Responses of the Kuroshio and the Kurosh

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Responses of the Kuroshio and the Kuroshio Extension to global

warming in a high-resolution climate model

Takashi T. Sakamoto,1Hiroyasu Hasumi,2 Masayoshi Ishii,1Seita Emori,1,3 Tatsuo Suzuki,1 Teruyuki Nishimura,1and 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 CO2 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

climate change by global atmosphere – ocean coupled gen-eral 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 high-resolution CGCM that runs on the Earth Simulator, which is one of the most powerful super-computers. In this simula-tion, the Kuroshio does not overshoot.

2. Model and Runs

[6] The model used in the present study is a high-resolution 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 Simu-lator. 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 CO2doubling 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 (pre-industrial 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 concentra-tion is increased at the rate of 1% year 1 from the

pre-GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L14617, doi:10.1029/2005GL023384, 2005

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.}

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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.

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, upper-ocean 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

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 temper-ature 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 men-tioned above, a local change of wind stress is likely to be a reason why the Kuroshio and KE are accelerated. Differ-ences 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

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).

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: Sv106m3s 1) between the CO2-run and the control-run (former minus latter) in the North Pacific. Contour interval is 0.05 m.

L14617 SAKAMOTO ET AL.: KUROSHIO AND KUROSHIO EXTENSION L14617

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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 compar-ison 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

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 atmo-sphere 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 accel-eration of the Kuroshio and KE occurs as a result of the recirculation spin-up, which is brought about by the anti-cyclonic 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 subtrop-ical and subarctic gyres in the North Pacific have been spun-up 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 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).

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.

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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 green-house-gas forcing, submitted toJournal of the Meteorolog-ical 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 accel-erated 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 projec-tions 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.

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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. (teng@jamstec.go.jp)

S. Emori, National Institute for Environmental Studies, Ibaraki 305-8506, Japan.