC; Uda and Hasunuma 1969), where a general flow
toward the west is expected according to the classical
wind-driven circulation theory. The STCC runs from
1228E to at least 1608E nearly along the tropic of Cancer
and shifts slightly to the north downstream. Another
eastward current along 188N was reported by Nitani
(1972) who investigated statistically the distribution of
zonal components of the surface geostrophic flow with
respect to latitudes along 15 meridional sections across
the western subtropical gyre. The two eastward currents,
the STCC and the other to the south of it, were confirmed by Hasunuma and Yoshida (1978) based on both
the synoptic and the long-term mean maps of the dynamic topography in the western North Pacific.
Corresponding author address: Dr. Toshio Suga, Department of
Geophysics, Graduate School of Science, Tohoku University, Aobaku, Sendai 980-8578, Japan.
E-mail: [email protected]
q 2002 American Meteorological Society
In spite of the early recognition, these two eastward
currents have not been further clarified from the observational point of view. This is at least partly because
of the large variability in the upper-flow field there. For
example, satellite altimetry reveals a regional maximum
of the sea surface height variability in the zonal band
from 198 to 258N, which extends from the western
boundary region to the Hawaiian Islands (Qiu 1999;
Kobashi and Kawamura 2001), indicating that energetic
mesoscale eddies dominate this region. These eddies
make it difficult to interpret an eastward flow in a given
meridional section as one of the eastward currents reported earlier. Consequently, the overall distribution of
the shallow currents in the central and southern part of
the western subtropical gyre is still not understood satisfactorily.
On the other hand, as the STCC has been regarded
as one of the permanent features of the upper flow in
the subtropical gyre, quite a few authors have theorized
about formation mechanisms of STCC and the accompanying front. Yoshida and Kidokoro (1967a,b) indicated that a trough in the wind stress curl found in the
lower subtropical latitudes could cause the eastward current. Several authors relate the STCC and its accompanying front to Ekman convergence induced by the
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JOURNAL OF PHYSICAL OCEANOGRAPHY
prevailing westeries and the trade winds (e.g., Roden
1975; Welander 1981; Cushman-Roisin 1981). However, neither of these mechanisms were essential for
formation of the STCC in Takeuchi’s (1984b) numerical
experiment. He succeeded in reproducing the eastward
current in a multilevel numerical model driven either
by the wind stress curl without a trough or by a hypothetical meridional wind stress whose curl field is
similar to that of the actual wind stress.
Alternatively, geostrophic current convergence has
been considered as a possible cause of the countercurrent and the front by several authors (Cushman-Roisin
1984; Takeuchi 1984a; Dewar 1992; Kubokawa 1995,
1997). These theories show that the subtropical frontlike
structure can occur through the geostrophic current convergence if some specific density structure is imposed
on a boundary, that is, a surface boundary, an eastern
boundary, or a western boundary. While the earlier theories by Cushman-Roisin (1984) and Takeuchi (1984a)
depend on the rather artificial eastern boundary ventilation, a theory excluding the eastern boundary ventilation was firstly proposed by Dewar (1992). He used
a two-layer planetary geostrophic ventilated thermocline model to discuss the frontogenesis caused by nonlinear Rossby waves and produced the subtropical countercurrent/front if the outcrop line slants northeastward.
Kubokawa (1995) expanded Dewar’s theory and showed
that the stationary Rossby wave emanating from the
western boundary region can also cause the subtropical
frontlike structure using a two-layer model, although a
somewhat unrealistic density structure is required for
the western boundary to be a source of midocean fronts.
Subsequently, however, Kubokawa (1997) formulated a
two-level model that can represent the density and flow
structure in the northwestern subtropical gyre more realistically. It was then shown that, if the northwestern
region has a small north–south gradient of surface density and weak vertical shear, the stationary Rossby wave
generated from this region possibly produces the subtropical countercurrent/front. Such structure in the
northwestern region compares well with Takeuchi’s
(1984b) numerical experiment.
Meanwhile, Kubokawa and Inui (1999) discussed the
generation mechanism of the STCC reproduced in an
ocean general circulation model with simple geometry,
driven by surface wind stress and surface buoyancy
forcing. According to their analysis, the low potential
vorticity (PV) fluid formed around the intersection of
the outcrop line and the mixed layer front defined as
the narrow transition zone of the mixed layer depth in
the northern subtropical gyre. When advected southwestward, the low PV fluids on different isopycnals tend
to converge in the horizontal plane and pile up vertically. As a result, a thick subsurface low PV pool is
formed in the central subtropical gyre. This thick layer
elevates the base of the surface layer, and a countercurrent occurs along the southern edge of this pool.
Using an ideal fluid thermocline model, Kubokawa
VOLUME 32
(1999) examined the influence of the mixed layer depth
distribution on the structure of the ventilated thermocline and supported Kubokawa and Inui’s scenario.
From the observational point of view, the low PV
fluid north of the STCC has indeed been noticed. Uda
and Hasunuma (1969) reported that the STCC flows
along the southern edge of the North Pacific Subtropical
Mode Water (STMW), which is the vertically homogeneous water mass in the northwestern subtropical gyre
(Masuzawa 1969). Suga et al. (1989) further suggested
the importance of STMW in maintenance of the STCC
by analyzing repeat hydrographic data along the 1378E
meridian. While these early observational views were
based on rather limited data, the low PV water north of
the countercurrent/front both in the ocean general circulation models by Takeuchi (1984b) and Kubokawa
and Inui (1999) and in the analytical models by Kubokawa (1997) and Kubokawa (1999) resembles STMW
to some extent.
It should be noted, however, that the model setup and
the interpretation of the low PV are somewhat different
among those models. Especially, Kubokawa (1997)
modeled the low PV water as originating near the northwestern corner of the subtropical gyre and showed that
its southward/southwestward intrusion maintained the
STCC. On the other hand, Kubokawa and Inui (1999)
found low PV water forming along the mixed layer front
extending well into the Sverdrup interior. The STCC in
their model was maintained by the stack of the low PV
water with different densities from different locations
along the mixed layer front. In order to examine whether
the STCC in those models are related to that in the real
ocean, we need a better description of the STCC, the
accompanying front, and the associated low PV water
based on observation. Moreover, although none of those
models distinguish the two separate countercurrents/
fronts, better observational description of these frontal
features would help to relate more clearly some models
to one front and others to the other.
The present paper reexamines the above observational notions in a more systematic way, using more
comprehensive hydrographic data, to clarify the relationship between the countercurrent/front and the low
PV water. We confirm the existence of two distinct subsurface temperature/density fronts accompanied by the
respective low PV water to the north: the northern front
corresponding to the STCC front of Uda and Hasunuma
(1969) and the southern one corresponding to the front
reported by Nitani (1972) mentioned above. These two
fronts are referred to as the northern and southern subsurface subtropical fronts in the present study. Since our
analysis is confined in the central and southern part of
the subtropical gyre, we do not intend to test the entire
scenario of STCC generation proposed by Kubokawa
(1997) or Kubokawa and Inui (1999). Our purpose here
is to examine whether the countercurrent/front is an inherent boundary in the ventilated thermocline suggested
by those authors.
AUGUST 2002
TABLE 1. Hydrographic repeat sections used in the present study.
Lon
1308E
1378E
1448E
1558E
Time of the
year
Jun
Jan
Jun–Aug
Feb
Jun
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AOKI ET AL.
Duration
Maintained by
1974, 1980–86
1967–98
1972–96, 1998
1984–96
1973, 1975–77,
1989–93
JMA
JMA
JMA
HD/JCG
JMA
The remainder of this paper is organized as follows.
The data and analytical methods used in the present
study are explained in section 2. In section 3, the repeat
hydrographic sections in the western part of the subtropical gyre are analyzed to identify the northern and
southern subsurface subtropical fronts and extract the
mean, or typical, thermocline structure associated with
those fronts. In section 4, the two fronts are identified
also in the recent high-resolution hydrographic sections
over the North Pacific to demonstrate their basinwide
characters. In section 5, we discuss implications of our
observations of the subsurface subtropical fronts and
their associated thermocline structure relative to the previously proposed theories about the subtropical countercurrents/fronts, followed by concluding remarks.
2. Data and processing
Three datasets are used in the present study. The first
comes from several repeat hydrographic sections meridionally crossing the western North Pacific, maintained by the Japan Meteorological Agency (JMA) and
the Hydrographic Department of Japan Coast Guard
(HD/JCG) as listed in Table 1. The station spacing of
these repeat sections is close to one degree in latitude.
The second set consists of available data from the World
Ocean Circulation Experiment (WOCE) Hydrographic
Program (WHP) listed in Table 2. Finally, we prepared
an isopycnally averaged climatology by using North Pacific HydroBase (Macdonald et al. 2001). While the
original climatology by Macdonald et al. is based on
their quality-controlled version of observed bottle data
from World Ocean Atlas 1994 (Levitus 1994), and part
of the WHP data, we updated the climatology by adding
other WHP data and the CTD data from NOAA Pacific
Marine Environmental Laboratory (PMEL) cruises
(Johnson and McPhaden 1999).
The hydrographic data from the Japanese repeat sections are low-resolution profiles consisting of discrete
samples. Each profile of temperature, salinity, or oxygen
is interpolated onto a vertical grid with a 10 dbar interval
using a shape-preserving local spline (Akima 1970) after visual quality check. The WHP data are high-resolution profiles. To reduce small-scale variability, individual profiles are smoothed with the 10 dbar half-width
Hanning filter and subsampled at 10-dbar intervals. The
resultant 10-dbar gridded profile data both from the re-
TABLE 2. WHP sections used in the present study.
Cruise
Lon
P9
P10
P13C
P14N
P15N
P16N
P16C
P17C
1378E
1498E
1658E
1798E
1658W
1538W
Typical
station
spacing
Survey period
309
409
18
309
309
409
309
309
Jul–Aug 1994
Oct–Nov 1993
Aug–Oct 1991
Jul–Sep 1993
Oct–Nov 1994
Mar 1991
Sep 1991
Jun–Jul 1991
1358W
peat section and WHP are used to calculate several derived properties at 10-dbar intervals. The resultant profiles of the observed and derived properties are further
interpolated linearly onto a series of isopycnal surfaces
with a 0.05s u interval.
The main tools for delineating the ventilated thermocline structure associated with the subsurface subtropical fronts in our analysis are PV and apparent oxygen utilization (AOU). A finite difference form of PV,
PV 5
f Dsu
,
r Dz
is calculated at each depth level using the fixed depth
increment Dz 5 60 m, where D s u is the variable potential density increment, f is the Coriolis parameter,
and r is the in situ density. AOU is the difference in
oxygen concentration of a water parcel from its saturation value. AOU has a tendency to increase due to the
consumption of oxygen by organic process after isolation from the atmosphere.
Geostrophic velocities are estimated relative to 1000
dbar for each station pair of the repeat sections using
the 10-dbar gridded profiles. The total eastward transports associated with the subsurface subtropical fronts
are evaluated by integrating all the eastward speeds over
the sectional area from the sea surface to 1000 dbar
within a 58 latitudinal band centered at the front.
3. Mean structure of the thermocline associated
with the subsurface subtropical fronts
Our purpose is to examine ‘‘typical’’ thermocline
structure associated with the subsurface subtropical
fronts. This is not, however, an easy task because mesoscale features and/or other spatiotemporal variability
tend to hide such typical structure. In fact, it is often
difficult even to identify the countercurrent/front relevant to the large-scale current structure. In order to extract the robust thermocline structure associated with
the fronts, we construct mean sections along several
meridians with the use of the repeat hydrographic observations (Table 1). For preserving the frontal structure
in the mean fields, we calculate the means with respect
to the frontal coordinate system where the front in a
given individual section is regarded as the origin of the
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VOLUME 32
FIG. 1. (a) Potential temperature section along 1558E in June 1977. Inverted triangles denote
the locations of the fronts. (b) Section of potential temperature standard deviations over the nine
observations along 1558E as listed in Table 1.
along-section coordinate for that particular section (e.g.,
Hanawa and Hoshino 1988). The actual procedure to
construct the mean sections is described below, followed
by presentation of the typical or robust thermocline
structure associated with the fronts.
An example of the synoptic section of potential temperature u along 1558E is shown in Fig. 1a. This example is the case where we can identify the two subsurface fronts unambiguously. The northern front is
found at 24.58N in the depth range of 50–300 m, and
the southern one at 18.58N in the depth range 100–250
m. When we averaged all nine available u sections along
1558E with respect to the geographical coordinate rather
than the frontal coordinate, the two fronts were still
discernible but considerably broadened and smoothed
(not shown). The section of the accompanying standard
deviations of u (Fig. 1b) captures the maxima near the
two fronts (Fig. 1a), illustrating that the fronts change
their positions from one section to another and thus the
frontal coordinate is beneficial.
To apply the frontal coordinate, we have to identify
the relevant fronts in individual sections. After some
trials, we chose the criteria to detect the fronts as follows. The northern (southern) front is defined as the
maximum in the magnitude of meridional gradients of
100–200 m depth averaged u in the latitudinal range
208–268N (148–208N). An example is given in Fig. 2 to
demonstrate how these criteria perform for the same u
section as shown in Fig. 1a. The meridional gradients
are evaluated between each adjacent pair of stations.
Note that temperature fronts are characterized by large
negative gradients since the meridional axis is positive
northward. The northern (southern) maximum magnitude is found at the same latitude as that of the northern
(southern) front identified by visually inspecting the u
section as described above. Furthermore, since the meridional gradients of vertically averaged u are virtually
comparable to the vertical shear of zonal velocity, the
maximum magnitude of negative gradients corresponds
with the upper eastward velocity maximum (Fig. 2a). It
should be also noted that similar criteria based on s u
give practically the same results as those using u (Fig.
2c) because salinity contributes little to the upper ocean
density field in the subtropics. Finally, we exclude from
the calculation of the means a few sections that have
only subtle gradient maxima with magnitudes smaller
than 18C/deg lat in order to ensure that the resulting
means adequately represent the frontal structure. The
mean properties of the fronts identified along each repeat section such as their latitudes, magnitudes of meridional gradients of the vertically averaged u, and associated total eastward transports are summarized in Tables 3 and 4. Since the whole procedure described above
is fairly mechanical, we may occasionally fail to detect
the right front among other mesoscale features. However, we emphasize that even the analysis based on these
simple criteria of ‘‘mere thermal fronts’’ reveal a robust
thermocline structure associated with the fronts.
The mean sections with respect to the northern front
along 1558E (Fig. 3) show the front as a sharp northward
shoaling of the upper isopycnals above 250 m, which
accounts for the maximum eastward geostrophic velocity, greater than 20 cm s 21 relative to 1000 dbar. A
relatively thick layer lies beneath the upper isopycnals
to the north of the front. This vertical PV minimum
layer is characterized by PV less than 2 3 10 210 m 21
s 21 at 25.3–25.6 s u centered at 25.4 s u . This low PV
water corresponds to STMW formed in the Kuroshio
Extension region to the north of this site (Suga and
Hanawa 1995). That is, the early notion of the corre-
AUGUST 2002
2303
AOKI ET AL.
FIG. 2. Meridional profiles, observed along 1558E in June 1977,
of (a) zonal geostrophic velocity at 50-m depth relative to 1000 dbar,
(b) meridional gradient of the potential temperature averaged vertically from depth 100 m to 200 m, and (c) meridional gradient of s u
averaged vertically from 100 to 200 m.
spondence between STCC and the southern edge of
STMW (Uda and Hasunuma 1969; Suga et al. 1989) is
confirmed in terms of the mean fields.
The AOU distribution in the mean section (Fig. 3d)
further supports this view. The sharp downward increase
of AOU below 26.3–26.5 s u suggests that this is the
base of the ventilated thermocline. The general southward increase of AOU along each isopycnal indicates
that the more aged water is in the south, as expected
from the ventilated thermocline theory (e.g., Luyten et
al. 1983; Huang and Russell 1994). As for the STMW
layer, the southward increase of AOU is not monotonic
but considerably sharper just north of the front, indicating the southern edge of the newer STMW there. In
addition, it is not surprising that AOU at the isopycnals
above the STMW layer rapidly decreases upward because those isopycnals are close to the sea surface.
Similar to the northern front, the mean sections along
1558E with respect to the southern front (Fig. 4) show
a sharp northward shoaling of the upper isopycnals
above 250 m. The eastward velocity of the countercurrent reaches a similar value, greater than 20 cm s 21 , to
that associated with the northern front. Thicker water
lies beneath the shoaling isopycnals north of the front,
compared with the water south of the front in the same
density range. The PV section captures the remarkable
character of the thermocline associated with the southern front. That is, the southern front coincides with the
sharp front of the isopycnal PV at 25.3–26.3 s u and the
southern edge of the lower PV water. The lower PV
water north of the front is above the sharp downward
increase in AOU at 26.3 s u that suggests the base of
the ventilated thermocline. The noticeably sharp southward increase in AOU especially around 25.5–26.3 s u
suggests that the front marks the boundary between the
differently ventilated regions. It should be noticed that
the lowest portion, around 26.0–26.3 s u , of the water
north of the southern front corresponds to North Pacific
Central Mode Water (CMW: Nakamura 1996; Suga et
al. 1997) whose origin is near the northern edge of the
subtropical gyre.
Although the correspondence between the southern
front and the southern edge of the CMW is similar to
that between the northern front and the southern edge
of STMW, there is a marked difference between the two.
The thick water responsible for the shoaling of the upper
isopycnals north of the southern front consists of not
only CMW but also the low PV fluids of a fairly wide
s u range, while the thick water north of the northern
front consists solely of STMW within a narrow s u range.
TABLE 3. Mean properties of the northern subsurface subtropical front and associated standard deviations.
Lon
Number of
observationsa
1308E
1378E
1448E
1558E
7/8
52/58
13/13
8/8c
Location (8N)
22.5
22.3
22.9
23.8
6
6
6
6
1.8
1.7
1.7
1.5
Magnitude of ]ū/]y
(8C/deg)
1.9
2.4
2.1
2.5
6
6
6
6
0.4
0.9
0.6
1.0
Eastward transportb
(Sv)
su range of low PV
water north of the
front
6
6
6
6
25.3–25.7
25.3–25.7
25.3–25.6
25.3–25.6
12.9
10.2
7.8
8.0
3.9
6.4
3.5
3.0
(Number of observations used for calculating the means)/(Total number of observations).
Total eastward transport evaluated by integrating all the eastward speeds relative to 1000 dbar over the sectional area from the sea surface
to 1000 dbar within a 58 lat band centered at the front.
c
One cruise has been excluded because of its larger station spacing.
a
b
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VOLUME 32
TABLE 4. Mean properties of the southern subsurface subtropical front and associated standard deviations.
Lon
Number of
observationsa
1308E
1378E
1448E
1558E
6/8
48/58
12/13
9/9
Location (8N)
17.2
17.6
18.0
18.3
6
6
6
6
1.6
1.5
0.8
0.7
Magnitude of ]ū/]y
(8C/deg)
2.1
2.1
2.1
2.4
6
6
6
6
0.6
0.8
0.7
0.6
Eastward transportb
(Sv)
su range of low PV
water north of the front
6
6
6
6
25.2–26.0
25.2–26.0
25.2–26.1
25.3–26.3
5.1
7.1
5.2
5.4
6.3
5.0
1.8
3.0
(Number of observations used for calculating the means)/(Total number of observations).
Total eastward transport evaluated by integrating all the eastward speeds relative to 1000 dbar over the sectional area from the sea surface
to 1000 dbar within a 58 lat band centered at the front.
a
b
FIG. 3. Mean sections along 1558E based on the frontal coordinate with respect to the northern
front: (a) s u , (b) eastward geostrophic velocity (cm s 21 ) relative to 1000 dbar, (c) potential vorticity
(10 210 m 21 s 21 ), and (d) AOU (ml l 21 ). Contour interval in (b) is 5 cm s 21 with the negative
contours broken and the zero contours thickened. Potential vorticity less than 2 3 10 210 m 21 s 21
is shaded in (c).
AUGUST 2002
AOKI ET AL.
The stack of low PV fluids in a rather wide s u range is
similar to that in the scenario proposed by Kubokawa
and Inui (1999).
The mean sections with respect to the northern and
southern fronts at 1308E, 1378E, and 1448E are constructed in the same manner as for the 1558E section.
They reveal essentially the same thermocline structure
associated with the fronts as found in the 1558E section.
The northern front is characterized as the southern
boundary of the thick and thus low PV water corresponding to STMW within a relatively narrow s u range,
while the southern front is characterized as the southern
boundary of the low PV water spanning a wider s u
range. The s u ranges of the low PV water north of the
two fronts at each repeat section are summarized along
with the other mean properties in Tables 3 and 4.
Since the periods over which the mean values were
calculated are different from one section to another, it
is difficult to deduce a meaningful mean zonal variation
in frontal properties, so we discuss this matter only briefly. STMW north of the northern front appears to diffuse
from east to west; its s u range becomes wider and the
minimum PV at its core increases to the west (not
shown), as partly captured by its wider s u range in the
1378E and 1308E sections (Table 3). The westward
weakening of the low PV signature is consistent with
Suga and Hanawa’s (1995) observation that STMW dissipates considerably along its southwestward/westward
advective path within a year from its formation. In contrast to the westward weakening of STMW, the eastward
transport increases to the west, while the magnitude of
the 100–200 m averaged temperature gradient at the
front shows no systematic zonal trend. The larger transport in the west may be an artifact due to the higher
eddy activity there (e.g., Aoki and Imawaki 1996). If
there are more eddies in the west, we may inadequately
regard the northern flank of anticyclonic eddy as the
front more frequently there. Since anticyclonic eddies
tend to have more intense ‘‘eastward currents’’ than the
actual countercurrent, the mean transport can be overestimated in the west. Nevertheless the mean eastward
transport range of 8 to 13 Sv (Sv [ 10 6 m 3 s 21 ) corresponds moderately to the 8–18 Sv estimated by Uda
and Hasunuma (1969). On the other hand, the mean
properties of the southern front appear much the same
for all four repeat sections (Table 4).
Finally, two other features are revealed by the frontal
composites. The first is the difference in vertical structure of the two fronts. The eastward geostrophic velocity
at the northern front at 1558E remains greater than 1
cm s 21 down to 500 m and close to zero below (roughly
captured in Fig. 3b) as implied by the isopycnals shoaling northward at 250–500 m and flattened below that
(Fig. 3a). On the other hand, the isopycnals at the southern front are almost flattened at 400 m and deepened
northward below that (Fig. 4a). As a result, the eastward
flow associated with the front is confined above 200 m
and the flow underneath is westward. The westward flow
2305
has a maximum speed of 4 cm s 21 at 400 m (not shown).
A similar difference in the vertical structure of the flow
associated with the two fronts is observed for all four
repeat sections (not shown). This feature may reflect a
difference in the formation mechanisms of the two
fronts.
The second point is that the southern (northern) frontal features appear on the composites of the northern
(southern) front. For example, the PV and AOU gradients at 268 latitude in Figs. 3c and 3d are associated
with the southern front, and the PV minimum north of
68 centered at 25.4–25.5 s u (Fig. 4c) is associated with
the northern front. A similar tendency is observed along
the other repeat sections (not shown). The concurrent
appearance of the two fronts on the composites indicates
that the meridional excursions of the two fronts tend to
preserve the relative distance between the two fronts.
These two features are interesting but beyond the scope
of the present study and thus left for future work.
In conclusion, based on the mean sections, we found
two subsurface subtropical fronts in the western subtropical gyre. The northern and the southern countercurrents/fronts are distinctive in the sense that their thermocline structures differ. Both fronts are zonally connected features, within the study area from 1308E to
1558E.
4. Basinwide distribution of the subsurface
subtropical fronts
The repeat hydrographic sections captured the northern and southern subsurface subtropical fronts and the
accompanying thermocline structure for the western part
of the subtropical gyre. Our purpose in the present section is to clarify the spatial extent of these two fronts
over the North Pacific. More specifically, we aim to
determine how far east we can identify these two fronts
accompanied by the same thermocline structure specified in the preceding section. While it is often difficult
to identify the subtropical fronts in a given synoptic
section, as mentioned above, the typical thermocline
features associated with the fronts provide criteria to
identify those fronts to a certain degree. We examine a
series of WHP meridional sections (Table 2). Every
frontal feature in the thermocline in each u section was
checked to see if it was the southern boundary of low
PV/AOU water. If it was, the feature was identified as
the subsurface subtropical front. The locations of the
northern and southern fronts identified in the WHP sections are shown in Fig. 5 along with their mean positions
in the repeat sections. The s u ranges of the low PV/
AOU water north of the fronts, which identify those
fronts in each section, are listed in Table 5.
The northern front, characterized by a thick layer of
STMW to the north, is detected on P9 (1378E), P10
(1498E), P13C (1658E), and P14N (1798E) but not on
the other sections farther east. The PV and AOU features
in the thermocline similar to those depicted in the mean
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VOLUME 32
FIG. 4. As in Fig. 3 but with respect to the southern front.
sections are found in the P9 and P10 sections, which
are within the longitudinal range covered by the repeat
lines (figures not shown). Similar features characterized
by the thick STMW of low PV and low AOU also appear
north of the front in the P13C section (Fig. 6a), which
is located outside the area covered by the repeat sections. As for the P14N section (Fig. 6b), the signature
of STMW in the PV and AOU fields is less marked but
still apparent north of 258N. We thus regard the front
at the STMW’s southern edge as the northern front.
STMW is absent from the sections farther east (Figs.
6c,d), which is consistent with the previous description
of STMW (e.g., Masuzawa 1969; Hanawa 1987; Talley
1988; Suga et al. 1997; Yasuda and Hanawa 1997).
Therefore it is reasonable to conclude that the eastward
extent of the northern front characterized as the southern
limit of STMW is somewhere between the international
date line and 1658W.
Kaneko et al. (1998) analyzed the P9 section and
identified three eastward currents at 188N, 228N, and
248N. Looking at temperature at 100–200 m, they regarded the eastward current at 228N as the STCC defined
by Uda and Hasunuma (1969). The 228N front also has
the deepest signature in the thermocline among the
three, reaching to 1000 m. On the other hand, we chose
the 248N front as it corresponds with the southern limit
of STMW. We do not think, however, that the difference
in the interpretation is critical because there may be
some eddies to hide the ‘‘typical’’ structure.
The southern front, characterized as the southern edge
of the lower PV/AOU with a relatively wide range of
s u , is detected at P9 (1378E), P10 (1498E), P13C
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AOKI ET AL.
FIG. 5. Locations of the northern (open symbols) and southern (closed symbols) subsurface
subtropical fronts along the repeat sections and the WHP sections. The mean locations of the
fronts along the repeat sections are indicated by circles with the bars spanning twice the standard
deviation. The locations of the fronts along the WHP line are indicated by stars. Stations along
each section are shown by dots.
(1658E), P14N (1798E), and P15N (1658W) but not in
the other sections farther east. As in the case of the
northern front, the two sections (P9 and P10) within the
longitudinal range covered by the repeat sections show
the features of PV and AOU similar to those found in
the mean sections associated with the southern front
(figures not shown). The southern front is also clearly
identified along 1658E (Fig. 6a) and 1798E (Fig. 6b)
accompanied by low PV/AOU to the north. Especially
the southern front at 1798E corresponds to a remarkable
front in AOU, reflecting the southern front’s role as the
southern boundary of well-ventilated water in the thermocline.
The PV distribution associated with the southern front
along 1798E is noticeably different from that along the
other sections to the west. Unlike the other western
sections, the 1798E does not show the broad meridional
PV minimum extending from the southern front to the
north on the isopycnals centered at 26.0–26.3 s u . The
PV at these isopycnals along 1798E monotonically decreases to the north, which means the low PV water
corresponding to CMW apparently recedes to the north.
As a result, the pile of the low PV waters of different
s u is not as vertically aligned as observed in the western
section.
This tendency for the low PV waters of different s u
to stack unevenly becomes much more enhanced along
1658W (Fig. 6c). While the PV between 25.6 s u and
26.3 s u gradually decreases from 158–178N to the north,
we identify the southern front at 24.58N where the relatively sharp northward decrease of isopycnal PV and
AOU at 25.6–26.3 s u occurs. The sections farther to the
east do not show the sharp meridional changes in the
isopycnal PV that characterize the southern front (Fig.
6d). We thus conclude that the eastward extent of the
southern front is somewhere between 1658W and
1538W.
The locations of the two fronts identified above are
plotted on the climatological maps of isopycnal PV on
25.4 s u for the northern front and on 26.0 s u for the
southern front (Fig. 7). The former isopycnal corresponds with the core of STMW. The northern front is
consistently located along the southern boundary of the
low PV region. The latter isopycnal corresponds to the
central isopycnal of the rather wide s u range of the low
PV/AOU water and also to the upper portion of CMW.
TABLE 5. The northern and southern subsurface subtropical fronts in the WHP sections.
Northern front
WHP section
Location
su range of low PV
water north of the front
P9(1378E)
P10(1498E)
P13C(1658E)
P14N(1798E)
P15N(1658W)
P16N/C(1538W)
P17C(1358W)
24.08N
21.58N
24.58N
25.08N
—
—
—
25.0–25.5su
25.2–25.5su
25.3–25.6su
25.4–25.6su
—
—
—
Southern front
Location
su range of low PV
water north of the front
18.58N
17.08N
17.58N
17.08N
24.58N
—
—
25.1–25.9
25.2–26.1
25.3–26.3
25.6–26.3
25.6–26.3
—
—
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VOLUME 32
FIG. 6. WHP sections of s u (left panel), potential vorticity (10 210 m 21 s 21 : middle panel) and AOU (ml l 21 : right panel) along (a) P13C
(1658E), (b) P14N (1798E), (c) P15N (1658W), and (d) P17C (1358W). The locations of the fronts in each section are indicated by inverted
triangles. Potential vorticity less than 2 3 10 210 m 21 s 21 is shaded.
AUGUST 2002
AOKI ET AL.
2309
FIG. 7. Locations of (a) the northern front and (b) the southern front superposed on the climatological maps of potential vorticity on the 25.4 s u and 26.0 s u surfaces, respectively. The
areas north of the wintertime outcrops are hatched.
The southern front is also consistently located along the
southern edge of the low PV region. The correspondence
between the fronts and the two dimensional extent of
low PV region generally supports the eastern limits of
the two fronts inferred above.
5. Discussion and concluding remarks
The two previously documented subsurface temperature/density fronts and their accompanying eastward
currents in the central and southern North Pacific subtropical gyre are identified using repeat hydrographic
sections in the western part of the gyre and the WHP
sections over the whole gyre. The northern countercurrent corresponds to the STCC reported by Uda and Hasunuma (1969). The countercurrent/front is located typically near 248N and extends from the western boundary
to the international date line. The early suggestion that
the STCC is located at the southern edge of the STMW
is confirmed. The front appears as the southern boundary of the lower PV/AOU on the isopycnals within the
STMW layer. The southern countercurrent corresponds
to the eastward current reported by Nitani (1972). The
countercurrent/front is located near 188N west of the
date line, but shifts northward east of the date line. Its
eastern limit is around 1658W near 258N. The southern
front corresponds to the southern boundary of the lower
PV/AOU waters of a wide s u range. While CMW contributes significantly to this low PV/AOU layer, the contribution from other waters is still significant.
We now discuss what these observations imply for
previously proposed theories about the formation mechanism of these fronts. Although the theories and numerical simulations focusing on the STCC have been
aimed at the single countercurrent/front noted by Uda
and Hasunuma (1969), the present study captures two
distinct countercurrents/fronts with associated features
in the thermocline. This does not mean, however, that
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JOURNAL OF PHYSICAL OCEANOGRAPHY
the previous theories are unrelated to the fronts in the
real ocean.
The southern countercurrent/front, corresponding to
the eastward current noted by Nitani (1972), is located
along the southern edge of low PV/AOU water vertically
spanning a range of s u , as in Kubokawa and Inui’s
(1999) numerical simulation and Kubokawa’s (1999)
analytical model. In their models, the low PV fluids of
different densities subduct from the northern zone of
the subtropical gyre; those on lighter isopycnals originate from the western region and those on denser isopycnals from the eastern region. Advected along the
anticyclonic gyre, the low PV fluid on each isopycnal
overlaps that on another isopycnal and makes a thick
low PV pool in the western central subtropical gyre.
The low PV fluids push up the isopycnals above, causing
the eastward current at the southern edge of the pool.
We also observe the low PV pile is less vertically coherent in the WHP sections upstream of the west/southwestward flow, which is also favorable for their scenario. However, since we examine only the southern
and central part of the subtropical gyre, we cannot tell
whether the low PV fluids are formed at the intersection
of the mixed layer front and the outcrop line.
The northern countercurrent/front is located along the
southern edge of the thick STMW layer. STMW has
much more pronounced vertical homogeneity compared
with the low PV water associated with the southern
front. Its spatial distribution is more confined in the
northwestern part of the gyre compared with the southern counterpart. These features match the countercurrent/front in Kubokawa’s (1997) two-level model. Kubokawa modeled STMW as the dense and vertically and
horizontally homogeneous fluid in the northwestern corner of the subtropical gyre, from which a stationary
Rossby wave emanates to produce the countercurrent.
This homogeneous fluid resembles the observed STMW
much better than the low PV fluids with wide s u range
found in Kubokawa and Inui’s (1999) simulation. Our
observations thus suggest that the formation mechanism
proposed by Kubokawa (1997) is more likely for the
northern front.
The observed features of the two fronts and the related theories of the frontogenesis suggest that both
fronts are lateral boundaries in the ventilated thermocline. It is thus implied that variability of these fronts
can be related to variations in the thermocline circulation, the subduction or water mass formation processes
associated with the wintertime air–sea interaction, etc.
It is also implied that numerical models of the upper
circulation and water properties should reproduce these
fronts in order to simulate the real ocean better. As
reviewed in Hanawa and Talley (2001), STMW exists
in all the subtropical gyres of the World Ocean. Therefore, it is expected that all the subtropical gyres may
have an STCC. Whether it is true or not is another good
subject to be addressed in future study.
The present results are mostly based on meridional
VOLUME 32
sections. Further clarification of the formation mechanisms of the fronts may be possible through analysis of
the three-dimensional PV/AOU structure of the ventilated thermocline. Such analysis with the full use of the
North Pacific HydroBase is now under way.
Acknowledgments. We thank the members of the
Physical Oceanography Group, Tohoku University for
helpful comments and useful discussion. The Japanese
repeat hydrographic data were obtained through JODC
Data Online Service System (J-DOSS) and Data Report
of Oceanographic Observation CD-ROM compiled by
JMA. Most of the WHP data were obtained through the
WHP Office. Dr. M. J. McPhaden kindly provided the
CTD data collected by NOAA/PMEL. Financial support
was provided by Japan Society for Promotion of Science
[Grant-in-Aid for Scientific Research (C) 10640418 and
(B) 13440138].
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