J Oceanogr (2012) 68:355–360
DOI 10.1007/s10872-011-0098-0

SHORT CONTRIBUTION

Decadal seesaw of the Central and Subtropical Mode Water
formation associated with the Kuroshio Extension variability
Eitarou Oka • Bo Qiu • Shinya Kouketsu •Kazuyuki Uehara • Toshio Suga

Received: 30 September 2011 / Revised: 14 December 2011 / Accepted: 20 December 2011 / Published online: 20 January 2012Ó The Oceanographic Society of Japan and Springer 2012

Abstract Available Argo profiling float data from 2002
to 2011 were analyzed to examine the effect of the Kuroshio Extension (KE) current system variability on the formation of the Central Mode Water. Just north of the
upstream portion of the KE at 140–152°E, formation of alighter variety of the Central Mode Water in winter was
active during the unstable period of the upstream KE in
2006–2009 and was reduced when the upstream KE was in
the stable period of 2002–2005 and 2010–2011. This decadal formation variability is out of phase with that of the
Subtropical Mode Water just south of the KE.
Keywords Decadal variability  North Pacific Mode waters  Kuroshio Extension  Mesoscale eddies

E. Oka (&)
Atmosphere and Ocean Research Institute,
The University of Tokyo, Kashiwa 277-8564, Japane-mail: eoka@aori.u-tokyo.ac.jp
B. QiuDepartment of Oceanography, University of Hawaii at Manoa,
Honolulu, HI 96822, USA
S. Kouketsu  T. Suga
Research Institute for Global Change,
Japan Agency for Marine-Earth Science and Technology,
Yokosuka 237-0061, JapanK. Uehara
Department of Marine Science, School of Marine Scienceand Technology, Tokai University, Shizuoka 424-8610, Japan
T. Suga
Department of Geophysics, Graduate School of Science,
Tohoku University, Sendai 980-8578, Japan

1 Introduction
Accumulation of satellite altimeter sea surface height dataover the past 19 years has revealed decadal variability of
the Kuroshio Extension (KE) jet and its associated mesoscale eddy field (Qiu and Chen 2005, 2011; Qiu et al.
2007). This decadal variability is remotely forced by largescale wind stress curl forcing in the central North Pacificcentered at 40°N, 160°W (Qiu 2003; Taguchi et al. 2007),
whose strength fluctuates in association with the Pacificdecadal oscillations (PDOs; Mantua et al. 1997). Specifically, when the PDO index is positive (negative), negative(positive) sea surface height and permanent thermocline
depth anomalies are generated and propagate westward at
the speed of first-mode baroclinic Rossby waves. When thewind-induced negative (positive) anomalies reach theregion east of Japan, the KE jet weakens (strengthens),
accompanied by a weak (strong) southern recirculation
gyre and high (low) regional eddy activity (Fig. 1a, b, d).1
During its weakened (strengthened) period, the KE jet isunstable (stable) in its upstream portion at 140–152°E andstable (unstable) in its downstream portion at 152–165°E.
The decadal KE variability has recently been recognized
to exert a significant impact on various processes ofneighboring water masses, thanks to the development of
the Argo profiling float array (Roemmich et al. 2009;Freeland et al. 2010). In contrast with our traditional viewthat the mode water thickness is determined by the surface
heat flux forcing, thinner (thicker) Subtropical Mode Water(STMW; Masuzawa 1969) is formed as deep winter mixed
layer in the southern recirculation gyre when the upstream
KE is in the unstable (stable) state (Qiu and Chen 2006).
1

The sea surface height anomalies along 32–34°N best explain therelation between the PDOs and the KE system variability.

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Fig. 1 a Time series of eddy
kinetic energy (EKE) in the
upstream KE region of
32–38°N, 141–153°E. Here,EKE is calculated using theweekly sea surface heightanomalies from the satellite
altimeter data compiled by the
Collocte Localisation Satellites(CLS) Space Oceanographic
Division of Toulouse, France(Ducet et al. 2000). b, c Sea
surface height anomalies(SSHa) along the zonal bands ofb 32–34°N and c 36–38°N.
d PDO index from the Joint
Institute for the Study of the
Atmosphere and Ocean Web
site (available online at
http://jisao.washington.edu/pdo/
PDO.latest)

This is because during an unstable period, both the active
southward transport of high potential vorticity water from
the Mixed Water region across the KE due to stronger eddy
activity (Qiu and Chen 2006; Qiu et al. 2007) and thehigher background stratification associated with the shallower permanent pycnocline in the recirculation gyre
(Sugimoto and Hanawa 2010) are unfavorable for thedevelopment of deep winter mixed layers. The decadal KE
variability also affects the STMW circulation (Oka et al.
2011a) and possibly its dissipation (Oka and Qiu 2012). In
addition to STMW, the decadal KE variability also influences the modulation of the North Pacific Intermediate
Water (NPIW; Reid 1965). NPIW south of the downstream
KE was fresher during 2002–2005 (when the downstream
KE was unstable and the upstream KE was stable) than
2006–2009 (the opposite phase), because the stronger eddy
activity along the downstream KE during the former period
led to a larger transport of fresh water from the Mixed
Water region in the NPIW density range, while the stronger
upstream KE acted as a barrier for the meridional waterexchange during both periods (Qiu and Chen 2011).
Motivated by these recent observational studies, it isnatural to ask whether the decadal KE variability also
affects water mass formation/transformation processes
north of the KE. The structure of deep winter mixed layer
in the mode water formation regions north of the KE has
been recently described using Argo float data from2003–2008 (Oka et al. 2011b; Fig. 2a). Specifically, twozonally elongated regions of deep winter mixed layer

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extend along 33–39°N and 39–43°N from the east coast ofJapan to 160°W; the southern region corresponds to theformation region of the lighter variety of Central Mode
Water (L-CMW; Tsujino and Yasuda 2004; Oka and Suga
2005), and the northern region to those of the denser
variety of Central Mode Water (D-CMW) and the Transition Region Mode Water (TRMW; Saito et al. 2007). On
the other hand, an ongoing analysis of dense shipboard
observation data north of the KE in spring 2003 (Oka et al.,manuscript in preparation) found no L-CMW in the western part of its formation region west of 160°E. This suggests a large temporal variability in the L-CMW formation
with the possibility of its relation to the decadal KE
variability.
The aforementioned study by Oka et al. (2011b) analyzed the float data from both periods of the KE stability todescribe the deep winter mixed layer structure in the mode
water formation regions. In this paper, we reexamine themixed layer structures using the data from 2002–2011,
focusing on the differences between the stable versus
unstable periods of the KE.

2 Results
Temperature and salinity data of 2002–2011 from the Argo
profiling floats, downloaded from the ftp site of the Argo
Global Data Assembly Center and edited as outlined in
Oka et al. (2007), are used to plot the mixed layer depths

Decadal seesaw of the Central and Subtropical Mode Water formation

357

Fig. 3 Time series of frequency of MLD greater than 150 dbar at
a 36–40°N, 140–155°E and b 31–35°N, 140–155°E in February andMarch from the float data. Black (white) circles correspond to theperiod when the upstream KE was stable (unstable)

Fig. 2 a Distributions of mixed layers deeper than 150 dbar with theproperties of STMW (white circles), L-CMW (black squares),
D-CMW (white triangles), and TRMW (black stars), based on the
float data in February–April of 2003–2008 (adapted from Oka et al.
2011b). b, c Distributions of MLD in February–April of b 2002–2005,
2010–2011 and c 2006–2009 from the float data. Note that gray dots
for MLD \150 dbar are smaller than the other dots

(MLDs) in late winter months of February–April for the
stable (2002–2005, 2010–2011) and unstable (2006–2009)
periods of the upstream KE (Fig. 2b, c). Here, MLD is
defined as the shallower value of the depth at which thepotential density increases by 0.03 kg m-3 from 10-dbar
depth and that at which the potential temperature changes
by 0.2°C from 10-dbar depth, following the definition of deBoyer Montégut et al. (2004) and Oka et al. (2007). During
the unstable period, deep mixed layers ([150 dbar) wereformed all over the region east of Japan between 28° and
43°N (Fig. 2c), as demonstrated by Oka et al. (2011b) forthe period of 2003–2008 (Fig. 2a). In contrast, during thestable period, there was a gap of deep mixed layers at
36–40°N, west of 158°E (Fig. 2b). This area, located justnorth of the KE (*35°N), corresponds to the western partof the L-CMW formation region (Oka et al. 2011b;
Fig. 2a), as confirmed by plotting temperature, salinity, anddensity distributions for the deep mixed layers (not shown).
The time series of frequency of deep mixed layers in thisarea exhibits a clear tendency of relatively high values
during the unstable period and low values during the stable
period (Fig. 3a). The averaged frequency of deep mixed
layers during the unstable period (18.0%) is more than
twice as high as that during the stable period (7.0%). Sucha tendency in the western part of the L-CMW formation
region is consistent with the nonexistence of L-CMW in
spring 2003 during the stable period (Oka et al., manuscript
in preparation), and is opposite to that in the STMW formation region between 140° and 155°E where deep wintermixed layers were more frequently observed during thestable period than the unstable period (Figs. 2, 3b).
The winter MLD maps composited for the two periods
highlight the impact of the decadal KE variability on themode water formation. During the stable period, MLD waslarge east of Japan at 31–35°N and 40–42°N,

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corresponding to the formation regions of STMW and
D-CMW/TRMW, respectively, while it had a minimum at
36–39°N, reflecting the inactive L-CMW formation(Fig. 4a). During the unstable period, on the other hand,
three comparable maxima of MLD existed in the threemode water formation regions at 30–33°N and 37–39°N
east of Japan and at 40–43°N, 154–162°E, and the meridional contrast of MLD was much smaller than during thestable period (Fig. 4b). The composite MLD difference
between the two periods delineates a striking dipole pattern
sandwiching the KE between 140° and 155°E, indicating

Fig. 4 Distributions of MLD in February and March of a 2002–2005,
2010–2011 and b 2006–2009, averaged for each 1° 9 1° grid box
using the float data from a 3° 9 3° grid box centered by the 1° 9 1°
grid box and weight function of d-2 (d is the distance in degree fromthe center of the 1° 9 1° grid box) for observation points whered [ 1°. c Difference of the averaged MLD between a and b (former
minus latter)

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enhanced formation of STMW during the stable period andof L-CMW during the unstable period (Fig. 4c).

3 Discussion
The above analyses demonstrated that the L-CMW production in the western part of its formation region is high(low) during the unstable (stable) period of the upstream
KE and is out of phase with the STMW production on thesouthern side of the KE. Since this variability occurs inalmost the same longitude range as the upstream KE variability (140–152°E), its mechanism can be reasonably
explained as follows. During the unstable period, theregional eddy activity is high along the upstream KE,
where nonlinear anticyclonic eddies actively detach fromthe KE to its north. These eddies bring low potential vorticity water of the southern recirculation gyre into the
L-CMW formation region, reduce the background stratification in the L-CMW formation region by increasing the
average permanent pycnocline depth, and generate a precondition favorable for deep winter mixed layer development. In addition, these eddies themselves have been
recently considered as important formation sites of
L-CMW. A composite analysis of Argo float data andsatellite sea surface height data demonstrated that roughly
half of deep winter mixed layers in the L-CMW formation
region are formed inside the anticyclonic eddies (Kouketsu
et al. 2012). This result is supported by past shipboard
observations (Tomosada 1986; Yasuda et al. 1992)
revealing that while the anticyclonic eddies detached fromthe KE survive through winter in the Mixed Water region,
the southern-origin STMW trapped within them is modified
to L-CMW because of the convective winter cooling andinteraction with the ambient water. Moreover, during an
unstable period, the KE tends to take a southerly position
and the Kuroshio bifurcation front tends to develop (Qiu
and Chen 2011), which also provides a favorable environment for the L-CMW formation. Thus, the KE variability has an opposite effect on the deep winter mixed
layer development between the L-CMW and STMW formation regions. In the latter region, the enhanced eddy
activity during the unstable period inhibits the mixed layer
development because the eddies bring high potential vorticity water from the Mixed Water region (Qiu and Chen
2006; Qiu et al. 2007).
Given the large spatial scale of atmospheric forcing, thewind stress curl forcing in the central North Pacific generates sea surface height (and permanent pycnocline depth)
anomalies of the same sign near 160°W in both latitude
ranges of 32–34°N and 36–38°N corresponding to the
STMW and L-CMW formation regions, respectively(Fig. 1b–d). The anomalies generated at 36–38°N

Decadal seesaw of the Central and Subtropical Mode Water formation

propagate westward at a lower speed and tend to reach
150°E roughly a year later than those at 32–34°N becauseof the smaller Coriolis parameter and weaker stratification(e.g., Qiu 2003). If they extend all the way to the east coastof Japan, the wind-induced anomalies are expected to raise(lower) the permanent pycnocline and enhance (reduce) the
background stratification in the L-CMW formation region
during an unstable (stable) period, therefore weakening thedecadal L-CMW formation variability. However, thepropagating anomalies along 36–38°N change their signnear the east coast of Japan and turn positive (negative)
during the unstable (stable) period (Fig. 1c). Such areversal of anomalies is not observed along 32–34°N(Fig. 1b), and is likely due to the more (less) active
detachment of nonlinear anticyclonic eddies from the KE
during the unstable (stable) period. Thus, in the westernpart of the L-CMW formation region, the sea surface
height and permanent pycnocline depth anomalies appear
to be more determined by the nonlinear eddy signals
relating to the decadally varying KE stability than by thewind-induced sea surface height anomalies propagatingfrom the east.2 It is this dominance by eddy signals that is
responsible for the observed decadal seesaw between the
L-CMW and STMW changes.
It might be worth noting that during the stable period,
the winter mixed layer is deeper in the region between 40°
and 42°N, west of 160°E, which corresponds to the formation region of D-CMW and TRMW (mostly the latter inthis longitude range; Figs. 2a, 4c). This seems to be consistent with the following findings in recent years: thick
TRMW is locally formed just south of a quasistationary jet
associated with the subarctic front (Isoguchi et al. 2006;
Saito et al. 2007; Tomita et al. 2011); the jet is stronger(weaker) when the upstream KE takes a northerly (southerly) position (T. Wagawa, personal communication); andthe upstream KE tends to take a northerly (southerly)
position during the stable (unstable) period (Qiu and Chen
2011). There might be a relation that during the stable
period, the stronger quasistationary jet transports morewarm water from the south, enhances the deep winterconvection to its south, and leads to the formation of
thicker TRMW.
The decadal L-CMW formation variability identified inthis study has a wide range of implications from the physicaland biogeochemical perspectives. Changes in the L-CMW
formation alter the upper thermal structure in the Mixed
Water region, which can modulate the sea surface temperature and possibly feedback onto the atmosphere. Through2

In the STMW formation region, the sea surface height anomalies
propagating from the east and those due to the nonlinear eddy signals
have the same sign, which explains why the reversal of anomalies was
not observed there.

359

changes in winter MLD, the L-CMW formation variability
can affect subsurface dissolved oxygen distribution and airto-sea CO2 flux that is high in the mode water formation
regions in winter (Takahashi et al. 2009). Moreover, a deeper(shallower) winter mixed layer in the L-CMW formation
region might lead to a larger (smaller) nutrient supply fromdeeper layers during the unstable (stable) period, although
this vertical transport effect might be cancelled out by thehorizontal transport of more (less) oligotrophic water fromthe southern recirculation gyre.
In addition to these local effects, the influence of thedecadal L-CMW formation variability in the western partof its formation region on the L-CMW formation andsubduction in the eastern part of the formation region (Okaet al. 2011b), and further on the upper thermal structure in
the subtropical gyre and decadal climate variability in the
Pacific region, needs to be examined in future studies. If
L-CMWs are largely formed in nonlinear anticycloniceddies, they are not likely to be advected eastward by themean flow, as speculated by Oka et al. (2011b), but move
westward as isolated eddies (Itoh and Yasuda 2010). The
standard density of Argo floats (one float per 3° square) is
obviously insufficient to study detailed processes of how
the eddies contribute to the L-CMW formation and subduction. High-resolution shipboard observations (e.g., Oka
et al. 2009, 2011a) and concentrated deployment of the
profiling floats in the L-CMW formation region in combination with eddy-resolving ocean general circulation
models are strongly desired.
Acknowledgments The authors thank Tsuyoshi Ohira for help in
preparing the Argo float data and two anonymous reviewers forhelpful comments on the manuscript. The Argo float data used in thisstudy were collected and made freely available by the International
Argo Project and the national programs that contribute to it(http://www.argo.ucsd.edu, http://argo.jcommops.org). This study
was motivated by discussions between EO and BQ in summer 2010
when EO visited the University of Hawaii at Manoa for 1 monthunder the Overseas Internship Program for Outstanding Young Earthand Planetary Researchers provided by the Department of Earth andPlanetary Science, the University of Tokyo. EO, KU, and TS aresupported by the Japan Society for Promotion of Science (KAKENHI,
Grant-in-Aid for Scientific Research (B), no. 21340133) and theMinistry of Education, Culture, Sports, Science and Technology,
Japan (MEXT; Grant-in-Aid for Scientific Research on Innovative
Areas under grant no. 22106007). BQ is supported by NSF throughGrant OCE-0220680 and NASA through contract 1207881.

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