RESEARCH LETTER
10.1029/2021GL097250
Key Points:• O cean modeling suggests that, due
to an enhanced North Equatorial
Current, Kuroshio was stronger during
last glacial period than at present• Southward migration of the Kuroshio
Extension during the last glacial
period was between 0.5 and 3.5°
depending on the longitudes• Model results indicate that the vertical
thermal gradient in the upper ocean
is not a suitable indicator of theKuroshio intensity
Supporting Information:

Supporting Information may be found in
the online version of this article.
Correspondence to:

X. Guo,guoxinyu@sci.ehime-u.ac.jp

Citation:

Yang, H., Guo, X., Miyazawa, Y.,
Varlamov, S. M., Abe-Ouchi, A., & Chan,
W.-L. (2022). Changes in the Kuroshio
path, surface velocity and transport
during the last 35,000 years. GeophysicalResearch Letters, 49, e2021GL097250.
https://doi.org/10.1029/2021GL097250
Received 29 NOV 2021
Accepted 29 JAN 2022
Author Contributions:

Formal analysis: Haiyan Yang
Supervision: Xinyu Guo
Writing – original draft: Haiyan Yang
Writing – review & editing: Xinyu
Guo, Yasumasa Miyazawa, Sergey M.
Varlamov, Ayako Abe-Ouchi, Wing-Le
Chan© 2022. American Geophysical Union.
All Rights Reserved.

YANG ET AL.

Changes in the Kuroshio Path, Surface Velocity and Transport
During the Last 35,000 Years
Haiyan Yang1 , Xinyu Guo2,3 , Yasumasa Miyazawa3
Ayako Abe-Ouchi4 , and Wing-Le Chan4, Sergey M. Varlamov3,

1
Graduate School of Science and Engineering, Ehime University, Matsuyama, Japan, 2Center for Marine Environmental
Studies, Ehime University, Matsuyama, Japan, 3Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan,
4
Atmosphere and Ocean Research Institute, University of Tokyo, Tokyo, Japan

Abstract We consider the sea level, air–sea heat flux, and wind stresses and use an ocean model toinvestigate the evolution of the Kuroshio path and intensity during the last 35,000 years. Relative to the present,
the Kuroshio during the last glacial period traveled the same path albeit with higher surface velocity in the
East China Sea, while it migrated northward south of Japan and southward at the Kuroshio Extension (KE).
The southward migrations of the KE axis were closely related to the positions of zero wind-stress curl. To acertain extent, stronger glacial trade winds enhanced the North Equatorial Current. Consequently, Kuroshiotransport increased in the southern and middle Okinawa Trough. Regarding Kuroshio strength, we suggest thatthe horizontal gradient of the subsurface temperature would be a better indicator than the upper-ocean vertical
thermal gradient, which is a commonly used index in paleoceanography.
Plain Language Summary The last glacial age was characterized by millennial-scale climate
variations and sea-level changes. Although the path and intensity of the Kuroshio during the last glacial age
have been studied extensively, the evolution of the Kuroshio is still poorly understood because of limited
observations and incomplete model configurations. Using an improved ocean model and paleoclimate air–sea
forcing field, we simulated the changes in the Kuroshio and Kuroshio Extension (KE) from 35,000 years ago
to the present. Compared to the present, the North Pacific subtropical gyre including its southern and northernboundaries (i.e., North Equatorial Current and KE) shifted southward during the glacial age, which was
associated with the changes in the wind-stress curl over the North Pacific. The North Equatorial Current andits northern branch (i.e., Kuroshio) was enhanced during the last glacial period. Consequently, the Kuroshiotransport increased with a higher surface velocity in the southern and middle East China Sea. With sea level
lowering and climatic variations at glacial times, the Kuroshio path south of Japan shifted northward. The KE
shifted southward with a range of 0.5–3.5°, which depended on the longitudes. The Kuroshio Current at 6 ka
became similar to that of modern day.
1. Introduction
The Kuroshio Current (KC) is a strong western boundary current in the North Pacific, entering the Okinawa
Trough via the Yonaguni Depression off the eastern coast of Taiwan, flowing along the continental slope of theEast China Sea (ECS), and eventually turning eastward through the Tokara Strait. After passing the area south ofJapan, the Kuroshio separates from the coast of Japan and flows eastward as the Kuroshio Extension (KE) (Figure 1a). The Kuroshio transports warm water from the tropics to the mid-latitudes, and variations in its path andvolume transport may influence profoundly the climate in East Asia (Gallagher et al., 2015; S. Kim et al., 2017).
In the last few decades, there has been increasing interest in the Kuroshio variations during the past 40,000 years.
The KC in the Last Glacial Maximum (LGM; ∼21 ka BP) was initially thought to be deflected to the east into
the Pacific because of the land bridge between the Ryukyu Arc and Taiwan (Ahagon et al., 1993; H. Ujiié
et al., 1991). However, many proxies in the Okinawa Trough, such as planktonic foraminiferal assemblages, oxygen isotopes, and sediment characteristics, indicated the inflow of the KC into the ECS during the last glacial
period (Amano & Itaki, 2016; D. W. Li et al., 2018; Kawahata & Ohshima, 2004; Xu & Oda, 1999). In addition tothis pathway, it has been suggested that the KC intrusion into the Okinawa Trough was reduced during the LGM
and early deglaciation (D. W. Li et al., 2018; Ijiri et al., 2005; Xiang et al., 2007; Y. Ujiié et al., 2016). However,1 of 10

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Figure 1. (a) Northwest Pacific Ocean bathymetry (shaded colors; unit: m) and ocean circulation (semitransparent blue
arrows) map. Red lines indicate sections used for volume transports in Table S2 in Supporting Information S1. The NEC,
NECC, MC, Kuroshio Current (KC), and Kuroshio Extension (KE) acronyms stand for North Equatorial Current, North
Equatorial Countercurrent, Mindanao Current, KC, and KE, respectively. (b) Domain and bathymetry of the model. Colors
indicate model bathymetry (unit: m). (c) Annual mean surface current (0–100 m, unit: m/s) in the Northwest Pacific. Shading
indicates the area where current speed exceeds 0.1 m/s. (d) Kuroshio axes in five different experiments. Gray dotted lines
indicate the East China Sea depths; other dotted lines indicate the northern KE limits. Red lines indicate sections for checking
the velocity cores and water temperatures. Black circles (from 1 to 8) are the sediment cores used in our discussion.

based on the temperature and nutrient conditions, the Kuroshio transport could be stronger (Kubota et al., 2017).
Thus far, it remains unclear whether the paleo-KC was stronger or weaker.
The KE is at the northern boundary of the North Pacific subtropical gyre. Early studies of planktic foraminiferal
assemblages reported that the boundary region of the subtropical-subarctic gyre and the KE shifted southward
during the last glacial period (Gallagher et al., 2015; Harada et al., 2004; Kawahata & Ohshima, 2002; Seoet al., 2018; Thompson, 1981; Thompson & Shackleton, 1980; Yamamoto, 2009). However, Sawada and Handa (1998) suggested northward Kuroshio migrations off the coast of Central Japan from 26 to 24.5 and 21 to
19 ka BP. To date, the paleo-KC path and intensity have been investigated and debated, but observations havevery limited spatial and temporal scales.

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In addition to these observations, ocean models have significantly improved our paleo-KC understanding. Earlier models have shown that the KC entered the Okinawa Trough with a stronger surface current and a reduced
volume transport during the LGM (Kao et al., 2006; Lee et al., 2013; Vogt-Vincent & Mitarai, 2020; Zheng
et al., 2016), but the mechanism was not adequately explained. Most modeling studies considered only the effectof sea-level lowering on the KC, using low-resolution models that were unable to resolve mesoscale eddies adequately. Moreover, earlier regional models, which relied greatly on the specified boundary conditions, focused on
the ECS but excluded the KE system. It was indicated that the westerly jet stream over the North Pacific during
the LGM migrated southward compared to the present position (Gray et al., 2020; N. Wang et al., 2018; Yanase& Abe-Ouchi, 2010). However, few of the existing modeling studies explicitly explored the relative extent andstrength of the paleo-KE under the combined influences of sea level and climate. A large-scale ocean model forstudying the Kuroshio region including the KE system is therefore necessary.
For the Kuroshio and its adjacent regions, the condition of the KC is affected by millennial-scale changes in theatmosphere and equatorial Pacific (Y. Ujiié et al., 2016). The KE dynamics are influenced by wind forcing overthe mid-latitude North Pacific, and the KE variability is mainly due to westward propagating mesoscale eddies
from the central and eastern North Pacific (Qiu, 2003, 2014; Sasaki & Schneider, 2011). Here, our ocean model
configuration considers the climatic (i.e., air–sea heat flux and wind stresses) and sea-level changes over theentire North Pacific. We simulate how the KC responded to the combined changes during five historical ages (0(i.e., present), 6, 21 (i.e., LGM), 30, and 35 ka BP), particularly in terms of the Kuroshio path, surface velocity,and transport. Using the simulated water-temperature data, we assess whether the vertical temperature gradientis a reliable KC intensity indicator.

2. Materials and Methods
The ocean model is the Stony Brook Parallel Ocean Model (sbPOM), and the model domain covers the entire
North Pacific and part of the South Pacific (30°S–62°N, 100°E−90°W), with a horizontal resolution of 1/12°
and 47 sigma levels in the vertical direction (Figure 1b in Supporting Information S1). Based on the global meansea level given by Yokoyama et al. (2018) for the past 35 ka, the reductions in water depth due to lowering of sea
level in four experiments from 35 ka to 6 ka were decided (Table S1 in Supporting Information S1). Among theseexperiments, the Tsushima Strait is closed at LGM (Figure S1 in Supporting Information S1).
The meteorological and oceanic data used for the four paleo-experiments were provided by the fully coupledatmosphere–ocean general circulation model MIROC4m (Chan et al., 2011; in Supporting Information S1). To
reduce the potential impact of model bias on our model results, we modified the MIROC4m paleo-data (i.e., air–sea heat flux and wind stresses) with the standard 0 ka BP data (in Supporting Information S1).
We compared the air–sea heat flux differences between the paleo-ages and 0 ka BP (Figure S2 in Supporting
Information S1) and checked the zero wind-stress curl positions (Figure S3 in Supporting Information S1). In theglacial ages (LGM, 30, and 35 ka BP), the ocean lost more heat (>20 W/m2) to the atmosphere in the Kuroshio
region, whose cooling effect has been indicated from the variations in subtropical SSTs (Sagawa et al., 2011).
Meanwhile, the westerly jet stream shifted southward by ∼1.0° in the North Pacific.

3. Results
3.1. Kuroshio Path
The annual mean Kuroshio axis for the five ages is defined by the maximum velocity along the Kuroshio (Figures 1d and Ambe et al., 2004). Compared to the modern KC, the KC at 6 ka BP exhibits almost the same positioneast of Taiwan (Figure S6a in Supporting Information S1), along the ECS shelf slope and south of Japan. However, the glacial KC axes east of Taiwan shift shoreward compared to the modern KC (Figure S6a in Supporting
Information S1). The location of the KC axis at the PN section has no migration at 35 ka BP but shifts seaward
during the LGM and at 30 ka BP (Figure S7a in Supporting Information S1). The glacial KC tends to shift seaward in the Tokara Strait (Figure S6b in Supporting Information S1). In addition, the seaward shift distance islikely proportional to the lowered sea level because the KC axis exhibits the southernmost displacement during
the LGM when the sea level is the lowest.
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The Kuroshio path south of Japan (i.e., 131.0–136.5°E) is very similar among the five cases (Figure 1c), especially in the region south of Shikoku (e.g., the ASUKA section; Figure S8 in Supporting Information S1). However, the glacial KC axes southeast of Japan (i.e., 136.5–142.0°E) migrate northward (Figure S9 in Supporting
Information S1), and present the largest migration of ∼1.5° at 140°E. The KE migrates northward at 142°E at
6 ka BP, LGM, and 35 ka BP (Figure S10 in Supporting Information S1), and southward at 146°E (Figure S7b
in Supporting Information S1) for 1.0 and 2.0° ranges in 6 ka BP and glacial ages, respectively. In the KE region(i.e., 142–150°E). By comparison, the southward migration of the glacial KE axes range from 0.5 to 3.5° at different longitudes. Compared to the latitudinal KE amplitude at 0 ka BP, the paleo-KE axes meander with smaller
amplitudes, and the smallest meandering amplitude occurs at 35 ka BP.
3.2. Kuroshio Surface Velocity
In the area east of Taiwan, there is little difference in the KC surface velocity between 6 and 0 ka BP. The glacial
KC surface velocities, whose maximum values are located closer to the shore (Figure S6a in Supporting Information S1), are stronger. In the central ECS (PN section in Figure S7a in Supporting Information S1), the KC surface
current also shows the following two regimes: A weak one (i.e., ∼0.9 m/s) at 0 and 6 ka BP and a strong one (i.e.,∼1.4 m/s) in the glacial ages. As the KC exits the ECS from the Tokara Strait, the glacial KC surface current isstill stronger than the modern one (Figure S6b in Supporting Information S1). The KE current speed at the 146°E
section (Figure S7b in Supporting Information S1) exceeds 0.4 m/s at 0 ka BP, reduces by ∼0.2 m/s at 6 ka BP
and by ∼0.1 m/s at 30 and 35 ka BP; however, it increases by ∼0.1 m/s during the LGM.
3.3. Kuroshio Transport
Based on the annual mean current, the upper 1000 m integrated volume transport stream function was calculated𝑥𝑥
0
𝐴𝐴 as Ψ(𝑥𝑥𝑥 𝑥𝑥) = ∫𝑥𝑥 ∫−1000 𝑚𝑚 𝑣𝑣(𝑥𝑥𝑥 𝑥𝑥𝑥 𝑥𝑥)𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝐴𝐴where Ψ is the stream function, xE denotes the eastern boundary of the
𝐸𝐸
Pacific, and v (x, y, z) is the northward velocity at the point with x, y, and z (i.e., longitude, latitude, and depth,
respectively) coordinates. In Figure 2, contours at 0 and 6 ka BP have similar distributions and values. As the 20
and 30 Sv contours show, the KC transports into the continental shelf break northeast of Taiwan. In the region
southeast of Kyushu, the KC merges with the Ryukyu Current, so the Kuroshio volume transport is significantly
increased after exiting the ECS through the Tokara Strait. In the region south of Japan, anticyclonic recirculation
exists all the time and contributes to the Ryukyu Current. Compared to the stream-function contours at 0 ka BP,
the southern boundary of the subtropical gyre in the glacial ages is located further south. In addition, there is alarger amount of water transported by the NEC to mid-latitude regions in the glacial ages, indicating stronger
subtropical gyres in the glacial ages compared to those at 0 and 6 ka BP.
We calculated the mean volume transports in the upper 1,000 m at seven sections (i.e., S1–S7 in Supporting Information S1; Figure 1a) across the main currents in the northwest Pacific (Table S2 in Supporting Information S1).
The volume transport values at 0 ka BP are comparable to the observed values, and they appear to be very similar
to the values at 6 ka BP. However, the NEC (i.e., the origin of the Kuroshio) exhibits the following large significant increases in westward volume transports: 16.4, 17.1 and 13.6 Sv at LGM, 30 and 35 ka BP, respectively.
This is also evident in the volume-transport stream function (Figure 2). Based on the values at S1 to S3 (Table S2
in Supporting Information S1), the Kuroshio bifurcates from the NEC, thereby further increasing the northward
volume transports, which are 13.4, 14.0, and 9.3 Sv at the LGM, 30 and 35 ka BP, respectively, all of which are
greater than the 0 ka BP volume transport. Conversely, the glacial Mindanao Current exhibits slightly decreased
volume transport. Therefore, in the glacial ages, most of the increased NEC transport is supplied to the KC.
The Kuroshio flows northward along the east Philippine coast, and then exchanges water with the South China
Sea through the Luzon Strait. The 0 ka BP volume transport in the upper 1,000 m across the Luzon Strait is westward and eastward at 25.1 and 18.5 Sv, respectively. Net volume transports are westward and amount to 6.6, 6.0,
1.4, 1.9, and 3.8 Sv at 0, 6 ka BP, LGM, 30 and 35 ka BP, respectively. The KC east of Taiwan is separated into
two branches, one of which enters the ECS as the Kuroshio mainstream, and the other flows eastward along thesouth edge of the Ryukyu Islands. The volume transports of both branches in the glacial ages are ∼4.0 Sv larger
than those at 0 ka BP. In the ECS, the KC outflow through the Kerama Gap forms an anticyclonic recirculation in
the region southeast of the Kerama Gap, while its volume transport increases by ∼2.0 Sv in glacial ages comparedto 0 ka BP. Meanwhile, there is a slight increase by ∼1.1 Sv in the glacial Kuroshio transport at the PN section.
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Figure 2. Upper 1000 m integrated volume-transport stream function (unit: Sv; 1 Sv = 106 m3/s) in the 5 ages.

Eventually, the KC in the ECS and the Ryukyu Current merge and form the KC south of Japan. The net volumetransport increases to 31.6 Sv (i.e., 0 ka BP) at section ASUKA. The volume transports there reach 39.2, 36.8, and34.1 at the LGM, 30 and 35 ka BP, respectively; such increased meridional transports are evident also in Figure 2.

4. Discussion
4.1. Kuroshio Path, Surface Velocity, and TransportCompared to the present location of the KC in the ECS, the KC axes shift slightly seaward at the LGM and 30 ka
BP (Figure S7a in Supporting Information S1). Such southeastward shift has also been reported by another ocean
model study (Vogt-Vincent & Mitarai, 2020) and a sediment core study (Kubota et al., 2017).
Up to now, a limited number of cores (i.e., stations 14, 19, 20 and MD01-2421; Figure 1d) suggested that the KC
off the coast of central Japan and KE moved northward during the last glacial period (Sawada & Handa, 1998;
Yamamoto, 2009) and the KE in the Central Pacific exhibited a southward shift (Kawahata & Ohshima, 2002).
In our study, the KC axes southeast of Japan (136.5–142.0°E) migrate northward by about 0.8° at 138.5°E during
the glacial ages. The northern limit of the KE axis at 6 ka BP, LGM, 30 and 35 ka BP exhibits southward migration of 1.1, 1.9, 2.0, and 2.4°, respectively. Similarly, the zonal mean wind-stress curl (Figure S3 in Supporting
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Information S1) reveals that the zero wind-stress curl at 0 ka BP is locatedat 41.2°N, and it migrates southward by 0.2, 1.1, 0.9, and 1.1° at 6 ka BP,
LGM, 30 and 35 ka BP, respectively. Therefore, the southward migrations of
the KE axes are likely closely related to the zero wind-stress curl positions.
Different latitudinal displacements of the KE should change the distributionof the glacial sea surface temperature, which will be reported elsewhere. We
note that the KC path south of Japan depends more on the local topography
and upstream conditions, rather than the position of the zero wind-stress curl.

Figure 3. Zonally integrated values in Sv from the east coast to 137°E ofmodeled volume transport in the upper 1,000 m (solid lines, with reference
to the black axes) and wind-driven Sverdrup transport from the east coast
to 137°E (dashed lines, with reference to the red axes). All the values areprocessed with 0.25° running average.

As the wind fields (Figure S3 in Supporting Information S1) shift southward
in the glacial ages, the NEC and KE latitudes exhibit southward displacements, indicating a southward shift of the North Pacific subtropical gyre. A
more southerly position of the NEC bifurcation results in stronger Kuroshiotransport (Hu et al., 2015; Y. L. Wang & Wu, 2018; Weiss et al., 2021). In
our simulations, the glacial Kuroshio transport in the southern and middle
Okinawa Trough increases with a higher surface velocity, which is due to thestronger NEC, as shown in Section 3.3. Compared to the present, the Kuroshio intrusion into the northern Okinawa Trough is reduced with a weaker current speed in the glacial ages, which is consistent with reconstructed results(Chang et al., 2008; D. W. Li et al., 2018; Ijiri et al., 2005; Shi et al., 2014;
Vats et al., 2020). The Tsushima Strait is closed at the LGM and narrow at
30 and 35 ka BP, leading to a decrease in the transport through the Tsushima
Strait. Moreover, the KC axis exhibits a southward displacement in the northern Okinawa Trough, which also reduces the northward Kuroshio transport
separation.

To quantify the subtropical circulation changes in 5 ages, we zonally integrated the volume transport from the east coast to 137°E for the annual meantransport stream function at different latitudes (solid lines in Figure 3). At 0 and 6 ka BP, the meridional transport
is northward south of 14°N, 0 at 14°N, and southward north of 14°N. In the 3 glacial ages, the meridional transport was 0 at 13°N, while exhibiting a southward shift (Figure 3), with values increasing by approximately 10 Sv
from 13 to 25°N and indicating a stronger subtropical gyre. Previous sediment cores and an ensemble of generalcirculation models suggested a stronger North Pacific subtropical gyre during the LGM (Gray et al., 2020; Sawada & Handa, 1998). Therefore, the North Pacific subtropical gyre tends to be stronger and shifts southward inglacial ages.
We also calculated the corresponding wind-driven Sverdrup transport from the east coast to 137°E (dashed lines
in Figure S3 in Supporting Information S1). The enhanced North Pacific subtropical gyre in the glacial ages canbe partly explained by the stronger wind-stress curl and trade winds (Figure S3 in Supporting Information S1).
From 13 to 25°N, the wind-driven Sverdrup transport agrees with gyre strengthening in the glacial ages. However, the Sverdrup transport north of 25°N is weaker in the glacial ages than at 0 and 6 ka BP, which is not consistent with gyre strengthening (Figure 3). Hogg and Gayen (2020) suggested that surface buoyancy fluxes (i.e.,
mainly heating and cooling in the ocean surface) are essential for the ocean-gyre transport. A significant amountof cooling (Figure S2 in Supporting Information S1) is evident over the Kuroshio region during the glacial ages
in our model forcing. Therefore, it is possible that the circulation north of 25°N is driven by the buoyancy forcing
rather than the wind stresses.
4.2. A Kuroshio Intensity Indicator
The KC intensity strongly affects the Okinawa Trough water temperatures. Using reconstructed Okinawa Trough′water temperatures, many studies have examined the Kuroshio intensity variation since the LGM.
𝐴𝐴 The UK37 proxyis usually used as an annual mean SST index (R. A. Kim et al., 2015) and the TEX86 proxy most likely reflects
subsurface (i.e., <200 m) water temperature (D. Li et al., 2013; Ruan et al., 2017; Zhao et al., 2015). Thesestudies considered that a weaker KC leads to a shallow mixed layer and smaller vertical thermal gradient. The′
vertical thermal gradient can be estimated from the difference between𝐴𝐴
UK37 and TEX86 derived temperature, that
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Table 1Modeled Sea Surface Temperature (SST), Subsurface Temperature (SubST), Their Difference (i.e., ΔT = SST – SubST; Unit:°C), and Surface Velocity (Unit: m/s) at the ODP1202B and M063-05 Sediment Cores
ODP1202B
Time

SST (0–30 m)

SubST(30–80 m)

M063-05
Velocity(0–80 m)ΔT

SST (0–30 m)

SubST(30–200 m)

Velocity(0–200 m)

ΔT

0 ka

25.95

23.01

0.58

2.93

24.91

19.78

0.67

5.14

6 ka

25.79

22.85

0.59

2.94

24.74

19.62

0.66

5.11

LGM

22.67

19.27

1.10

3.40

23.06

16.44

1.20

6.61

30 ka

23.04

19.21

1.04

3.83

23.84

16.91

1.24

6.93

35 ka

23.68

19.82

1.34

3.86

23.68

17.15

1.16

6.52

Note. The depth of SubST at the ODP1202B core is from Ruan et al. (2017) while that at the M063-05 core is from Li
et al. (2020).

𝐴𝐴
ΔT has been used as an indicator of the KC intensity and evolution (Li𝐴𝐴 is, ΔT = SSTUK′ − SubSTTEX86; hence,37
et al., 2020; Ruan et al., 2017).

The surface thermal buoyancy flux and local wind speed also affect the depth of the thermocline or the surface
mixed layer (L. Wang et al., 2016). To confirm the relationship between ∆T and KC intensity, we compared themodeled annual mean ∆T and the modeled surface-current speeds at the two core sites (Table 1). According toTable 1, a stronger KC appears to correspond with a higher ∆T but not a lower ∆T.
The idea that a stronger KC results in a lower ∆T value is not compatible with ocean dynamics. According to thethermal wind relation, the vertical geostrophic-flow variation is proportional to the horizontal water-temperature
gradient; a larger horizontal temperature gradient corresponds to a stronger KC. As we described earlier, the SST
in the Kuroshio region is affected not only by the Kuroshio speed, but also by the air–sea heat flux and local
wind forcing. Consequently, the subsurface water temperature, rather than the SST, is sensitive to the Kuroshio
intensity variations (Kubota et al., 2017).
We hypothesized that the subsurface horizontal thermal gradient across the KC is more appropriate as a KC
intensity index than the vertical thermal gradient. To confirm this idea, we calculated the difference between
the modeled subsurface (i.e., 200 m) water temperature (i.e., ΔTH = SubSTDGKS9603–SubSTDGKS9604) between the
DGKS9604 and DGKS9603 cores (Figure 1d). The ΔTH values are 0.49, 0.50, 2.39, 2.00, and 1.49°C at 0, 6 ka
BP, LGM, 30 and 35 ka BP, respectively. The maximum current speeds in the upper 200 (500) m are 0.92 (0.07),
0.93 (0.07), 1.47 (0.19), 1.44 (0.17), and 1.40 (0.13) m/s at 0, 6 ka BP, LGM, 30 and 35 ka BP, respectively. There
was a significant correlation between ΔTH and KC speeds; therefore, it is practical to evaluate the KC surface
strength by using the horizontal subsurface temperature gradient across the KC. If it is difficult to find two corestations at the direction normal to the Kuroshio axis, the subsurface temperature gradient at the longitudinal orlatitudinal direction is also a good index for the KC intensity with the condition that a core station is on both sidesof the KC.

5. Conclusions
Considering the changes in sea level, air–sea heat flux, and wind stresses, our model results provide the evolutionof the Kuroshio path, transport, and strength from 35 ka BP to the present. The subtropical gyre in the last glacial
period becomes stronger and migrates southward, which is related to the wind-stress curl in the North Pacific.
The 6 ka BP KC has basically the same path, transport, and strength as the modern KC. It has been confirmed
that, the glacial Kuroshio transport increases and KC axis migrates slightly seaward in the ECS. The glacial KC
appears to shift southward when it exits the ECS through the Tokara Strait, and exhibits the southernmost shiftduring the LGM. Moreover, the glacial KC off the southeastern coast of Japan migrates northward with a small
meander. The southward migration of the glacial KE varies between 0.5 and 3.5°.
In search for an appropriate KC intensity indicator, we found that the upper-ocean vertical thermal gradient usedin some studies is unsatisfactory; therefore, we suggest that it is better to evaluate the KC surface strength using

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the horizontal subsurface-temperature gradient across the KC. This conclusion can facilitate future studies on the
Kuroshio path and strength variations.

Data Availability Statement
Major simulated data are available at https://doi.org/10.5281/zenodo.5733861.
Acknowledgments
This work was supported by JSPS KAKENHI Grant No. 17H02959, 17H06104,
and 17H06323. H. Yang thanks the China
Scholarship Council (CSC) for supporting
her stay in Japan.

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