Decadal Variability of the Kuroshio Extension: Observations and an Eddy-Resolving
Model Hindcast*
BUNMEI TAGUCHI,
⫹,##
SHANG-PING XIE,
#
NIKLAS SCHNEIDER,
@
MASAMI NONAKA,
&
H
IDEHARU SASAKI,** AND YOSHIKAZU SASAI
&
⫹
Department of Meteorology, SOEST, University of Hawaii at Manoa, Honolulu, Hawaii
#
IPRC, and Department of Meteorology, SOEST, University of Hawaii at Manoa, Honolulu, Hawaii
@
IPRC, and Department of Oceanography, SOEST, University of Hawaii at Manoa, Honolulu, Hawaii
&
Frontier Research Center for Global Change, JAMSTEC, Yokohama, Japan
**
Earth Simulator Center, JAMSTEC, Yokohama, Japan
(Manuscript received 20 June 2006, in final form 17 October 2006)
ABSTRACT
Low-frequency variability of the Kuroshio Extension (KE) is studied using observations and a multidec-
adal (1950–2003) hindcast by a high-resolution (0.1°), eddy-resolving, global ocean general circulation
model for the Earth Simulator (OFES). In both the OFES hindcast and satellite altimeter observations,
low-frequency sea surface height (SSH) variability in the North Pacific is high near the KE front. An
empirical orthogonal function (EOF) analysis indicates that much of the SSH variability in the western
North Pacific east of Japan is explained by two modes with meridional structures tightly trapped along the
KE front. The first mode represents a southward shift and to a lesser degree, an acceleration of the KE jet
associated with the 1976/77 shift in basin-scale winds. The second mode reflects quasi-decadal variations in
the intensity of the KE jet. Both the spatial structure and time series of these modes derived from the
hindcast are in close agreement with observations.
A linear Rossby wave model forced by observed wind successfully reproduces the time series of the
leading OFES modes but fails to explain why their meridional structure is concentrated on the KE front and
inconsistent with the broadscale wind forcing. Further analysis suggests that KE variability may be decom-
posed into broad- and frontal-scale components in the meridional direction—the former following the linear
Rossby wave solution and the latter closely resembling ocean intrinsic modes derived from an OFES run
forced by climatological winds. The following scenario is suggested for low-frequency KE variability:
basin-scale wind variability excites broadscale Rossby waves, which propagate westward, triggering intrinsic
modes of the KE jet and reorganizing SSH variability in space.
1. Introduction
The Kuroshio Extension (KE) is a swift eastward
inertial jet formed after the Kuroshio separates from
the Japanese coast at around 35°N, 142°E. The KE
region has the largest sea surface height (SSH) variabil-
ity on both mesoscale and interannual time scales in the
extratropical North Pacific Ocean (Qiu 2002a). The
Kuroshio–Oyashio Extension (KOE) region, including
the KE and the Oyashio subarctic front, has recently
been identified as the window in which subsurface
ocean variability strongly affects sea surface tempera-
ture (SST; Xie et al. 2000). This subsurface feedback on
SST occurs in winter when a deep mixed layer devel-
ops, by vertical entrainment (Schneider et al. 2002) or
horizontal advection by the varying Kuroshio and
Oyashio Extensions (Qiu 2000; Seager et al. 2001; To-
mita et al. 2002; Scott and Qiu 2003). Thus, ocean dy-
namics are an important mechanism for low-frequency
variability in SST, and possibly the overlying atmo-
sphere via surface turbulent fluxes (Tanimoto et al.
* International Pacific Research Center Publication Number
432 and School for Ocean and Earth Science and Technology
Publication Number 7099.
##
Current affliation: Earth Simulator Center, JAMSTEC,
Yokohama, Japan.
Corresponding author address: Dr. Bunmei Taguchi, Earth
Simulator Center, Japan Agency for Marine-Earth Science and
Technology, 3173-25 Showa-machi, Kanazawa-ku, Yokohama,
Kanagawa 236-0001, Japan.
E-mail: bunmei@jamstec.go.jp
1J
UNE 2007 T AGUCHI ET AL. 2357
DOI: 10.1175/JCLI4142.1
© 2007 American Meteorological Society
JCLI4142
2003) in the KOE region. Coupled ocean–atmosphere
model studies support this notion (Pierce et al. 2001;
Schneider et al. 2002). In a multivariant regression
analysis, Schneider and Cornuelle (2005) show that on
decadal time scales KE intensity variability is an impor-
tant contributor to the observed Pacific decadal oscil-
lation (PDO) along with the Aleutian low and El Niño–
Southern Oscillation (ENSO). All these studies indi-
cate the importance of understanding the mechanisms
for decadal variability in upper-ocean dynamical fields
such as SSH.
Two schools of thought exist with conflicting views
regarding low-frequency variations of upper-ocean cir-
culation. The first school points to the large internal
variability in double-gyre circulation simulated in
simple idealized models forced by steady wind forcing
(Jiang et al. 1995; Dijkstra and Ghil 2005). This view
appears consistent with snapshot satellite images of
SST and SSH, which show a turbulent and chaotic
KOE. The recirculation and narrow western boundary
current extension themselves are generally considered
as resulting from the eastward advection of potential
vorticity anomalies from the western boundary layer
(Cessi et al. 1987). The nonlinear interaction of recir-
culation, PV advection, and eddies cause the double-
gyre circulation to vacillate between a straight and pen-
etrative inertial jet and a meandering and westward
confined one (e.g., McCalpin and Haidvogel 1996). Us-
ing a three-layer quasigeostrophic model, Dewar (2003)
and Hogg et al. (2005) identify a low-frequency (⬎10
yr) mode of intrinsic variability characterized by shifts
in the inertial jet position and slow adjustment in po-
tential vorticity. They consider this mode as being dis-
tinct from the linear Rossby wave adjustment. The sec-
ond school of thought views the KOE variability as
deterministic and consistent with the linear baroclinic
Rossby wave adjustment to wind variability. Analyzing
historical observations of subsurface temperature, De-
ser et al. (1999) detected westward-intensified struc-
tures in decadal changes associated with the 1976/77
climate regime shift, which they suggested are due to
the Sverdrup adjustment to basin-scale wind changes.
This result was confirmed by ocean general circulation
model (OGCM) studies (Miller et al. 1998; Xie et al.
2000; Seager et al. 2001). Indeed, the slow westward
propagation of wind-forced baroclinic Rossby waves
has been exploited for skillful prediction of ocean pres-
sure variability in the KOE region with a lead time up
to a year (Schneider and Miller 2001).
While the school of wind-forced Rossby waves has
been successful in explaining broadscale (⬎1000 km)
subsurface anomalies, most of observational and OGCM
studies on low-frequency variability in the North Pacific
Ocean circulation are limited by the coarse resolution
of their data/models that does not resolve the oceanic
fronts and recirculations. It remains to be seen whether
the same success can be extended to studying the vari-
ability of the inertial KE jet and the Kuroshio recircu-
lation, both highly nonlinear phenomena full of internal
variability.
Opportunities are emerging to study low-frequency
variability of the narrow KE jet owing to the accumu-
lation of satellite altimeter observations and a recent
multidecadal, eddy-resolving (0.1° horizontal resolu-
tion) OGCM hindcast performed on Japan’s Earth
Simulator supercomputer. From 11-yr-long satellite
SSH data, Qiu (2003) found decadal modulations in the
KE jet’s intensity. Analyzing the Earth Simulator hind-
cast, Nonaka et al. (2006) showed that the well-known
basinwide cooling during the early 1980s is accompa-
nied by the southward shift and intensification of two
separate oceanic fronts: the KE and the subarctic
Oyashio fronts. Although Nonaka et al. (2006) attrib-
uted the cooling along the KE to linear Rossby wave
adjustment, the relevant dynamics and other possible
modes of variability were not fully explored.
The present study uses satellite SSH observations
and the Earth Simulator hindcast to study decadal vari-
ability of the KE jet and the recirculation to the south.
In particular, we are interested in what determines the
spatiotemporal structure of this variability. Specific
questions to be addressed include the following: How
well does the high-resolution model hindcast capture
the observed variability in the KE jet? How is the ocean
variability related to atmospheric wind forcing? What
are the role and contribution of oceanic intrinsic non-
linearity to this variability? We will show that the
model hindcast is remarkably consistent with satellite
and other in situ observations. With its realism estab-
lished, the model hindcast offers a powerful tool to
study the nature and mechanism of ocean variability
prior to the satellite altimetry era. Our analysis shows
that while the linear Rossby wave theory explains the
temporal variability, nonlinear ocean dynamics play an
essential role in organizing the spatial structure,
thereby reconciling two conflicting schools of thought
reviewed above. The present study will rely mostly on
empirical analysis to derive the spatiotemporal struc-
ture and relate it to wind forcing; thus, important in-
formation for future studies to further decipher the un-
derlying mechanisms and dynamics.
The rest of the paper is organized as follows. Section
2 describes models and observational data used in this
study. Section 3 examines the spatiotemporal structure
of low-frequency variability in the KE and recirculation
while section 4 studies the processes that determine this
2358 JOURNAL OF CLIMATE VOLUME 20
structure. Section 5 discusses our results in light of pre-
vious modeling studies and their implications while sec-
tion 6 is a summary.
2. Model output and observational data
a. Eddy-resolving model
We analyze a multidecadal hindcast by the OGCM
for the Earth Simulator (OFES; Masumoto et al. 2004;
Nonaka et al. 2006; Sasaki and Nonaka 2006). The
OFES is based on the Third Modular Ocean Model
(MOM3; Pacanowski and Griffies 1999) and has been
substantially modified for optimal performance on the
Earth Simulator. The model covers a near-global do-
main extending from 75°Sto75°N, with a horizontal
resolution of 0.1° and 54 vertical levels. It is forced by
surface wind stress, heat, and freshwater fluxes derived
from the daily-mean National Centers for Environmen-
tal Prediction–National Center for Atmospheric Re-
search (NCEP–NCAR) reanalysis (Kalnay et al. 1996).
Surface turbulent heat flux is calculated using the
model SST and meteorological variables from the
NCEP–NCAR atmospheric reanalysis while sea surface
salinity is restored to the observed monthly climatol-
ogy. Within 3° from the model’s southern and northern
boundaries at 75°Sto75°N, temperature and salinity
are restored to the monthly climatology of the World
Ocean Atlas 1998 (WOA98; Boyer et al. 1998a,b,c).
From an initial condition at rest with observed (WOA98)
annual-mean temperature and salinity, the model was
first spun up for 50 yr with monthly climatological at-
mospheric forcing, followed by a 54-yr hindcast integra-
tion forced with the daily-mean reanalysis data from
1950 to 2003 (hindcast run). This is the first multidec-
adal hindcast that resolves fronts and eddies from the
Tropics to the midlatitude in the World Ocean, provid-
ing a unique opportunity to study interannul to decadal
variability of narrow western boundary currents and
their eastward extensions. Nonaka et al. (2006), for ex-
ample, show that the shift around the early 1980s in the
northwestern Pacific east of Japan displays distinct ver-
tical structures along the KE and Oyashio fronts, with
deep subsurface anomalies in the former and large SST
anomalies in the latter regions, respectively. In this
study, we analyze the hindcast run for a 42-yr period
from 1962 to 2003, regarding the first 12 yr of the hind-
cast as the spinup, during which the model adjusts to
changes in atmospheric forcing from climatological to
1950 values. All our analysis uses a subsampled output
on a 0.5° grid to reduce the data size.
In addition to the hindcast run, two runs forced with
monthly climatological atmospheric forcings are carried
out: a climatological control run that continues the
spinup for another 48 yr (Sasai et al. 2004), and a pre-
diction
1
run initialized with the hindcast run on 1 Janu-
ary 2000 and integrated forward for another 3 yr.
b. Observational data
The OFES hindcast is compared to the following ob-
servational data. We use SSH anomaly maps compiled
from the Ocean Topography Experiment (TOPEX)/
Poseidon (T/P), Jason, and the European Remote Sens-
ing satellite (ERS-1/2) altimeter observations, distrib-
uted by the Archiving, Validation and Interpretation of
Satellite Oceanographic Data (AVISO; Ducet et al.
2000). The data are mapped on the Mercator grid with
1
⁄
3
° resolution in longitude and varying spacing in lati-
tude (from 37 km at the equator to 18.5 km at 60°N).
Monthly mean maps are used, calculated from the
original weekly mean data that cover an 11-yr period
from January 1993 to December 2003.
To validate the OFES hindcast prior to the satellite
altimeter era, we use the monthly ocean temperature
data compiled from expendable bathythermograph
(XBT) observations at the Joint Environmental Data
Analysis Center of the Scripps Institution of Oceanog-
raphy (White 1995). For the period during1955–2004,
the XBT observations are optimally interpolated on a
2° latitude by 5° longitude grid with 2.5° latitude by 5°
longitude decorrelation scales.
3. Modes of KE variability
This section examines the dominant modes of KE
variability and compares them with satellite and in situ
observations. In particular, we investigate the effect of
the sharp KE jet on the modal structure and how these
modes are related to variability in the recirculation.
a. Variance
Figure 1 compares the standard deviation of interan-
nual SSH variations. For both the hindcast and T/P
observations, the calculation of the standard deviation
is based on monthly anomalies defined as deviations
from the 11-yr (1993–2003) monthly climatology. A 12-
month low-pass filter is applied to suppress mesoscale
eddies. Also shown in contours are the absolute mean
SSH fields from the hindcast and the estimate of Niiler
et al. (2003) using surface drifter and satellite altimeter
observations. The OFES hindcast captures the salient
1
The term “prediction” is used in the sense that no interannual
history in the surface wind and buoyancy forcing is provided to
the model during the additional 3-yr integration after 2000.
1J
UNE 2007 T AGUCHI ET AL. 2359
features of both the mean and interannual variance of
observed SSH. In the mean SSH field, the KOE, de-
picted as a broad eastward current in coarse-resolution
(1° or less) models, consists of two distinct fronts of the
KE and Oyashio Currents. Qu et al. (2002) note such a
separation of the KOE into two distinct narrow jets in
a 0.25° OGCM. In both the OFES and observations, a
sharp KE front is found nearly zonally at around 34°–
35°N with quasi-stationary meanders east of Japan in
the upstream KE at 141°–150°E. High interannual vari-
ance is concentrated within a narrow latitudinal band
along the mean KE front. Specifically, west of 170°E,
the standard deviation exceeds 20 cm within a frontal
band, over which the mean SSH drops 40 cm (delin-
eated by two thick contour lines in Fig. 1), while in most
of the area outside the band, the standard deviation is
FIG. 1. (a) Standard deviation of 12-month low-pass-filtered SSH observed by satellite altimeters (shaded) and the mean absolute sea
level (contours at 10-cm intervals; the 60- and 100-cm contours thickened to delineate the KE frontal zone). (b) Same as in (a), but for
the OFES hindcast (the 30- and 70-cm contours for the mean SSH are thickened).
2360 JOURNAL OF CLIMATE VOLUME 20
Fig 1 live 4/C
less than 10 cm. In the OFES, there is a band of sec-
ondary maximum in SSH variance along the Oyashio
front around 41°N, which is not obvious in the T/P
observations.
Despite the good agreement of the slow frontal vari-
ability between the OFES hindcast and altimeter ob-
servations, there are also significant discrepancies. In
the observations, the KE front shifts southward by 1°–
1.5° around 152°–154°E east of the KE’s second sta-
tionary meander, and is associated with a southward
displacement of the interannnual variance. The hind-
cast SSH does not show this meridional shift of the
mean front and its variability. Nevertheless, the overall
agreement of the mean and interannual variance with
observations encourages us to look further into the
simulated variability.
b. Leading mode and comparison with satellite
altimetry
Now we examine the spatial and temporal structures
of the KE’s low-frequency variations. Since both the
mean and variance fields of SSH are zonally enlon-
gated, we focus on the meridional structure of the KE
variability by zonally averaging variables in 142°E–
180°.
2
The zonal averaging suppresses mesoscale vari-
ability while emphasizing low-frequency variability. We
apply the empirical orthogonal function (EOF) analysis
to the zonal mean in a latitudinal domain 30°–45°Nso
that the EOF patterns and the associated principal
components (PCs) represent the spatial and temporal
variations, respectively. The EOF analysis is applied
independently to the hindcast and observations, and
the results are then compared.
Satellite altimeters, with high meridional resolution,
provide an unprecedented opportunity to observe the
meridional structure of SSH variability and validate
model simulations. Figure 2 compares the first EOF
mode between the OFES and the altimeter analyzed
from 1993 to 2003. The OFES hindcast successfully
captures the observed EOF-1, both with large loading
concentrated near the KE jet. The model EOF peaks
more sharply around the mean KE jet than in observa-
tions, reflecting the more zonal KE jet in the OFES
simulation than in the altimeter observation as men-
tioned in section 3a. Dependence of the EOF-1 pattern
on the mean flow structure is discussed in section 3c.
Zonal current anomalies at 100 m associated with the
EOF-1 of simulated SSH represent a northward shift of
the KE jet (Figs. 2b,c), with the acceleration to the
north of the jet slightly larger than the deceleration to
the south.
The first EOF mode explains about 50% of the
zonal-mean SSH variability within 30°–45°N in both
observations and the OFES hindcast. The hindcast
PC-1 tracks the observations remarkably well, and both
show a full decadal cycle. This close agreement with
observations demonstrates the skills of the eddy-re-
solving OFES hindcast in simulating the low-frequency
variability of the narrow inertial KE jet. In particular,
the hindcast captures the following KE variability docu-
mented in Qiu and Chen (2005): the gradual weakening
and southward shift of the KE jet from 1993 to 1996,
and the steady strengthening and northward migration
of the jet after 1997.
c. The KE jet’s signature in modal structure
The strong trapping of SSH variance and the first
EOF pattern by the KE jet suggests that the mean jet
exerts a strong influence on the meridional pattern of
ocean variability. To corroborate this point, we take
advantage of the zonal variations of the mean KE front
in the altimeter observation and divide the KE region
into three 10°-wide longitudinal sectors at 142°–152°E,
156°–166°E, and 170°E–180°. The observed KE front
varies its meridional structure significantly among these
longitudinal sectors. In the upstream sector the sharp
KE front is located around 34.5°N but shifts to around
33°N in the middle sector, and there is no distinct front
in the downstream sector (Fig. 3, upper left). We cal-
culate the meridional EOF modes of the T/P SSH
within 30°–45°N separately for each longitudinal sector.
The meridional pattern of the first mode (Fig. 3, upper
right) displays a clear dependency on the mean flow,
with its peak tracking the KE front as the latter shifts
southward from the upstream to the middle sector. The
EOF shows a much broader pattern in the downstream
sector where the mean flow broadens in the meridional
direction without a sharply defined front in SSH.
We further calculate the two-dimensional EOF for
the observed SSH variability in a domain of 140°E–180°
and 30°–45°N and show the first mode in color shaded
in the upper-left panel of Fig. 3. The leading EOF ex-
plains 34% of the total variance, with large loading over
or near the mean KE front west of 170°E, consistent
with the one-dimensional EOF modes over 30°–45°N.
Near the date line, the mean eastward flow broadens,
and so does the positive loading in the two-dimensional
2
Although the KE’s mean frontal structure and interannual
variability are not exactly uniform in the zonal direction within
this sector, we first highlight the meridional structure of the KE
frontal variability representative of the whole KOE region rang-
ing from the Japanese coast to the date line. The dependence of
the meridional structure of the variability on the choice of the
zonal sector is discussed in section 3c.
1J
UNE 2007 T AGUCHI ET AL. 2361
