Responses of the Kuroshio and the Kurosh
GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L14617, doi:10.1029/2005GL023384, 2005
Responses of the Kuroshio and the Kuroshio Extension to global
warming in a high-resolution climate model
Takashi T. Sakamoto,1 Hiroyasu Hasumi,2 Masayoshi Ishii,1 Seita Emori,1,3
Tatsuo Suzuki,1 Teruyuki Nishimura,1 and Akimasa Sumi2
Received 30 April 2005; revised 15 June 2005; accepted 20 June 2005; published 27 July 2005.
[1] Using a high-resolution atmosphere – ocean coupled
climate model, responses of the Kuroshio and the Kuroshio
Extension (KE) to global warming are investigated. In a
climate change experiment with atmospheric CO 2
concentration ideally increased by 1% year 1, the current
velocity of the Kuroshio and KE increases, while the
latitude of the Kuroshio separation to the east of Japan does
not change significantly. The increase of the current velocity
is up to 0.3 m s 1 at 150°E. This acceleration of the
Kuroshio and KE is due to changes in wind stress over the
North Pacific and consequent spin-up of the Kuroshio
recirculation gyre. The acceleration of the currents may
affect sea level along the southern coast of Japan and
northward heat transport under global warming.
Citation: Sakamoto, T. T., H. Hasumi, M. Ishii, S. Emori,
T. Suzuki, T. Nishimura, and A. Sumi (2005), Responses of the
Kuroshio and the Kuroshio Extension to global warming in a
high-resolution climate model, Geophys. Res. Lett., 32, L14617,
doi:10.1029/2005GL023384.
1. Introduction
[2] The Kuroshio is a western boundary current of the
subtropical ocean gyre in the North Pacific, and one of the
strongest ocean currents in the world. The Kuroshio and
the Kuroshio Extension (hereafter, KE), the latter of which
is the extended eastward current of the Kuroshio, have been
frequently investigated from various aspects: the dynamics
of the Kuroshio current path change south of Japan, the
variability of volume transport [e.g., White and McCreary,
1976; Kagimoto and Yamagata, 1997; Isobe and Imawaki,
2002; Tanaka et al., 2004], low-frequency variability of the
KE [e.g., Qiu, 2003], and influences on heat and momentum
fluxes through air– sea interaction [Qiu, 2002; Nonaka and
Xie, 2003; Tanimoto et al., 2003] which play an important
role in the regional climate around Japan and may also have
a significant impact on the global climate. Therefore,
variability and changes of the Kuroshio and KE are very
important issues to be investigated.
[3] Despite of such importance of the Kuroshio and KE,
their responses in global warming projections have little
been investigated. It is mostly because coarse-resolution
ocean models have been used in projections of long-term
1
Frontier Research Center for Global Change, Japan Agency for
Marine-Earth Science and Technology, Kanagawa, Japan.
2
Center for Climate System Research, University of Tokyo, Chiba,
Japan.
3
National Institute for Environmental Studies, Ibaraki, Japan.
Copyright 2005 by the American Geophysical Union.
0094-8276/05/2005GL023384$05.00
climate change by global atmosphere – ocean coupled general circulation models (CGCMs). In coarse-resolution
ocean models, the Kuroshio cannot be resolved enough,
and the latitude of the Kuroshio separation (hereafter, LKS)
overshoots to the north in comparison with observation
[e.g., Choi et al., 2002]. Therefore, such coarse-resolution
ocean models may not be relevant to argue the change of the
Kuroshio and KE under global warming. However, there is
no study with a high-resolution global CGCM so far
because of limited computer resource.
[4] One of solutions to overcome this problem could be a
‘‘time slice experiment’’ with a regional high-resolution
ocean general circulation model (OGCM), in which the
OGCM is integrated with atmospheric fields obtained from
a global warming experiment using a coarse-resolution
CGCM. In a coarse-resolution model, however, the
Kuroshio is suspected to overshoot as mentioned above.
Therefore, the simulated atmospheric fields would be
biased, and heat and water fluxes calculated from such a
biased atmosphere would be unsuitable to force an OGCM
in a time slice experiment.
[5] The purpose of this paper is to show responses of the
Kuroshio and KE to global warming projected by a highresolution CGCM that runs on the Earth Simulator, which is
one of the most powerful super-computers. In this simulation, the Kuroshio does not overshoot.
2. Model and Runs
[6] The model used in the present study is a highresolution setup of the Model for Interdisciplinary Research
on Climate (MIROC) version 3.2 [K-1 Model Developers,
2004], which is optimized for the run on the Earth Simulator. The atmospheric component is a T106 global spectral
model with 56 vertical sigma levels, and the oceanic
component consists of an OGCM of 0.28° (zonally)
0.19° (meridionally) resolution with 48 vertical levels and
a dynamic-thermodynamic sea-ice model. The land and
river models have 0.56° 0.56° and 0.5° 0.5° resolution,
respectively. Note that the equilibrium climate sensitivity of
the atmospheric part of this model coupled to a slab ocean
model responding to CO2 doubling is 4.3 K.
[7] Results of two experiments using the high-resolution
CGCM are shown in this study. One is a simulation for a
control climate state (hereafter control-run), which is carried
out by fixing the external forcing at the year 1900 (preindustrial condition), in terms of solar and volcanic forcing,
greenhouse gases concentration, various aerosols emissions,
and land-use. The other is a global warming experiment
(hereafter CO2-run), where the atmospheric CO2 concentration is increased at the rate of 1% year 1 from the pre-
L14617
1 of 4
L14617
SAKAMOTO ET AL.: KUROSHIO AND KUROSHIO EXTENSION
Figure 1. Long-term mean current velocities at 100-m
depth (vectors, unit: m s 1) and dynamic sea surface height
(contours, unit: m) relative to 2048-m depth in (a) the
control-run, (b) the CO2-run, and (c) their difference of
those between the CO2-run and the control-run (former
minus latter). Contour intervals are 0.2 m in (a) and (b), and
0.05 m in (c).
industrial condition. Both of the control- and CO2-runs are
initiated after 109 years spin-up of the coupled model, and
are integrated for 100 years and 90 years, respectively. The
spin-up run is conducted under the pre-industrial condition.
The control climate state is defined as averaged fields for
the entire period (100 years) of the control-run, and the
global warming state is defined as averaged fields for the
last 20 years of the CO2-run.
L14617
changed largely (Figure 1). The acceleration of the current
speed is large in the KE region, especially in area of 35°–
37°N and 145°– 155°E where the increase of the current
speed is about 0.2– 0.3 m s 1. The standard deviation of the
13-month running mean velocity along the KE averaged
over 145° – 155°E is about 0.07 m s 1 in the control-run.
Therefore the increase of the KE velocity in the CO2-run is
significantly larger than the inherent variability of the
CGCM. Note that there is a weak trend of ocean temperature in the control-run, including the Kuroshio and KE
region, but no trend is found in the velocity of the Kuroshio
and KE.
[10] Since the Kuroshio is a western boundary current of
a wind-driven ocean gyre, spin-up by the subtropical wind
in the North Pacific is a possible cause of the velocity
increase. However, the whole of the subtropical gyre is not
spun-up in the CO2-run but only the recirculation is
(Figure 2). The latter is an anti-cyclonic circulation in the
northwestern corner of the subtropical gyre induced by
vorticity balance in a narrow western boundary current
[Cessi et al., 1990].
[11] Associated with the spin-up of the recirculation,
differences of dynamic sea surface height referenced to
2048-m depth between the control- and the CO2-runs are
large south and southeast of Japan (Figure 1c). This is
brought by intensification of the anti-cyclonic recirculation
south of the Kuroshio and KE. This implies that the sea
level rise along the southern coast of Japan can be relatively
small comparing to that in the offshore if the geostrophic
balance is valid.
3.2. Why is the Kuroshio Recirculation Spun-Up?
[12] Because only the recirculation is spun-up as mentioned above, a local change of wind stress is likely to be a
reason why the Kuroshio and KE are accelerated. Differences in wind stress between the control and the CO2-runs
yield negative wind stress curl over the northwestern North
Pacific, but that of the opposite sign in the southwestern and
northeastern regions (Figure 3). The former is associated
with weakening of the northeasterly trade winds in the
model and forces the subtropical gyre to spin-down. The
latter indicates intensification of the Aleutian Low and it
will be discussed in the Section 4. Between these regions,
the negative change of wind stress curl occurs and it
3. Results
3.1. Mean State of the Kuroshio and KE in the
Control and the Warm Climates
[8] In the mean state of the control-run (Figure 1a), it is
well captured that the Kuroshio has two paths to the south
of Japan: one is a straight path along the southern coast of
Japan and the other is a meandering path flowing off the
southern coast between 135° – 140°E. This reflects the
bimodality of the Kuroshio current path [Shoji, 1972;
Kawabe, 1995]. The followings are realistically reproduced:
LKS, the velocity of the Kuroshio, the stationary meander
of the KE having two crests at 145°E and 150°E, and the
bifurcated KE paths to the east of 152°E [Qu et al., 2001].
[9] In the warm climate obtained by the CO2-run, upperocean velocities of the Kuroshio and KE to the west of
155°E apparently increase in comparison with the control
climate, while the positions of the currents have not
Figure 2. Differences of long-term mean dynamic sea
surface height relative to 2048-m depth (contour, unit: m)
and Sverdrup transport streamfunction (color shading, unit:
Sv 106 m3 s 1) between the CO2-run and the control-run
(former minus latter) in the North Pacific. Contour interval
is 0.05 m.
2 of 4
L14617
SAKAMOTO ET AL.: KUROSHIO AND KUROSHIO EXTENSION
Figure 3. Differences of long-term mean wind stress
(vectors, unit: N m 2) and the curl (color shading, unit:
10 8 N m 3) between the CO2-run and the control-run
(former minus latter).
enhances the subtropical gyre near Japan. Also, there is a
narrow latitude band of negative change in wind stress curl
over 30°– 38°N extending from 140°E to 150°W. As a
result of these wind stress changes, westward-intensified
Sverdrup transport appears in the Kuroshio, the KE, and
their recirculation regions (Figure 2). This intensified
Sverdrup transport means strengthening of its compensating
currents, i.e., the Kuroshio and KE here. It should be noted
that the relationship between the KE acceleration and wind
stress changes over the North Pacific was discussed by
some previous numerical studies in the context of the
climate regime shift occurred in the 1970s [e.g., Xie et al.,
2000; Seager et al., 2001; Schneider et al., 2002].
[13] Although the linear Sverdrup theory supports the
acceleration of the Kuroshio and KE, the change of the
circulation depicted by dynamic sea surface height response
differs from what is expected from the linear theory
(Figure 2). This implies that the linear theory is not
sufficient to account for the response of the Kuroshio and
KE. The discrepancy between the response and the linear
theory can be explained by the recirculation spin-up by a
non-linear effect [Taguchi et al., 2005] induced by the
acceleration of the Kuroshio and KE. This spin-up of the
recirculation results in large acceleration of the KE. In
the CO2-run, the vertically integrated volume transport of
the KE averaged over 145° –155°E where the acceleration
of the current is large increases by about 29 Sv in comparison with the control-run (figure not shown), but increase of
Sverdrup transport calculated from wind stress is 5 – 6 Sv
(Figure 2). Difference of the eastward volume transport
excluding the contribution from the intensified recirculation
is about 5.8 Sv when averaged over 145° –155°E (figure not
shown). This is consistent with the change of Sverdrup
transport. Thus, the recirculation gyre is indeed intensified
in the CO2-run.
[14] This response of wind stress seen in the global
warming experiment resembles that of surface wind induced
by El Nin˜os during boreal winters [see Tanimoto et al.,
2003, Figure 7], which is a remote response to tropical sea
surface temperature anomalies called ‘‘atmospheric bridge’’
[Lau and Nath, 1996]. In the present model, an El Nin˜o-like
response in the tropical Pacific is projected (figures not
shown), as in many other global warming simulations [e.g.,
Cubasch et al., 2001; Meehl et al., 2000; M. Kimoto,
Simulated change of the east Asian circulation under the
global warming, submitted to Geophysical Research
Letters, 2005]. The wind change over the subtropical North
L14617
Pacific can be caused by such a response in the tropics.
However other factors should be considered for the surface
wind change over the subtropical North Pacific since the
mechanism of the mid-latitude atmosphere is complicated.
The El Nin˜o-like response under global warming still
remains to be verified as well. These problems call for
further investigation. Note that such responses in the atmosphere and the Kuroshio occur in the other global warming
projection experiments conducted with our model under
Special Report on Emissions Scenarios (SRES) A1B and
B1 [Intergovernmental Panel on Climate Change, 2000],
hence the results mentioned above are robust in our model.
4. Summary and Discussion
[15] In the present study, it is shown that the Kuroshio
and KE is accelerated in a global warming experiment, with
a new high-resolution climate model. The position where
the Kuroshio separates does not largely change. The acceleration of the Kuroshio and KE occurs as a result of the
recirculation spin-up, which is brought about by the anticyclonic change in wind stress curl over the western region
of the subtropical North Pacific under global warming.
[16] Acceleration of the KE is also recognized in
observational data from 1965 to 2003. Dynamic sea
surface height relative to 500-m depth, which is calculated
from an objective analysis of historical ocean temperature
(revised dataset of Ishii et al. [2003]) and climatological
salinity data [Boyer et al., 2001], shows that the Kuroshio
recirculation has an intensifying trend (Figure 4). The
results in this study seem to be supported by the
observation.
[17] In recent studies, it has been shown that the subtropical and subarctic gyres in the North Pacific have been spunup and spun-down simultaneously [Hanawa, 1995; Ishi and
Hanawa, 2005]. Ishi and Hanawa [2005] pointed out that
the Kuroshio and the Oyashio transports calculated from
wind stress curl with the Sverdrup relationship have high
correlation with the Aleutian Low activity and they have a
positive trend in the last decade of the 20th century. In our
simulation, the Oyashio is also accelerated in the warm
climate state (Figure 1c). We speculate that it is related to
the intensification of the Aleutian Low, although the trend
Figure 4. Distribution of linear trend of dynamic sea
surface height from 1965 to 2003 estimated from a
historical temperature analysis [Ishii et al., 2003]. Contour
interval is 0.5 10 6 m year 1 and the dashed lines
indicate negative values.
3 of 4
L14617
SAKAMOTO ET AL.: KUROSHIO AND KUROSHIO EXTENSION
of the atmospheric circulation around Japan is anti-cyclonic
(Figure 3).
[18] Choi et al. [2002] and Y. Sato et al. (Response of
North Pacific ocean circulation in a Kuroshio-resolving
ocean model to an arctic oscillation (AO)-like change in
Northern Hemisphere atmospheric circulation due to greenhouse-gas forcing, submitted to Journal of the Meteorological Society of Japan, 2005, hereinafter referred to as Sato et
al., submitted manuscript, 2005) presented a northward shift
of LKS in their global warming experiments. According to
Sato et al. (submitted manuscript, 2005), the northward shift
is caused by intensified (reduced) Sverdrup transport in the
mid- (high-) latitudes under global warming, and it is
induced by an atmospheric change like Arctic Oscillation.
This atmospheric change differs from that of the present
study. Therefore, it is necessary to discuss more how the
atmosphere will change under global warming.
[19] Sea level rise (SLR) is one of the most serious
subjects on global warming. According to the present study,
sea level along the southern coast of Japan may be affected
by the accelerated Kuroshio in future. Note that the accelerated Oyashio mentioned above may cause relatively high
SLR along the northeastern coast of Japan to that in the
offshore for the same reason. This is reported by T. Suzuki
et al. (Future projection of sea level and its variability in a
high resolution climate model, submitted to Geophysical
Research Letters, 2005) who discuss the global and local
SLR by thermal expansion led by global warming projections using the same CGCM as in this study.
[20] With a high-resolution atmosphere –ocean coupled
model, it is possible to simulate local climate changes. The
present study is an example of this, and the advance of
climate models toward higher resolution is promising in this
regard.
[21] Acknowledgment. This work is supported by the first subject of
the Kyousei project – ‘‘Project for Sustainable Coexistence of Human,
Nature, and the Earth’’, which is produced by Ministry of Education,
Culture, Sports, Science and Technology of Japan.
References
Boyer, T. P., et al. (2001), World Ocean Database 2001, vol. 2, Temporal
Distribution of Bathythermograph Profiles [CD-ROM], NOAA Atlas
NESDIS 43, 119 pp., NOAA, Silver Spring, Md.
Cessi, P., R. V. Condie, and W. Young (1990), Dissipative dynamics of
western boundary currents, J. Mar. Res., 48, 677 – 700.
Choi, B.-H., D.-H. Kim, and J.-W. Kim (2002), Regional responses of
climate in the northwestern Pacific Ocean to gradual global warming
for a CO2 quadrupling, J. Meteorol. Soc. Jpn., 80, 1427 – 1442.
Cubasch, U., G. A. Meehl, G. J. Boer, R. J. Stouffer, M. Dix, A. Noda, C. A.
Senior, S. Raper, and K. S. Yap (2001), Projections of future
climate change, in Climate Change 2001: The Scientific Basis, edited
by J. T. Houghton et al., pp. 525 – 582, Cambridge Univ. Press, New
York.
Hanawa, K. (1995), Southward penetration of the Oyashio water system
and the wintertime condition of midlatitude westerlies over the North
Pacific, Bull. Hokkaido Natl. Fish. Res. Inst., 59, 103 – 119.
Intergovernmental Panel on Climate Change (2000), Special Report on
Emissions Scenarios, edited by N. Nakicenovic and R. Swart, 612 pp.,
Cambridge Univ. Press, New York.
L14617
Ishi, Y., and K. Hanawa (2005), Large-scale variabilities of wintertime wind
stress curl field in the North Pacific and their relation to atmospheric
teleconnection patterns, Geophys. Res. Lett., 32, L10607, doi:10.1029/
2004GL022330.
Ishii, M., M. Kimoto, and M. Kachi (2003), Historical ocean subsurface
temperature analysis with error estimates, Mon. Weather Rev., 131, 51 –
73.
Isobe, A., and S. Imawaki (2002), Annual variation of the Kuroshio transport in a two-layer numerical model with a ridge, J. Phys. Oceanogr., 32,
994 – 1009.
K-1 Model Developers (2004), K-1 coupled model (MIROC) description,
K-1 Tech. Rep., 1, edited by H. Hasumi and S. Emori, 34 pp., Cent. for
Clim. Syst. Res., Univ. of Tokyo, Tokyo.
Kagimoto, T., and T. Yamagata (1997), Seasonal transport variations of the
Kuroshio: An OGCM simulation, J. Phys. Oceanogr., 27, 403 – 418.
Kawabe, M. (1995), Variations of current path, velocity, and volume transport of the Kuroshio in relation with the large meander, J. Phys. Oceanogr., 25, 3103 – 3117.
Lau, N.-C., and M. J. Nath (1996), The role of the ‘‘atmospheric bridge’’ in
linking tropical Pacific ENSO events to extratropical SST anomalies,
J. Clim., 9, 2036 – 2057.
Meehl, G. A., W. D. Collins, B. A. Boville, J. T. Kiehl, T. M. L. Wigley,
and J. M. Arblaster (2000), Response of the NCAR Climate System
Model to increased CO2 and the role of physical processes, J. Clim.,
13, 1879 – 1898.
Nonaka, M., and S.-P. Xie (2003), Covariations of sea surface temperature
and wind over the Kuroshio and its extension: Evidence for ocean-toatmosphere feedback, J. Clim., 16, 1404 – 1413.
Qiu, B. (2002), The Kuroshio Extension system: Its large-scale variability
and role in the midlatitude ocean-atmosphere interaction, J. Oceanogr.,
58, 57 – 75.
Qiu, B. (2003), Kuroshio Extension variability and forcing of the Pacific
decadal oscillations: Responses and potential feedback, J. Phys. Oceanogr., 33, 2465 – 2482.
Qu, T., H. Mitsudera, and B. Qiu (2001), A climatological view of the
Kuroshio/Oyashio system east of Japan, J. Phys. Oceanogr., 31, 2575 –
2589.
Schneider, N., A. J. Miller, and D. W. Pierce (2002), Anatomy of North
Pacific decadal variability, J. Clim., 15, 586 – 605.
Seager, R., Y. Kushnir, N. H. Naik, M. A. Cane, and J. Miller (2001), Winddriven shifts in the latitude of the Kuroshio – Oyashio Extension and
generation of SST anomalies on decadal timescales, J. Clim., 14,
4249 – 4265.
Shoji, D. (1972), Time variation of the Kuroshio south of Japan, in Kuroshio—Its Physical Aspects, edited by H. Stommel and K. Yoshida, pp.
217 – 234, Univ. of Tokyo Press, Tokyo.
Taguchi, B., S.-P. Xie, H. Mitsudera, and A. Kubokawa (2005), Response
of the Kuroshio Extension to Rossby waves associated with the 1970s
climate regime shift in a high-resolution ocean model, J. Clim., in press.
Tanaka, K., M. Ikeda, and Y. Masumoto (2004), Predictability of interannual variability in the Kuroshio transport south of Japan based on wind
stress data over the North Pacific, J. Oceanogr., 60, 283 – 291.
Tanimoto, Y., H. Nakamura, T. Kagimoto, and S. Yamane (2003), An active
role of extratropical sea surface temperature anomalies in determining
anomalous turbulent heat flux, J. Geophys. Res., 108(C10), 3304,
doi:10.1029/2002JC001750.
White, W. B., and J. P. McCreary (1976), On the formation of the Kuroshio
meander and its relationship to the large-scale ocean circulation, Deep
Sea Res., 23, 33 – 47.
Xie, S.-P., T. Kunitani, A. Kubokawa, M. Nonaka, and S. Hosoda (2000),
Interdecadal thermocline variability in the North Pacific for 1958 – 97: A
GCM simulation, J. Phys. Oceanogr., 30, 2798 – 2813.
H. Hasumi and A. Sumi, Center for Climate System Research, University
of Tokyo, Chiba 277-8568, Japan.
M. Ishii, T. Nishimura, T. T. Sakamoto, and T. Suzuki, Frontier Research
Center for Global Change, Japan Agency for Marine-Earth Science and
Technology, Kanagawa 236-0001, Japan. ([email protected])
S. Emori, National Institute for Environmental Studies, Ibaraki 305-8506,
Japan.
4 of 4
Responses of the Kuroshio and the Kuroshio Extension to global
warming in a high-resolution climate model
Takashi T. Sakamoto,1 Hiroyasu Hasumi,2 Masayoshi Ishii,1 Seita Emori,1,3
Tatsuo Suzuki,1 Teruyuki Nishimura,1 and Akimasa Sumi2
Received 30 April 2005; revised 15 June 2005; accepted 20 June 2005; published 27 July 2005.
[1] Using a high-resolution atmosphere – ocean coupled
climate model, responses of the Kuroshio and the Kuroshio
Extension (KE) to global warming are investigated. In a
climate change experiment with atmospheric CO 2
concentration ideally increased by 1% year 1, the current
velocity of the Kuroshio and KE increases, while the
latitude of the Kuroshio separation to the east of Japan does
not change significantly. The increase of the current velocity
is up to 0.3 m s 1 at 150°E. This acceleration of the
Kuroshio and KE is due to changes in wind stress over the
North Pacific and consequent spin-up of the Kuroshio
recirculation gyre. The acceleration of the currents may
affect sea level along the southern coast of Japan and
northward heat transport under global warming.
Citation: Sakamoto, T. T., H. Hasumi, M. Ishii, S. Emori,
T. Suzuki, T. Nishimura, and A. Sumi (2005), Responses of the
Kuroshio and the Kuroshio Extension to global warming in a
high-resolution climate model, Geophys. Res. Lett., 32, L14617,
doi:10.1029/2005GL023384.
1. Introduction
[2] The Kuroshio is a western boundary current of the
subtropical ocean gyre in the North Pacific, and one of the
strongest ocean currents in the world. The Kuroshio and
the Kuroshio Extension (hereafter, KE), the latter of which
is the extended eastward current of the Kuroshio, have been
frequently investigated from various aspects: the dynamics
of the Kuroshio current path change south of Japan, the
variability of volume transport [e.g., White and McCreary,
1976; Kagimoto and Yamagata, 1997; Isobe and Imawaki,
2002; Tanaka et al., 2004], low-frequency variability of the
KE [e.g., Qiu, 2003], and influences on heat and momentum
fluxes through air– sea interaction [Qiu, 2002; Nonaka and
Xie, 2003; Tanimoto et al., 2003] which play an important
role in the regional climate around Japan and may also have
a significant impact on the global climate. Therefore,
variability and changes of the Kuroshio and KE are very
important issues to be investigated.
[3] Despite of such importance of the Kuroshio and KE,
their responses in global warming projections have little
been investigated. It is mostly because coarse-resolution
ocean models have been used in projections of long-term
1
Frontier Research Center for Global Change, Japan Agency for
Marine-Earth Science and Technology, Kanagawa, Japan.
2
Center for Climate System Research, University of Tokyo, Chiba,
Japan.
3
National Institute for Environmental Studies, Ibaraki, Japan.
Copyright 2005 by the American Geophysical Union.
0094-8276/05/2005GL023384$05.00
climate change by global atmosphere – ocean coupled general circulation models (CGCMs). In coarse-resolution
ocean models, the Kuroshio cannot be resolved enough,
and the latitude of the Kuroshio separation (hereafter, LKS)
overshoots to the north in comparison with observation
[e.g., Choi et al., 2002]. Therefore, such coarse-resolution
ocean models may not be relevant to argue the change of the
Kuroshio and KE under global warming. However, there is
no study with a high-resolution global CGCM so far
because of limited computer resource.
[4] One of solutions to overcome this problem could be a
‘‘time slice experiment’’ with a regional high-resolution
ocean general circulation model (OGCM), in which the
OGCM is integrated with atmospheric fields obtained from
a global warming experiment using a coarse-resolution
CGCM. In a coarse-resolution model, however, the
Kuroshio is suspected to overshoot as mentioned above.
Therefore, the simulated atmospheric fields would be
biased, and heat and water fluxes calculated from such a
biased atmosphere would be unsuitable to force an OGCM
in a time slice experiment.
[5] The purpose of this paper is to show responses of the
Kuroshio and KE to global warming projected by a highresolution CGCM that runs on the Earth Simulator, which is
one of the most powerful super-computers. In this simulation, the Kuroshio does not overshoot.
2. Model and Runs
[6] The model used in the present study is a highresolution setup of the Model for Interdisciplinary Research
on Climate (MIROC) version 3.2 [K-1 Model Developers,
2004], which is optimized for the run on the Earth Simulator. The atmospheric component is a T106 global spectral
model with 56 vertical sigma levels, and the oceanic
component consists of an OGCM of 0.28° (zonally)
0.19° (meridionally) resolution with 48 vertical levels and
a dynamic-thermodynamic sea-ice model. The land and
river models have 0.56° 0.56° and 0.5° 0.5° resolution,
respectively. Note that the equilibrium climate sensitivity of
the atmospheric part of this model coupled to a slab ocean
model responding to CO2 doubling is 4.3 K.
[7] Results of two experiments using the high-resolution
CGCM are shown in this study. One is a simulation for a
control climate state (hereafter control-run), which is carried
out by fixing the external forcing at the year 1900 (preindustrial condition), in terms of solar and volcanic forcing,
greenhouse gases concentration, various aerosols emissions,
and land-use. The other is a global warming experiment
(hereafter CO2-run), where the atmospheric CO2 concentration is increased at the rate of 1% year 1 from the pre-
L14617
1 of 4
L14617
SAKAMOTO ET AL.: KUROSHIO AND KUROSHIO EXTENSION
Figure 1. Long-term mean current velocities at 100-m
depth (vectors, unit: m s 1) and dynamic sea surface height
(contours, unit: m) relative to 2048-m depth in (a) the
control-run, (b) the CO2-run, and (c) their difference of
those between the CO2-run and the control-run (former
minus latter). Contour intervals are 0.2 m in (a) and (b), and
0.05 m in (c).
industrial condition. Both of the control- and CO2-runs are
initiated after 109 years spin-up of the coupled model, and
are integrated for 100 years and 90 years, respectively. The
spin-up run is conducted under the pre-industrial condition.
The control climate state is defined as averaged fields for
the entire period (100 years) of the control-run, and the
global warming state is defined as averaged fields for the
last 20 years of the CO2-run.
L14617
changed largely (Figure 1). The acceleration of the current
speed is large in the KE region, especially in area of 35°–
37°N and 145°– 155°E where the increase of the current
speed is about 0.2– 0.3 m s 1. The standard deviation of the
13-month running mean velocity along the KE averaged
over 145° – 155°E is about 0.07 m s 1 in the control-run.
Therefore the increase of the KE velocity in the CO2-run is
significantly larger than the inherent variability of the
CGCM. Note that there is a weak trend of ocean temperature in the control-run, including the Kuroshio and KE
region, but no trend is found in the velocity of the Kuroshio
and KE.
[10] Since the Kuroshio is a western boundary current of
a wind-driven ocean gyre, spin-up by the subtropical wind
in the North Pacific is a possible cause of the velocity
increase. However, the whole of the subtropical gyre is not
spun-up in the CO2-run but only the recirculation is
(Figure 2). The latter is an anti-cyclonic circulation in the
northwestern corner of the subtropical gyre induced by
vorticity balance in a narrow western boundary current
[Cessi et al., 1990].
[11] Associated with the spin-up of the recirculation,
differences of dynamic sea surface height referenced to
2048-m depth between the control- and the CO2-runs are
large south and southeast of Japan (Figure 1c). This is
brought by intensification of the anti-cyclonic recirculation
south of the Kuroshio and KE. This implies that the sea
level rise along the southern coast of Japan can be relatively
small comparing to that in the offshore if the geostrophic
balance is valid.
3.2. Why is the Kuroshio Recirculation Spun-Up?
[12] Because only the recirculation is spun-up as mentioned above, a local change of wind stress is likely to be a
reason why the Kuroshio and KE are accelerated. Differences in wind stress between the control and the CO2-runs
yield negative wind stress curl over the northwestern North
Pacific, but that of the opposite sign in the southwestern and
northeastern regions (Figure 3). The former is associated
with weakening of the northeasterly trade winds in the
model and forces the subtropical gyre to spin-down. The
latter indicates intensification of the Aleutian Low and it
will be discussed in the Section 4. Between these regions,
the negative change of wind stress curl occurs and it
3. Results
3.1. Mean State of the Kuroshio and KE in the
Control and the Warm Climates
[8] In the mean state of the control-run (Figure 1a), it is
well captured that the Kuroshio has two paths to the south
of Japan: one is a straight path along the southern coast of
Japan and the other is a meandering path flowing off the
southern coast between 135° – 140°E. This reflects the
bimodality of the Kuroshio current path [Shoji, 1972;
Kawabe, 1995]. The followings are realistically reproduced:
LKS, the velocity of the Kuroshio, the stationary meander
of the KE having two crests at 145°E and 150°E, and the
bifurcated KE paths to the east of 152°E [Qu et al., 2001].
[9] In the warm climate obtained by the CO2-run, upperocean velocities of the Kuroshio and KE to the west of
155°E apparently increase in comparison with the control
climate, while the positions of the currents have not
Figure 2. Differences of long-term mean dynamic sea
surface height relative to 2048-m depth (contour, unit: m)
and Sverdrup transport streamfunction (color shading, unit:
Sv 106 m3 s 1) between the CO2-run and the control-run
(former minus latter) in the North Pacific. Contour interval
is 0.05 m.
2 of 4
L14617
SAKAMOTO ET AL.: KUROSHIO AND KUROSHIO EXTENSION
Figure 3. Differences of long-term mean wind stress
(vectors, unit: N m 2) and the curl (color shading, unit:
10 8 N m 3) between the CO2-run and the control-run
(former minus latter).
enhances the subtropical gyre near Japan. Also, there is a
narrow latitude band of negative change in wind stress curl
over 30°– 38°N extending from 140°E to 150°W. As a
result of these wind stress changes, westward-intensified
Sverdrup transport appears in the Kuroshio, the KE, and
their recirculation regions (Figure 2). This intensified
Sverdrup transport means strengthening of its compensating
currents, i.e., the Kuroshio and KE here. It should be noted
that the relationship between the KE acceleration and wind
stress changes over the North Pacific was discussed by
some previous numerical studies in the context of the
climate regime shift occurred in the 1970s [e.g., Xie et al.,
2000; Seager et al., 2001; Schneider et al., 2002].
[13] Although the linear Sverdrup theory supports the
acceleration of the Kuroshio and KE, the change of the
circulation depicted by dynamic sea surface height response
differs from what is expected from the linear theory
(Figure 2). This implies that the linear theory is not
sufficient to account for the response of the Kuroshio and
KE. The discrepancy between the response and the linear
theory can be explained by the recirculation spin-up by a
non-linear effect [Taguchi et al., 2005] induced by the
acceleration of the Kuroshio and KE. This spin-up of the
recirculation results in large acceleration of the KE. In
the CO2-run, the vertically integrated volume transport of
the KE averaged over 145° –155°E where the acceleration
of the current is large increases by about 29 Sv in comparison with the control-run (figure not shown), but increase of
Sverdrup transport calculated from wind stress is 5 – 6 Sv
(Figure 2). Difference of the eastward volume transport
excluding the contribution from the intensified recirculation
is about 5.8 Sv when averaged over 145° –155°E (figure not
shown). This is consistent with the change of Sverdrup
transport. Thus, the recirculation gyre is indeed intensified
in the CO2-run.
[14] This response of wind stress seen in the global
warming experiment resembles that of surface wind induced
by El Nin˜os during boreal winters [see Tanimoto et al.,
2003, Figure 7], which is a remote response to tropical sea
surface temperature anomalies called ‘‘atmospheric bridge’’
[Lau and Nath, 1996]. In the present model, an El Nin˜o-like
response in the tropical Pacific is projected (figures not
shown), as in many other global warming simulations [e.g.,
Cubasch et al., 2001; Meehl et al., 2000; M. Kimoto,
Simulated change of the east Asian circulation under the
global warming, submitted to Geophysical Research
Letters, 2005]. The wind change over the subtropical North
L14617
Pacific can be caused by such a response in the tropics.
However other factors should be considered for the surface
wind change over the subtropical North Pacific since the
mechanism of the mid-latitude atmosphere is complicated.
The El Nin˜o-like response under global warming still
remains to be verified as well. These problems call for
further investigation. Note that such responses in the atmosphere and the Kuroshio occur in the other global warming
projection experiments conducted with our model under
Special Report on Emissions Scenarios (SRES) A1B and
B1 [Intergovernmental Panel on Climate Change, 2000],
hence the results mentioned above are robust in our model.
4. Summary and Discussion
[15] In the present study, it is shown that the Kuroshio
and KE is accelerated in a global warming experiment, with
a new high-resolution climate model. The position where
the Kuroshio separates does not largely change. The acceleration of the Kuroshio and KE occurs as a result of the
recirculation spin-up, which is brought about by the anticyclonic change in wind stress curl over the western region
of the subtropical North Pacific under global warming.
[16] Acceleration of the KE is also recognized in
observational data from 1965 to 2003. Dynamic sea
surface height relative to 500-m depth, which is calculated
from an objective analysis of historical ocean temperature
(revised dataset of Ishii et al. [2003]) and climatological
salinity data [Boyer et al., 2001], shows that the Kuroshio
recirculation has an intensifying trend (Figure 4). The
results in this study seem to be supported by the
observation.
[17] In recent studies, it has been shown that the subtropical and subarctic gyres in the North Pacific have been spunup and spun-down simultaneously [Hanawa, 1995; Ishi and
Hanawa, 2005]. Ishi and Hanawa [2005] pointed out that
the Kuroshio and the Oyashio transports calculated from
wind stress curl with the Sverdrup relationship have high
correlation with the Aleutian Low activity and they have a
positive trend in the last decade of the 20th century. In our
simulation, the Oyashio is also accelerated in the warm
climate state (Figure 1c). We speculate that it is related to
the intensification of the Aleutian Low, although the trend
Figure 4. Distribution of linear trend of dynamic sea
surface height from 1965 to 2003 estimated from a
historical temperature analysis [Ishii et al., 2003]. Contour
interval is 0.5 10 6 m year 1 and the dashed lines
indicate negative values.
3 of 4
L14617
SAKAMOTO ET AL.: KUROSHIO AND KUROSHIO EXTENSION
of the atmospheric circulation around Japan is anti-cyclonic
(Figure 3).
[18] Choi et al. [2002] and Y. Sato et al. (Response of
North Pacific ocean circulation in a Kuroshio-resolving
ocean model to an arctic oscillation (AO)-like change in
Northern Hemisphere atmospheric circulation due to greenhouse-gas forcing, submitted to Journal of the Meteorological Society of Japan, 2005, hereinafter referred to as Sato et
al., submitted manuscript, 2005) presented a northward shift
of LKS in their global warming experiments. According to
Sato et al. (submitted manuscript, 2005), the northward shift
is caused by intensified (reduced) Sverdrup transport in the
mid- (high-) latitudes under global warming, and it is
induced by an atmospheric change like Arctic Oscillation.
This atmospheric change differs from that of the present
study. Therefore, it is necessary to discuss more how the
atmosphere will change under global warming.
[19] Sea level rise (SLR) is one of the most serious
subjects on global warming. According to the present study,
sea level along the southern coast of Japan may be affected
by the accelerated Kuroshio in future. Note that the accelerated Oyashio mentioned above may cause relatively high
SLR along the northeastern coast of Japan to that in the
offshore for the same reason. This is reported by T. Suzuki
et al. (Future projection of sea level and its variability in a
high resolution climate model, submitted to Geophysical
Research Letters, 2005) who discuss the global and local
SLR by thermal expansion led by global warming projections using the same CGCM as in this study.
[20] With a high-resolution atmosphere –ocean coupled
model, it is possible to simulate local climate changes. The
present study is an example of this, and the advance of
climate models toward higher resolution is promising in this
regard.
[21] Acknowledgment. This work is supported by the first subject of
the Kyousei project – ‘‘Project for Sustainable Coexistence of Human,
Nature, and the Earth’’, which is produced by Ministry of Education,
Culture, Sports, Science and Technology of Japan.
References
Boyer, T. P., et al. (2001), World Ocean Database 2001, vol. 2, Temporal
Distribution of Bathythermograph Profiles [CD-ROM], NOAA Atlas
NESDIS 43, 119 pp., NOAA, Silver Spring, Md.
Cessi, P., R. V. Condie, and W. Young (1990), Dissipative dynamics of
western boundary currents, J. Mar. Res., 48, 677 – 700.
Choi, B.-H., D.-H. Kim, and J.-W. Kim (2002), Regional responses of
climate in the northwestern Pacific Ocean to gradual global warming
for a CO2 quadrupling, J. Meteorol. Soc. Jpn., 80, 1427 – 1442.
Cubasch, U., G. A. Meehl, G. J. Boer, R. J. Stouffer, M. Dix, A. Noda, C. A.
Senior, S. Raper, and K. S. Yap (2001), Projections of future
climate change, in Climate Change 2001: The Scientific Basis, edited
by J. T. Houghton et al., pp. 525 – 582, Cambridge Univ. Press, New
York.
Hanawa, K. (1995), Southward penetration of the Oyashio water system
and the wintertime condition of midlatitude westerlies over the North
Pacific, Bull. Hokkaido Natl. Fish. Res. Inst., 59, 103 – 119.
Intergovernmental Panel on Climate Change (2000), Special Report on
Emissions Scenarios, edited by N. Nakicenovic and R. Swart, 612 pp.,
Cambridge Univ. Press, New York.
L14617
Ishi, Y., and K. Hanawa (2005), Large-scale variabilities of wintertime wind
stress curl field in the North Pacific and their relation to atmospheric
teleconnection patterns, Geophys. Res. Lett., 32, L10607, doi:10.1029/
2004GL022330.
Ishii, M., M. Kimoto, and M. Kachi (2003), Historical ocean subsurface
temperature analysis with error estimates, Mon. Weather Rev., 131, 51 –
73.
Isobe, A., and S. Imawaki (2002), Annual variation of the Kuroshio transport in a two-layer numerical model with a ridge, J. Phys. Oceanogr., 32,
994 – 1009.
K-1 Model Developers (2004), K-1 coupled model (MIROC) description,
K-1 Tech. Rep., 1, edited by H. Hasumi and S. Emori, 34 pp., Cent. for
Clim. Syst. Res., Univ. of Tokyo, Tokyo.
Kagimoto, T., and T. Yamagata (1997), Seasonal transport variations of the
Kuroshio: An OGCM simulation, J. Phys. Oceanogr., 27, 403 – 418.
Kawabe, M. (1995), Variations of current path, velocity, and volume transport of the Kuroshio in relation with the large meander, J. Phys. Oceanogr., 25, 3103 – 3117.
Lau, N.-C., and M. J. Nath (1996), The role of the ‘‘atmospheric bridge’’ in
linking tropical Pacific ENSO events to extratropical SST anomalies,
J. Clim., 9, 2036 – 2057.
Meehl, G. A., W. D. Collins, B. A. Boville, J. T. Kiehl, T. M. L. Wigley,
and J. M. Arblaster (2000), Response of the NCAR Climate System
Model to increased CO2 and the role of physical processes, J. Clim.,
13, 1879 – 1898.
Nonaka, M., and S.-P. Xie (2003), Covariations of sea surface temperature
and wind over the Kuroshio and its extension: Evidence for ocean-toatmosphere feedback, J. Clim., 16, 1404 – 1413.
Qiu, B. (2002), The Kuroshio Extension system: Its large-scale variability
and role in the midlatitude ocean-atmosphere interaction, J. Oceanogr.,
58, 57 – 75.
Qiu, B. (2003), Kuroshio Extension variability and forcing of the Pacific
decadal oscillations: Responses and potential feedback, J. Phys. Oceanogr., 33, 2465 – 2482.
Qu, T., H. Mitsudera, and B. Qiu (2001), A climatological view of the
Kuroshio/Oyashio system east of Japan, J. Phys. Oceanogr., 31, 2575 –
2589.
Schneider, N., A. J. Miller, and D. W. Pierce (2002), Anatomy of North
Pacific decadal variability, J. Clim., 15, 586 – 605.
Seager, R., Y. Kushnir, N. H. Naik, M. A. Cane, and J. Miller (2001), Winddriven shifts in the latitude of the Kuroshio – Oyashio Extension and
generation of SST anomalies on decadal timescales, J. Clim., 14,
4249 – 4265.
Shoji, D. (1972), Time variation of the Kuroshio south of Japan, in Kuroshio—Its Physical Aspects, edited by H. Stommel and K. Yoshida, pp.
217 – 234, Univ. of Tokyo Press, Tokyo.
Taguchi, B., S.-P. Xie, H. Mitsudera, and A. Kubokawa (2005), Response
of the Kuroshio Extension to Rossby waves associated with the 1970s
climate regime shift in a high-resolution ocean model, J. Clim., in press.
Tanaka, K., M. Ikeda, and Y. Masumoto (2004), Predictability of interannual variability in the Kuroshio transport south of Japan based on wind
stress data over the North Pacific, J. Oceanogr., 60, 283 – 291.
Tanimoto, Y., H. Nakamura, T. Kagimoto, and S. Yamane (2003), An active
role of extratropical sea surface temperature anomalies in determining
anomalous turbulent heat flux, J. Geophys. Res., 108(C10), 3304,
doi:10.1029/2002JC001750.
White, W. B., and J. P. McCreary (1976), On the formation of the Kuroshio
meander and its relationship to the large-scale ocean circulation, Deep
Sea Res., 23, 33 – 47.
Xie, S.-P., T. Kunitani, A. Kubokawa, M. Nonaka, and S. Hosoda (2000),
Interdecadal thermocline variability in the North Pacific for 1958 – 97: A
GCM simulation, J. Phys. Oceanogr., 30, 2798 – 2813.
H. Hasumi and A. Sumi, Center for Climate System Research, University
of Tokyo, Chiba 277-8568, Japan.
M. Ishii, T. Nishimura, T. T. Sakamoto, and T. Suzuki, Frontier Research
Center for Global Change, Japan Agency for Marine-Earth Science and
Technology, Kanagawa 236-0001, Japan. ([email protected])
S. Emori, National Institute for Environmental Studies, Ibaraki 305-8506,
Japan.
4 of 4