J Oceanogr (2015) 71:389–400
DOI 10.1007/s10872-015-0300-x

ORIGINAL ARTICLE

Decadal variability of Subtropical Mode Water subduction
and its impact on biogeochemistry
Eitarou Oka1 · Bo Qiu2 · Yusuke Takatani3,4 · Kazutaka Enyo3 · Daisuke Sasano3,4 ·Naohiro Kosugi4 · Masao Ishii3,4 · Toshiya Nakano3,4 · Toshio Suga5,6Received: 20 January 2015 / Revised: 14 May 2015 / Accepted: 15 May 2015 / Published online: 4 June 2015
© The Oceanographic Society of Japan and Springer Japan 2015

Abstract Temperature and salinity data from Argo profiling floats during 2005–2014 were analyzed to examine
the decadal variability of the North Pacific SubtropicalMode Water (STMW) in relation to that of the Kuroshio
Extension (KE) system. The formation volume of STMW
in the southern recirculation gyre of KE in the coolingseason was larger during the stable KE period after 2010
than the unstable KE period of 2006–2009 by 50 %. As aresult, the volume and spatial extent of STMW increased(decreased) in the formation region during the stable(unstable) KE period, as well as in the southern, downstream region with a time lag of 1–2 years. The decadal
expansion and contraction of STMW were also detectedby shipboard observations conducted routinely in the mostdownstream region near the western boundary, in termsof not only physical, but also biogeochemical parameters.
After 2010, enhanced subduction of STMW consistently
increased dissolved oxygen, pH, and aragonite saturation* Eitarou Oka
eoka@aori.u‑tokyo.ac.jp
1

Atmosphere and Ocean Research Institute, The Universityof Tokyo, Kashiwa 277‑8564, Japan2

Department of Oceanography, University of Hawaii
at Manoa, Honolulu, HI 96822, USA

3

Global Environment and Marine Department, Japan
Meteorological Agency, Tokyo 100‑8122, Japan

4

Oceanography and Geochemistry Research Department,
Meteorological Research Institute, Tsukuba 305‑0052, Japan

5

Department of Geophysics, Graduate School of Science,
Tohoku University, Sendai 980‑8578, Japan

6

Research and Development Center for Global Change,
Japan Agency for Marine-Earth Science and Technology,
Yokosuka 237‑0061, Japan

state and decreased potential vorticity, apparent oxygen
utilization, nitrate, and dissolved inorganic carbon, among
which changes of dissolved inorganic carbon, pH, andaragonite saturation state were against their long-term
trends. These results indicate a new mechanism consisting of westward sea surface height anomaly propagation,
the KE state transition, and the STMW formation and subduction, by which the climate variability affects physicaland biogeochemical structures in the ocean’s interior andpotentially impacts the surface ocean acidification trend
and biological production.
Keywords North Pacific Subtropical Mode Water ·Subduction · Decadal variability · Kuroshio Extension ·Biogeochemistry

1 Introduction
One of the most striking features revealed by the satellite
altimeter sea surface height data during the past 22 years isthe decadal variability of the Kuroshio Extension (KE) jet
and its associated eddy field (Qiu and Chen 2005; Qiu et al.
2007, 2014; Figs. 1, 2) in association with the Pacific Decadal Oscillation (PDO; Mantua et al. 1997). When the PDO
index is positive (negative), namely when the Aleutian Low
is stronger (weaker), negative (positive) sea surface heightand permanent pycnocline depth anomalies are generated
in the central North Pacific, and then propagate westward
at the speed of first-mode baroclinic Rossby waves. Whenthe anomalies reach the area east of Japan after 3–4 years,
KE turns into an unstable (stable) state, in which it takesa southerly (northerly) position and is accompanied by aweakened (strengthened) southern recirculation gyre andhigh (low) regional eddy activity.

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40°N
36°N
32°N
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40°N

1993

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36°N
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140°E

160°E

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140°E

160°E

140°E

150°E

160°E

Fig. 1  Yearly paths of the Kuroshio and KE plotted every 14 days using satellite altimeter sea surface height data, as in Qiu et al. (2014). KE
was in a stable state in 1993–1994, 2002–2005, and 2010–, and in an unstable state in 1995–2001 and 2006–2009

3

KE index

2
1
0
-1
-2
-3
1993

96

98

2000

02

Fig. 2  Time series of the KE index, defined by Qiu et al. (2014)
as the average of four KE related parameters, normalized by theirrespective standard deviations. A positive (negative) KE index means

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10

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14

that KE has a stable (unstable) and northerly (southerly) path, anincreased (decreased) surface transport, and a strengthened (weakened) southern recirculation gyre

Decadal variability of Subtropical Mode Water subduction and its impact on biogeochemistryThis decadal variability of the KE system influences
the formation, transportation, and modification of adjacentwater masses, such as Subtropical Mode Water (STMW;
Qiu and Chen 2006) and Central Mode Water (Oka et al.
2012), which account for the major portion of the upperand lower permanent pycnocline ventilation in the North
Pacific subtropical gyre (Suga et al. 2008). An initial analysis of temperature profiles during 1993–2004 revealed
that STMW formed in the southern recirculation gyre of
KE was thicker (thinner) during the stable (unstable) KE
period and that its thickness had little correlation withthe oceanic heat loss to the atmosphere during the previous fall–winter (Qiu and Chen 2006). These findings wereconfirmed by subsequent studies with more recent observations (Miyazawa et al. 2009; Sugimoto and Hanawa 2010).
The decadal STMW formation variability was attributed
to water exchange across KE through the detachment of
nonlinear mesoscale eddies (Qiu and Chen 2006); during
the unstable KE period, more cyclonic eddies are detachedfrom KE to the southern recirculation gyre and decay there(Sasaki and Minobe 2015), supplying water with relatively
high potential vorticity north of KE to hinder the formationof deep winter mixed layers. Shallower permanent pycnocline and stronger background stratification during theunstable period also likely inhibit the formation of thick
STMW (Sugimoto and Hanawa 2010).
Given the evident decadal STMW variability in its formation region, it is natural to ask how the KE variability
influences the entire STMW distribution in the western partof the North Pacific subtropical gyre, whose variability isimportant not only physically (e.g., Sugimoto and Hanawa
2005; Xie et al. 2011; Kobashi and Kubokawa 2012; Kobashi et al. 2013), but also biogeochemically (e.g., Bateset al. 2002; Palter et al. 2005; Krémeur et al. 2009). After
forming as deep mixed layers in the recirculation gyrenorth of 28°N in late winter (Oka and Suga 2003), STMW
is advected southward across the mixed layer depth front at
28°N and is thereby subducted to the permanent pycnocline(Oka 2009; Oka et al. 2011), due partly to mesoscale eddy
activities (Rainville et al. 2007; Nishikawa et al. 2010; Xu
et al. 2014). Vigorous STMW formation during the stable
KE period is likely to increase the entire STMW volume,
while higher eddy activity during the unstable KE period
might enhance the eddy subduction of STMW and increase
its volume in the southern region (Oka and Qiu 2012).
The entire STMW distribution region has been monitored by a sufficient number of Argo profiling floats(Roemmich et al. 2001) since around 2005, thanks to intensive and continuous float deployment by various countries.
The collected temperature and salinity data have clarified
detailed seasonal and interannual variability of STMW
and their relation to mesoscale eddies (Oka and Qiu 2012,
and references therein), and have greatly improved our391

understanding of STMW from the pre-Argo era when
only climatological data and limited repeat hydrographic
sections were available (e.g., Suga et al. 1989; Suga and
Hanawa 1990, 1995a, b). Now that float data with dense
spatial coverage have been accumulated for 10 years, it isworth exploring decadal variability of STMW in its entire
distribution region. In this paper, we analyze the Argo float
data during 2005–2014 to investigate the decadal STMW
variability and its relation to the decadal KE variability. We
also examine how the decadal STMW variability affects thephysical and biogeochemical structures in the downstream,
western boundary regions, based on long-term shipboard
observation data.
In the past 10 years, KE was in a stable state from 2002
through 2005, switched to an unstable state in 2006–2009,
and returned to the stable state in 2010 (Figs. 1, 2). The
analysis period of 2005–2014 includes both the stable andunstable KE periods of nearly equal time lengths and is,therefore, well suited to examine the relationship between
the decadal variability of STMW and that of KE.

2 Data and method
Temperature and salinity data from Argo profiling floats in
the North Pacific from December 2004 through December
2014 were downloaded from the ftp site of the Argo Global
Data Assembly Center (ftp://usgodae.org/pub/outgoing/
argo, ftp://ftp.ifremer.fr/ifremer/argo) and edited as outlined in Oka et al. (2007). After discarding profiles shallower than 500 dbar and those in marginal seas, we vertically interpolated each profile onto a 1-dbar grid using the
Akima spline (Akima 1970), and calculated potential temperature (θ), σθ, and Q. Here, potential vorticity Q is definedas Q = gf ∂σθ/∂p, neglecting relative vorticity (e.g., Talley
1988; Oka et al. 2011), where f is the Coriolis parameter, g
the gravity acceleration, and p is pressure.
We followed our previous studies (Oka 2009; Oka et al.
2011) to define STMW as a layer where Q is lower than
2.0 × 10−10 m−1 s−1 and θ at the Q minimum is between
16 and 19.5 °C, except for those existing only at depths
less than 100 dbar (in order to exclude spring–fall mixed
layers with a thickness of several tens of meters). After
detecting STMW in each float profile using this definition,
we mapped the STMW thickness for each month according to the following procedure. First, the thickness values
from float profiles in each 1° × 1° grid box were averaged
to obtain the representative value at the center of the gridbox (hereafter called grid point). Then, the mapped thickness at each grid point was calculated as the average ofrepresentative values at grid points within a search radiusof 1° from the target grid point, if two or more representative values existed within the search radius. If only one or

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no representative value existed, the search radius increasedby 1°, up to a maximum of 5°, until we obtained two or
more representative values. In such cases, the representative values were averaged using a weight function of d−2,where d is the distance in degrees from each grid point to
the target grid point. This interpolation procedure is similar to those used for the HydroBase climatology (Lozier
et al. 1995; Macdonald et al. 2001; Suga et al. 2004) and
was adopted to preserve the water mass structure in the KE
frontal region as much as possible.
We used temperature and salinity data measured witha conductivity–temperature–depth profiler (CTD) in 32
cruises during 2005–2014 and dissolved O2 data measured with a RINKO rapid-response sensor (JFE Advantech, Co.; Uchida et al. 2008) and calibrated with the datafrom discrete samples in 12 cruises after spring 2010 at
the OK line. The data at 13 offshore CTD stations withthe water depth exceeding 2000 m were analyzed. We
also used temperature, salinity, and biogeochemical datacollected with a CTD-carousel multi-sampler system in
45 cruises during 2002–2013 at a station at 25°N in the
137°E repeat hydrographic section. These observations
were conducted routinely two to four times a year by
R/V Chofu-maru, R/V Keifu-maru, and R/V Ryofu-maruof the Japan Meteorological Agency (JMA), and the collected data are publicly available through the JMA web
site(http://www.data.jma.go.jp/gmd/kaiyou/db/vessel_
obs/data-report/html/ship/ship_e.php). Quality-controlled
datasets including dissolved inorganic carbon (DIC)
data are also available in the Pacific Ocean Interior Carbon Database (http://cdiac.ornl.gov/oceans/PACIFICA/).
Methods for the measurements of dissolved O2 and DIC
were also explained in our previous studies (Ishii et al.
2011; Takatani et al. 2012).

3 Decadal variability of Subtropical Mode Water
STMW thickness in its formation region between the Kuroshio/KE and 28°N in late winter exhibited a significant decadal variation in relation to that of KE (Fig. 3). In 2005,
STMW thicker than 300 dbar was formed at 142–156°E
and 165–168°E, with a maximum thickness >400 dbar. In
the following 4 years (2006–2009) of the unstable KE, the
STMW thickness gradually decreased, as demonstrated bya recent study (Rainville et al. 2014), and its maximum fell
down to 300 dbar in 2009. After the KE returned to the stable state in 2010, the STMW thickness gradually increased,
and its values in 2014 returned to those observed in 2005.
Thus, the thickness increased (decreased) during the stable (unstable) KE period, rather than being large (small)as described in the earlier studies (Qiu and Chen 2006;
Miyazawa et al. 2009; Sugimoto and Hanawa 2010).

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Late winter mixed layer depth distributions exhibit
mixed layers deeper than 150 dbar only north of 28°N
(Fig. 4), indicating that STMW observed south of 28°N inlate winter was formed in the formation region in preceding winters and was then transported southward, as demonstrated in our previous studies (Oka 2009; Oka et al.
2011). This downstream STMW, which was subducted
and advected southwestward to the western boundary(e.g., Suga and Hanawa 1995a; Oka 2009), also showeda decadal variation (Fig. 3). STMW thicker than 50 dbar
broadly existed as far south as 18°N in 2005–2007, shrank
considerably in 2010–2011, and recovered thereafter. (Itsarea south of 28°N was 2.7 × 1012 m2 in 2005 and 2006,
reduced to a minimum of 0.8 × 1012 m2 in 2011, andreturned to 2.3 × 1012 m2 in 2014.) This corresponds to the
STMW thickness variability in the formation region with alag of about 1 year.
As the season progressed each year, STMW was gradually eroded, and concurrently, thick STMW formed in theformation region shifted southward (Fig. 5). In fall, the central latitude of the thick STMW shifted southward to about
28°N (Fig. 6), which was the southern limit of the STMW
formation region in late winter (Fig. 3). The STMW thickness in fall also fluctuated decadally (Fig. 6). STMW
thicker than 100 dbar was distributed broadly between 135°
and 177°E in 2005, reduced to almost zero until 2010, andrecovered after 2011.
These temporal variations of STMW are well quantified by its volume (Fig. 7a), which was computed fromthe monthly thickness distributions such as Figs. 3, 5,
and 6. The total STMW volume exhibited both a seasonalvariation, maximum in late winter (February–April) andminimum in fall (October–December), and a clear decadal variation characterized by a decrease in 2005–2009
and an increase after 2010. Its annual averages in 2005
(0.77 × 1015 m3) and 2014 (0.81 × 1015 m3) were twice
as much as that in 2009 (0.40 × 1015 m3). The STMW volume in the formation region north of 28°N showed a similar variation to that in the whole distribution region, whilethat in the downstream region south of 28°N fluctuateddecadally with a small seasonal variation, reaching a maximum in 2005–2006 and a minimum in 2010. As mentioned
above, the STMW volume in the downstream region lagged
about 1 year behind that in the formation region, indicating that low-frequency STMW variation in the formation
region was transmitted to the downstream region throughadvection.
The decadal variation of STMW volume reflected thatof the formation volume, which was calculated for eachyear as the difference between the maximum volume inlate winter and the minimum volume in the previous fallin the formation region north of 28°N. The formation volume averaged in 2010/11–2013/14 during the stable KE

Decadal variability of Subtropical Mode Water subduction and its impact on biogeochemistry
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120˚E130˚E140˚E150˚E160˚E170˚E 180˚ 170˚W

Thickness (dbar)
025

50 100 150 200 250 300

Fig. 3  Distributions of STMW thickness in March, based on Argo float data. Contours are drawn at 25 dbar and every 50 dbar. Thick white line
denotes the OK line

period was 0.52 × 1015 m3, and is larger by 53 % than that
(0.34 × 1015 m3) averaged in 2006/07–2008/09 during theunstable KE period (Fig. 7b). As discussed in the previous
studies (Qiu and Chen 2006; Sugimoto and Hanawa 2010),
more vigorous water exchange across KE and stronger
background stratification in the STMW formation region
during the unstable KE period are believed to weaken the
STMW formation. Smaller outcropping area due to thesoutherly KE position and to masking by cyclonic eddies(Rainville et al. 2007) during the same period are also
expected to contribute to the smaller formation volume.
The erosion volume of STMW, calculated as the difference between the maximum volume in late winter andthe minimum volume in the following fall in the formation

region, was also larger in 2010–2014 during the stable KEperiod (0.45 × 1015 m3 on average) than in 2006–2009
during the unstable KE period (0.39 × 1015 m3) by 15 %.
Notice that the erosion volume during the stable (unstable)
KE period was smaller (larger) than the formation volume
during the same period by 0.07 (0.05) × 1015 m3, resulting in that the STMW volume in the formation region
increased (decreased) during the stable (unstable) KE
period (Fig. 7a). The erosion volume consists mainly of
volume dissipated in the formation region and that transported to the downstream region south of 28°N, the latter ofwhich contributed to the lagged volume increase (decrease)in the downstream region during the stable (unstable) KE
period.

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120˚E130˚E140˚E150˚E160˚E170˚E 180˚ 170˚W
120˚E130˚E140˚E150˚E160˚E170˚E 180˚ 170˚W
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Depth (dbar)
025

50 100 150 200 250 300

Fig. 4  Distributions of mixed layer depth in March, otherwise following Fig. 3. Mixed layer depth, which is defined as the shallower
value of the depth at which σθ increases by 0.125 kg m−3 from
10-dbar depth and that at which θ changes by 0.5 °C from 10-dbar

depth (e.g., Levitus 1982; Suga et al. 2004), was computed for eachfloat profile and then mapped using the same procedure as for STMW
thickness

4 Biogeochemical influence in the downstream
regiondecadal variation at the OK line seems to lag about
0.5 year behind that in the STMW downstream region
and about 1.5 years behind that in its formation region(Fig. 7a).1

Decadal variability of STMW formation and subduction
was also detected in the most downstream region at the
OK line just southeast of the Okinawa Island (thickwhite line in Figs. 3, 4, 5, and 6), where JMA has conducted shipboard observations routinely two to fourtimes a year. The STMW area in the OK section(Fig. 7c) exhibited a decadal variation, being large in
2006–2007, small in 2010, and large in 2012–2014,
with a shorter-term variation possibly related to mesoscale activities (e.g., Takikawa et al. 2005). This

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1

The correlation between the STMW area in the OK section(Fig. 7c) and the STMW volume south of 28°N (Fig. 7a, blue line)
is 0.5–0.6 with a time lag of 0–10 months and drops with a longer
time lag. The correlation with low-frequency variation of the STMW
volume north of 28°N (Fig. 7a, red line), which is obtained by subtracting its monthly average, has a peak of ~0.6 at a time lag of
13–16 months.

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Decadal variability of Subtropical Mode Water subduction and its impact on biogeochemistry

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Sep.
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Thickness (dbar)
025

50 100 150 200 250 300

Fig. 5  Seasonal evolution of STMW thickness in 2007 (left) and 2012 (right), otherwise following Fig. 3

In addition, concurrent sensor measurements of temperature, salinity, and dissolved oxygen (O2) available
after spring 2010 at the OK line reveal not only a potential vorticity (Q) decrease, but also a dissolved O2 increase
and an apparent oxygen utilization (AOU) decrease on the
isopycnals of σθ = 24.8–25.4 kg m−3 (σθ is potential density), which correspond to the isotherms of 16.5–19.4 °C
in the STMW temperature range (Fig. 8). The increased
advection of O2-rich STMW during the stable KE period
is thought to enhance the oxygen supply to the subsurface layer at the OK line. Such consistent and significant
changes of Q, dissolved O2, and AOU were limited to the
STMW-related isopycnals (Fig. 9). At depths greater thanσθ = 25.4 kg m−3, the change rates dropped quickly. At
depths less than 24.8 kg m−3, dissolved O2 increased and

AOU decreased at similar rates to the STMW layer, but Q
increased. It should be noted that the changes of Q, dissolved O2, and AOU were observed not only on isopycnals
corresponding to lighter STMW originating in the westernpart of the formation region south of Japan, but also on
those corresponding to denser STMW formed in the eastern part of the formation region. In other words, the hydrographic structure at the OK line in the western boundary
region reflected the decadal variation of STMW formationin its entire formation region that spreads zonally south of
the Kuroshio and KE (Oka and Suga 2003; Oka 2009).
The downstream influence of STMW advection was
further examined using the shipboard observation data at
25°N in the 137°E repeat hydrographic section of JMA,
located about 500 km east/upstream of the OK line, where

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15˚N
15˚N
120˚E130˚E140˚E150˚E160˚E170˚E 180˚ 170˚W
120˚E130˚E140˚E150˚E160˚E170˚E 180˚ 170˚W
40˚N

40˚N

35˚N
35˚N
30˚N
30˚N
25˚N
25˚N
20˚N
20˚N
2006
2011
15˚N
15˚N
120˚E130˚E140˚E150˚E160˚E170˚E 180˚ 170˚W
120˚E130˚E140˚E150˚E160˚E170˚E 180˚ 170˚W
40˚N

40˚N

35˚N
35˚N
30˚N
30˚N
25˚N
25˚N
20˚N
20˚N
2007
2012
15˚N
15˚N
120˚E130˚E140˚E150˚E160˚E170˚E 180˚ 170˚W
120˚E130˚E140˚E150˚E160˚E170˚E 180˚ 170˚W
40˚N

40˚N

35˚N

35˚N

30˚N
30˚N
25˚N
25˚N
20˚N
20˚N
2008
2013
15˚N
15˚N
120˚E130˚E140˚E150˚E160˚E170˚E 180˚ 170˚W
120˚E130˚E140˚E150˚E160˚E170˚E 180˚ 170˚W
40˚N

40˚N

35˚N
35˚N
30˚N
30˚N
25˚N
25˚N
20˚N
20˚N
2009
2014
15˚N
15˚N
120˚E130˚E140˚E150˚E160˚E170˚E 180˚ 170˚W
120˚E130˚E140˚E150˚E160˚E170˚E 180˚ 170˚W

Thickness (dbar)
025

50 100 150 200 250 300

Fig. 6  Distributions of STMW thickness in November, otherwise following Fig. 3

various biogeochemical parameters have been monitored(e.g., Ishii et al. 2001, 2011; Midorikawa et al. 2006;
Takatani et al. 2012). Six biogeochemical parameters
derived from discrete water samples taken at depths, specifically AOU, salinity-normalized dissolved inorganiccarbon (nDIC), preformed nDIC, pH, aragonite saturation
state index (Ωarag), and nitrate (NO3) concentration, wereinterpolated onto the σθ = 25.0 kg m−3 isopycnal for theperiod of 2002–2013 (Fig. 10). In 2011–2013, we observed
a remarkable AOU decrease due to the increased advection of more recently ventilated STMW, as in the OK line
(Fig. 8c). An nDIC decrease was also observed, in spite ofa steady increase in preformed nDIC throughout the observation period reflecting the increase of anthropogenic CO2
content. As a result of the nDIC decrease, pH and Ωarag

13

increased in 2011–2013, in contrast to their long-term
decreasing trends indicating ocean acidification. NO3 concentration decreased in the same period, which, together
with the decreases of AOU and nDIC, is expected to reflect
the reduction in the transformation of organic matters to
inorganic form, i.e., remineralization, as a result of theincreased STMW advection.
The influence of the STMW advection on the nutrient
concentration in the downstream has also been suggested
in the North Atlantic. At the Bermuda Atlantic Time Series
station located south of the formation region of North
Atlantic STMW, long-term measurements of physical andbiogeochemical parameters revealed a positive correlation
between Q and NO3 concentration in STMW (Palter et al.
2005). This relationship indicates that NO3 concentration

397

Decadal variability of Subtropical Mode Water subduction and its impact on biogeochemistry

STMW Volume (1015 m3)

1.1

6

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

1.2
1.0
0.9
0.8
0.7
0.6
0.5

24.8

4

24.9

3

25.1
25.0
25.4
25.2
25.3

2
1

0.4

2010

11

12

13

14

0.3

210

0.2
0.1

0.7
0.6

2005 06

07

08

09

10

11

12

13

14

(b)

0.5
0.4

195
190
185

40

2005 06

07

08

09

10

11

12

13

(c)

30
20
10
0

2005 0660

14

AOU (µmol/kg)

50

25.2
25.3
25.1
25.0
24.9
24.8
25.4

200

0.3
0.2

(b)

205

O2 (µmol/kg)

0.0

STMW Area (106 m2) STMW Volume (1015 m3)

(a)

5

07

08

09

10

11

12

13

14

Fig. 7  a Monthly time series of STMW volume at 15°–40°N (blackline), 28°–40°N (red line), and 15°–28°N (blue line) between 120°E
and 170°W, based on Argo float data. b Time series of formation
volume (red line) and erosion volume (blue line) of STMW north of
28°N, between 120°E and 170°W, based on Argo float data. c Time
series of STMW area in the vertical section of the OK line, based on
shipboard observationsin the downstream region tends to be lower when a more
recently ventilated STMW reaches the station. The downstream NO3 concentration fluctuation in the North Pacific
STMW might have also partly originated in the STMW
formation region, where NO3 concentration tends to be low(high) during the stable (unstable) KE period, probably due
to the inactive (active) water exchange between the NO3depleted oligotrophic subtropical gyre and the eutrophic
mixed water region across the KE (Kouketsu et al. 2015).
Such a contribution from the STMW formation region
potentially reinforces the concentration changes in thedownstream through advection, and needs to be quantified
in future studies.
The above changes of biogeochemical parameters at
25°N, 137°E were all consistent with one another and are

2010

11

12

13

14

(c)

50

25.4
25.3
25.2
25.1
25.0
24.9
24.8

40
30
20

2010

11

12

13

14

Fig. 8  Time series of a Q, b dissolved O2, and c AOU on the isopycnals of σθ = 24.8–25.4 kg m−3, based on shipboard observations at
the OK line

expected to reverse during the unstable KE period when
the STMW advection weakens. In fact, AOU exhibited aslight decrease from mid-2003 to mid-2007 and a gradualincrease from mid-2007 through 2010, lagging 1.5 years
behind the stable KE period in 2002–2005 and the unstable KE period in 2006–2009. If it had been measured bya sensor with 1-dbar resolution, we might have obtained
its clearer decadal variation. AOU changes in the coming unstable KE period should be verified by sensor
measurements.

5 Discussion
The findings of the present and previous (Qiu and Chen
2005, 2006; Qiu et al. 2007, 2014) studies point to an integrated decadal climate-physical-biogeochemical variability

13

E. Oka et al.

Q Change Rate (10-10 1/m/s/year)
-0.6
24.5

-0.3

0.0

0.3

0.6

26.0

AOU

-4

-3

-2

O2

-1

0

1

2

3

4

O2/AOU Change Rate (µmol/kg/year)
Fig. 9  Change rates of Q, dissolved O2, and AOU on the isopycnalsof σθ = 24.5–26.0 kg m−3, calculated from shipboard observation
data at the OK line during 2010–2014 using the least squares method

pref.nDIC (µmol/kg)

25.5

50
40

(a)

30
20
10

2060
2050
2040
2030
2020
2010
2000

nDIC (µmol/kg)

25.0

2030
2020

2002 03

04

05

06

07

08

09

10

11

12

13

04

05

06

07

08

09

10

11

12

13

04

05

06

07

08

09

10

11

12

13

04

05

06

07

08

09

10

11

12

13

04

05

06

07

08

09

10

11

12

13

04

05

06

07

08

09

10

11

12

13

(b)

2002 03

(c)

2010
2000
1990
8.12
8.10

pH

σθ (kg m-3)

Q

AOU (µmol/kg)

398

2002 03

(d)

8.08
8.06

13

8.04

Ωarag

2002 03
3.1
3.0
2.9
2.8
2.7
2.6
2.5

NO3 (µmol/kg)

across the North Pacific basin. Specifically, a strengthened(weakened) Aleutian Low in the positive (negative) PDO
phase destabilizes (stabilizes) the KE system 3–4 years
later, and the associated high (low) eddy activity modulates
the mixed layer processes in the southern recirculation gyreon a decadal time scale, leading to the formation of thinner (thicker) STMW. As a result, the volume and extent of
STMW decrease (increase) in the formation region during
the unstable (stable) KE period, as well as in the southern,
downstream region with a time lag of 1–2 years. In thelatter region, weakened (intensified) advection of young,O2-rich STMW increases (decreases) NO3 and nDIC andthereby decreases (increases) pH and Ωarag. To put it more
plainly, it is the intensification (weakening) of the Aleutian
Low that subsequently accelerates (slows down) acidification in the subsurface ocean near the Okinawa Island. The
observed fluctuations in biogeochemical parameters areimportant not only for understanding the mechanism ofsubsurface biogeochemical variations but also for quantifying correctly the long-term trends of ocean acidificationfrom records that only span for a decade or two (e.g., Doreet al. 2009; Midorikawa et al. 2010; Ishii et al. 2011).
The present results have broad implications. First, it isimportant to investigate how the observed biogeochemical variability in the subsurface affects the surface environment in the subtropical regions of the western North
Pacific, particularly the region near the Okinawa Island
known for its coral reefs. The increasing O2 trend at
depths less than σθ = 24.8 kg m−3 (Fig. 9) might indicatethat oxygen (and other substances) was supplied from

5
4

(e)

2002 03

(f)

3
2
1

2002 03

Fig. 10  Time series of a AOU, b nDIC, c preformed nDIC, d pH, eΩarag, and f NO3 concentration on the isopycnal of σθ = 25.0 kg m−3,
based on shipboard observations at 25°N, 137°E

the STMW layer to the shallower layers through diapycnal mixing. If the variability in NO3 concentration in the
STMW layer is transmitted to the surface layer, it might
have a notable impact on the primary production there. On
the other hand, considering that the consumption of NO3
by production is associated with that of nDIC, the variability in the STMW subduction and the resultant changes
in the upward NO3/nDIC flux might not influence directly
the rate of ocean acidification at the sea surface. Nevertheless, the formation and subduction of STMW play animportant role in transporting anthropogenic CO2 from

Decadal variability of Subtropical Mode Water subduction and its impact on biogeochemistry

the sea surface to the ocean interior in the subtropics,
and its erosion by diapycnal mixing in the downstream
region contributes to the reemergence of anthropogenic
CO2 to the sea surface. Secondly, the significant decadal
variability of STMW is expected to be accompanied bythat of thermal structure, which is reflected in the upperocean heat content in the western part of the subtropicalgyre and possibly modulates the sea surface temperature
through the generation of the Subtropical Countercurrent(Xie et al. 2011; Kobashi and Kubokawa 2012). Furthermore, the linkage between the climate and biogeochemical variability through the formation, subduction, andadvection of STMW might be applicable to other mode
waters in the world oceans (Hanawa and Talley 2001),
whose formation regions are characterized by high air-tosea CO2 flux (Takahashi et al. 2009). It is worth emphasizing finally that the decadal oscillation across the North
Pacific basin and its mechanism have been clarified thanks
to the sustained observations by the satellites, the Argo
float network, and the research vessels. Synthetic analysesof these data will further clarify decadal variations in theocean interior and their impact on biogeochemical variations and changes.
Acknowledgments The authors are grateful to the captain, crew,
and scientists of R/V Chofu-maru, R/V Keifu-maru, and R/V Ryofumaru of JMA for their efforts in long-term observations. They also
thank Kanako Sato and Kana Nakamoto for their assistance in preparing the Argo float data and Yosuke Iida, Atsushi Kojima, Tomoyuki
Kitamura, participants at the 2014 HYARC symposium on air–sea
interaction, and two anonymous reviewers for helpful comments on
the manuscript. E.O. and T.S. acknowledge support by JSPS throughGrant 21340133 and 25287118 and MEXT through 22106007. E.O. isalso supported by MEXT through 25121502. B.Q. acknowledges support by NASA through Grant NNX13AE51G and NSF through OCE0926594. D.S. acknowledges support by JSPS through 24241010.
M.I. acknowledges support by MRI’s key research fund C3 andMEXT through 24121003.

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