Nishikawa et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-020-00342-2(2020) 7:29

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open Access

Detection of the Oyashio and Kuroshio
fronts under the projected climate change
in the 21st century
Haruka Nishikawa1* , Shiro Nishikawa1, Hiroshi Ishizaki1, Tsuyoshi Wakamatsu2,3 and Yoichi Ishikawa1

Abstract
We evaluate the Oyashio and Kuroshio fronts latitudinal transition under the projected climate change scenarios
using eddy resolving regional ocean climate projection products from 1981 to 2100. The regional ocean climate
projections are produced based on dynamical downscaling of four CMIP5 models and two RCP experiments RCP2.6
and RCP8.5. Two approaches of the fronts detection methods are compared. One is conventional approach basedon subsurface specific isotherm and another is newly proposed approach based on the Oyashio and Kuroshio
water temperature and salinity (TS) profiles that may change as the global warming progresses. It is found that the
Oyashio TS profile rapidly shift to be high both in temperature and salinity in RCP8.5 cases while the Kuroshio TS
profile shows small change toward the end of the twenty-first century in all cases. Northward shift of the Oyashio
and Kuroshio fronts reaches 2° northward by the late twenty-first century in extreme case of RCP8.5 cases from thetemperature-based definition. On the other hand, the northward shifts of fronts are less than 1° when we use theTS-based definition. The comparison of both fronts positions with the surface velocity distributions suggest that theTS-based definition captures transitions of the fronts better than the temperature-based definition.
Keywords: Global warming, Climate projection, Kuroshio, Oyashio

Introduction
Oyashio is the western boundary current and its extensionof the western subarctic gyre in the North Pacific Ocean(Fig. 1). A part of East Kamchatka Current (EKC) flows
into the Okhotsk Sea in the northern Kuril Straits and isthen mixed with Okhotsk Sea Mode Waters (OSMW).
This is the origin of the Oyashio water. Oyashio flows
southward along Japanese coast and its south westernedge reaches off Tohoku region. Turning eastward off the
Tohoku region, Oyashio current follows the Sub-Arctic
Front (SAF) characterized by the temperature front andforms the Oyashio extension or the Subarctic Current(Zhang and Hanawa 1993). South of the Oyashio front lies
the Kuroshio front associated with Kuroshio and its* Correspondence: harukan@jamstec.go.jp
1
Japan Agency for Marine-Earth Science and Technology, 3173-25
Showa-machi, Kanazawa-ku, Yokohama, Kanagawa 236-0001, Japan
Full list of author information is available at the end of the article

extension, Kuroshio Extension (KE), a strong westward
boundary current of the North Pacific Ocean subtropicalcirculation. Area between the Oyashio front and the Kuroshio front, the Kuroshio-Oyashio Transition Zone
(KOTZ), is known to be highly productive fishinggrounds. Nutrient-rich Oyashio waters sustain large stocksof small pelagic fish (e.g., Pacific saury, Japanese sardine)
while the warm Kuroshio waters provide them winterspawning grounds. The small pelagic fish migrate seasonally between the Kuroshio waters for spawning and the
Oyashio waters for growing in KOTZ. In autumn, fishingvessels in Tohoku region can catch fat fish that is migrating from the Oyashio waters to the Kuroshio waters intheir neighborhood ocean. KOTZ is also important forfish stock itself. Recruitment of the Pacific saury varies
with the spring productivity in this region through the larvae survival rate (Ichii et al. 2018).

© The Author(s). 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License,
which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give
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Nishikawa et al. Progress in Earth and Planetary Science(2020) 7:29

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Fig. 1 Schematic diagram of the Kuroshio and Oyashio. Oyashio is a mixture of the East Kamchatka Current (EKC) and Okhotsk Sea Mode Water(OSMW) that outflows to the Pacific through the Kuril Straits. The black triangle represents the area where we picked up waters as the pure
Oyashio water and the shaded area below the Kuroshio (gray arrow) represent the area where we picked up waters as the pure Kuroshio water

Since KOTZ is defined as an area that lies between the
Oyashio and Kuroshio fronts, evaluation of its transition
under the climate change requires concurrent analysis ofboth fronts. If the both Oyashio and Kuroshio fronts
shift northward, the transition zone will shift northward,
and if the only Kuroshio front shifts north, the area oftransition zone will reduce. These changes should haveserious impacts on migration ecology and stock level offish and fishing industry. Inter-annual to decadal timescale variation of the Oyashio and Kuroshio fronts canbe explained mainly by linear dynamics of wind-driven
ocean circulation theory (Seager et al. 2001). There aresome studies on the future projection of Kuroshio front
under the climate change. Most of studies suggested that
the Kuroshio front will shift north along with the Kuroshio intensification (Sato et al. 2006; Yang et al. 2016; Liet al. 2017) and one study suggested that the Kuroshio
front location will not change (Sakamoto et al. 2005). In
contrast, there are no studies conducted on the Oyashio
front transition under the climate change to authors’knowledge.
In general, realistic representation of the Oyashio andKuroshio fronts and associated complex water massproperties in KOTZ in numerical simulation requires
eddy-resolving horizontal resolution (e.g., Hurlburt et al.
1996). However, the resolution of most of ocean components in CMIP5 models are in the range of 1–2° andtheir horizontal resolutions are too coarse to reproduce
KOTZ and its associated fronts. In this study, we investigated how the Oyashio and Kuroshio fronts will shift

under the climate change by using high-resolution future
ocean regional projections based on the fifth phase of
the Climate Model Intercomparison Project (CMIP5)
products. The new regional ocean projections have 10
km horizontal resolution which is high enough to represent the Oyashio and Kuroshio fronts in realistic manner. Combinations of four models and two experiments
are chosen from CMIP5 multi model ensemble datasetsand this is the first model intercomparison study of the
Oyashio and Kuroshio fronts transition under the climate change.
Since the Oyashio front and the Kuroshio front are bothcharacterized by strong surface current as extensions of subpolar and subtropical western boundary currents, the best
way to identify the location of both fronts are to follow theassociated strong surface current axis. The most reliable
Kuroshio front location, which is reported by the Japan Coast
Guard (JCG), is estimated by various observation data. They
see the velocity, temperature, and sea surface height (SSH)
distribution collected by national patrol boats, merchant vessels, buoys, radars, and satellites and finally they decide thelocation by hand. However, such works take a lot of time.
For that reason, the locations of the Oyashio and Kuroshio
fronts are conventionally defined based on respective specificisotherm at reference depth because both fronts are characterized by temperature fronts.
Under the current climate condition, the Oyashio front
roughly follows 5 °C isotherm at 100 m depth and theKuroshio front roughly follows 14–15 °C isotherm at
200 m depth (Kawai 1972). These definitions are theNishikawa et al. Progress in Earth and Planetary Science(2020) 7:29

most popular because they are more practical and robust
than other definitions (e.g., using sea surface height gradient or sea surface temperature) and the defined fronts
correspond well to the core of the surface velocity (Kidaet al. 2015). Many studies (e.g., Masujima et al. 2003;
Kouketsu et al. 2005; Kakehi et al. 2017), and government offices (e.g., Japan Meteorological Agency, Fisheries Research, and Education Agency Japan) used thesedefinitions. Also, previous studies for the climate change
adopt these definitions (e.g., Yokouchi et al. 2006). However, we expect that this conventional approach would
not be effective for front detection in the future projection products, since the specific index temperature associated with each front gradually changes as the climatechange progresses.
We propose new front detection method that uses bothtemperature and salinity profiles. Then we compare transition of KOTZ and associated fronts distribution evaluated by the two front detection methods, conventional
method (henceforth temperature-based) and newly proposed method (henceforth TS-based). We focused on
spring season (April–May) when the Oyashio front shows
seasonal southward shift and the period is an importantseason for growing and migration of small pelagic fish.
This paper is organized as follows. Production of the regional projection of ocean state by dynamical downscalingof CMIP5 products and the front detection methods aredescried in “Methods/Experimental” section. Evaluation of
the TS properties and detected front locations using theregional ocean projection data set are made in “Results”
section. General remarks on our front detection methods
and their impact of to the evaluation of the KOTZ production are briefly discussed.

Methods/Experimental
Future ocean regional projection datasets

High-resolution future ocean regional projection (FORP)
products over the North Pacific domain (Nishikawa
et al. 2020) are generated by dynamically downscaling
CMIP5 global ocean future projection products (Taylor
et al. 2012). Four models, MIROC5, MRI-CGCM3,
GFDL-ESM2M, and IPSL-CM5A-MR, and three experiments, historical (1960–2005), RCP2.6 (2006–2100), and
RCP8.5 (2006–2100), are selected from CMIP5 database
based on representativeness of Kuroshio separation and
KE latitudes judged from sea surface temperature map
and availability of three hourly atmospheric parameters
necessary to construct atmospheric external forcings todrive an ocean circulation model. For dynamical downscaling of the CMIP5 products, we configured ocean
general circulation model MRI.COMv4 (Tsujino et al.
2017) over the North Pacific domain (NP10: 15° S–70°
N, 100° E–75° W) with 10 km horizontal resolution and
54 vertical levels (Fig. 2). NP10 domain has three open

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boundaries at its west, south, and north boundaries andmodel temperature and salinity within 2° band alongeach boundary are restored to prescribed monthly mean
temperature and salinity data gridded on the World
Ocean Atlas (WOA) standard grid with 1° horizontal
resolution and 33 levels.
The monthly mean temperature and salinity data at
the model boundaries during CMIP5 historical run
period (1960–2005) are derived from World Ocean Atlas
94 (WOA94) database (Levitus and Boyer 1994; Levitus
et al. 1994). At the end of the historical run period, theboundary values are switched from WOA to an interannual monthly mean data constructed by merging
CMIP5 ocean products (1996–2100) and the WOA
product. First, CMIP5 temperature and salinity data areregridded to the common WOA grid and 21 years climatological monthly mean data over the period of
1996–2015 are derived. Merger of WOA and CMIP5
products is achieved by replacing the monthly mean
field of each CMIP5-derived monthly mean field by
WOA monthly mean field during RCP experimentperiod (2006–2100). The period is chosen so that jumpof boundary values from end of historical run to RCPruns after 2006 is minimized. At last, 11 years runningmean filter is applied to the merged boundary values at
each monthly bin.
Sets of external forcing for driving the ocean model isconstructed from CMIP5 surface atmospheric parameters: 2 m air temperature; 2 m specific humidity, surfacepressure, surface solar radiation flux, surface long wave
flux, precipitation flux; 10 m zonal wind; and 10 m meridional wind at three hourly temporal resolution andsea level pressure at six hourly resolution for each
CMIP5 model and experiment. The atmospheric parameters are regridded to common spherical coordinate of
0.5° horizontal resolution. Then 5 days running averagefilter is applied to the regridded parameters at each threehourly temporal bin for reducing high-frequency noisein order to increase stability of long-term integration of
the ocean model. Freshwater flux from precipitation rateis converted to salt flux internally in the ocean modeland the model integration is conducted with volumeconservation constraint.
Freshwater inputs from rivers are not considered andsea surface salinity (SSS) is restored to the WOA climatological monthly mean salinity in all experiments. The
SSS restoration is introduced for avoiding SSS drift inbasin scale commonly experienced by the ocean circulation model driven by climatological or reanalysis atmospheric forcing. We have used 2 days as the restoration
time scale. We do not restore the model SSS to the corresponding CMIP5 ocean salinity products for several
reasons. The main reason is that CMIP5 ocean model
has coarse horizontal resolution of order of 1° to 2° andNishikawa et al. Progress in Earth and Planetary Science(2020) 7:29

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Fig. 2 Model domain of FORP-NP10 configuration. Color shades are bottom topography and overlaid white box is area of analysis in this study

western boundary currents such as Kuroshio and Oyashio are not well represented compared to FORP-NP10
with resolution of order 1/10°. Because of this large discrepancy between the original CMIP5 ocean model and
FORP-NP10, CMIP5 SSS has spatial distribution unmatched to the circulation pattern represented in FORPNP10 and level of the discrepancy differs among theCMIP5 models and scenarios. In order to make thedownscaling experimental settings as common as possible, we chose the WOA SSS as a common SSS productfor the restoration.
Ocean model starts from the rest initialized with theWOA January mean temperature and salinity and integrated
with the CMIP5-derived atmospheric forcing from 1960 to
2100. After 20 years spinning-up run period, we saved
monthly mean oceanic parameters, temperature, salinity,
SSH, velocity fields, and surface forcing fields from 1981 to
2100. We call this regional future ocean projection product,
FORP-NP10, hereafter. In this study, we use monthly mean
temperature, salinity, and horizontal velocity fields from
FORP-NP10 over the period from 1981 to 2100. In order totest the newly proposed front detection method under thecurrent climate condition, we also use temperature, salinity,and velocity from four-dimensional variational ocean reanalysis for the Western North Pacific Ocean, FORA-WNP(Usui et al. 2017). FORA-WNP shares the same horizontal
grid and vertical grid coordinates with FORP-NP10 over the
analysis domain depicted in Fig. 2.

Front detection methods

The conventional approach, which regards the Oyashio
front roughly follows 5 °C isotherm at 100 m depth and
the Kuroshio front roughly follows 14–15 °C isotherm at
200 m depth, was applied to temperature data of FORAWNP on April 2002. As can be confirmed in Fig. 3, thedetected Oyashio and Kuroshio fronts match well with
the KE and the SAF accompanying eastward strong surface currents. However, in evaluating future transition of
the Oyashio and Kuroshio fronts, the conventional
method to detect front distribution based on target isotherm line would become infeasible due to basin-wide
upper ocean temperature rise expected under the projected climate change. Thus, detection of both fronts
would require alternative approach. Here, we propose a
new method for detecting the Oyashio and Kuroshio
fronts based on TS profiles of the Oyashio and Kuroshio
waters. We picked up waters in a triangle area (143.5–
146° E and 42–43° N) in Fig. 1 as the area where standard Oyashio water can be found due to its closeness to
the region where the EKC is mixed with OSMW to form
the Oyashio water. We define standard TS profile of thepure Oyashio water by water properties within the triangle. Detecting the KE axis from the surface velocity
field, we define wasters found within 0–50 km south of
the Kuroshio axis and within 140–160 E longitudinalband as standard TS profile of the pure Kuroshio waters(shaded area in Fig. 1). TS profiles of the pure Oyashio

Nishikawa et al. Progress in Earth and Planetary Science(2020) 7:29

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Fig. 3 Distribution of a the TS based Oyashio water (red shade) and TS based Kuroshio water (blue shade) and b surface absolute velocity in April 15,
2002. Isotherm of 5 °C represents the value at 100 m and is a traditional definition of the Oyashio front, and isotherm of 14.5 °C represents the value at
200 m and is a traditional definition of the Kuroshio front. Data are from FORA-WNP

and pure Kuroshio waters (solid lines in Fig. 4) defined
this manner from FORA-WNP during the period 1981–2015 correspond well to TS profiles of observed Oyashio
and Kuroshio waters (cross marks in Fig. 4).
We calculated each TS profiles of FORP-NP10 in April
and May from 2006 to 2100 and averaged them every 20

Fig. 4 Profiles of the current pure Oyashio waters (blue) and Kuroshio
waters (red) that were estimated by using FORA-WNP temperature andsalinity data. Color shade represents the Oyashio mixing ratio. Blue (red)crosses denote the pure Oyashio (Kuroshio) waters profiles estimated byobservation data (Yasuda et al. 2001). We defined the mixing ratio below
26.0 σθ as 0% or undefined by salinity

years period. But only first period is from 2006 to 2020.
Then we interpolated the TS profile sets (2006–2020, …,
2081–2100) to estimate the yearly TS profiles. The reason
why we use averaging and interpolation is that we focus
on describing decadal time scale transitions of the Oyashio
and Kuroshio fronts. Next, we calculated the ratio of
Oyashio-Kuroshio mixing ratio of the water at 300 m
depth over the rectangular domain (143–160° E, 30–50°
N) depicted in Fig. 2. Waters in the target area are the
Oyashio water, Kuroshio water, or mixture of these waters(see Fig. 4), and the Oyashio-Kuroshio mixing ratio is calculated by using TS profiles (Shimizu et al. 2001).
Since low-density Kuroshio water sometimes runs overhigh density Oyashio water (Nishikawa et al. 2016), thefronts position varies with depth. Also the TS profiles ofpure Oyashio and Kuroshio waters change slightly during
the transportation. For that reason, we tested the
Oyashio-Kuroshio mixing ratio in some depths to find themost suitable criteria for fronts detection. Consequently,
we defined the waters with larger than 90% of the
Oyashio-Kuroshio mixing ratio at 300 m depth as the
Oyashio waters and the waters with smaller than 5% of
the Oyashio-Kuroshio mixing ratio at 300 m depth as theKuroshio waters. This definition mostly corresponds to
the Oyashio and Kuroshio fronts distribution that are suggested by velocity, temperature, and salinity distribution.
Then we detect the Oyashio and Kuroshio fronts as south
and north limits of each waters in each longitude. Thisdefinition of fronts is named as “TS-based.” We also use
the conventional way to detect the fronts. We regarded
the isotherm of 5 °C at 100 m as the Oyashio front andregarded the isotherm of 14.5 °C at 200 m as the Kuroshio
front. This definition is named as “temperature-based.”
Then we analyze mean latitudes of the Oyashio andKuroshio fronts from 143° E to 160° E in this study.

Nishikawa et al. Progress in Earth and Planetary Science(2020) 7:29

We compared the “TS-based” and “temperaturebased” front detection methods in the current climate
state. Figure 3a is an example of the comparison madein April 2002 using FORA-WNP data. In the TS-based
definition, the Oyashio front in 145° E is at 38.7° N and
the Kuroshio front in 155° E is located at 38.0° N since
we picked up the minimum and maximum latitude of
the Oyashio and Kuroshio waters. The Oyashio andKuroshio fronts locations detected from the two
methods distribution are mostly the same in the current
climate state. It can be also confirmed that both Oyashio
and Kuroshio fronts locations are also consistent with
the KE and the SAF, respectively based on the surface
current structure (Fig. 3b).

Results
Sverdrup transport stream function of CMIP5 forcing

In order to characterize differences among the selected
CMIP5 forcing regarding their expected impact to the
Kuroshio and Oyashio fronts, the Sverdrup transport
stream function (STSF) is calculated from 10 years annual mean and winter (December, January, and February) mean wind stress for each CMIP5 forcing. Time
series of the latitude of zero STSF measured at 160° E
are plotted in Fig. 5. The latitude of zero STSF represents the border of sub-tropical and sub-polar gyres expected from the linear wind-driven ocean circulation
theory. More specifically, the zero STSF latitudes basedon winter mean wind stress and annual mean windstress correspond to the Kuroshio front and the subarctic (Oyashio) front (Hurlburt et al. 1996), respectively.

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The winter mean wind-stress derived zero STSF latitudes under RCP8.5 scenario during the future projection period (2005–2100) shows two distinct patterns: (i)no significant trend in MRI-CGCM3 and IPSL-CM5AMR and (ii) poleward shift of about 2 to 3° in MIROC5
and GFDL-ESM2M. The same patterns can be found in
the annual mean wind-stress derived STSF latitudes withlarger poleward shifts of about 4° in MIROC5. There isno clear trend in STSF latitudes in all cases under
RCP2.6 scenario. Thus from CMIP5 wind stress, significant poleward shift of both Kuroshio and Oyashio fronts
in the FORP-NP10 products is expected to be seen in
MIROC5 and GFDL-ESM2M under RCP8.5 scenario.
Transition of Oyashio and Kuroshio water properties
under the climate change

We show the future Oyashio and Kuroshio TS profiles
from the FORP-NP10 products. The average TS profilesof April and May during 2006–2020, 2041–2060, and
2081–2100 are plotted in Fig. 6. In the four RCP2.6 runs,
the scenario the emission decreased from mid-twentyfirst century; only slight changes can be seen both in the
Oyashio and the Kuroshio TS profiles. Among the fourmodels, clear shift of Oyashio temperature and salinity
shift to high in the mid-twenty-first century can befound in MRI-CGCM3 and MIROC5 in April and
MIROC5 in May. On the contrary in the RCP8.5 runs,
the scenario the emission keeps increasing toward theend of the twenty-first century; all products show that
Oyashio temperature and salinity drastically increase
and get closer to the Kuroshio TS profiles by the end of

Fig. 5 Time series of zero latitude of Sverdrup transport at 160° E calculated from MIROC5 (black lines), MRI-CGCM3 (red lines), IPSL-CM5A-MR(blue lines), and GFDL-ESM2M (green lines) a winter (December, January, and February) mean and b annual mean wind stress forcing from CMIP5
RCP2.6 (solid lines) and RCP8.5 (dashed lines) experiments.

Nishikawa et al. Progress in Earth and Planetary Science(2020) 7:29

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Fig. 6 TS profiles of the pure Oyashio waters (blue) and Kuroshio waters (red) in April and May with four models and two scenarios. Each curve iscorresponding to a mean value during 2006–2020, 2041–2060, and 2081–2100

the century. On the other hand, Kuroshio TS profiles
show very little change in all simulations throughtwenty-first century. We concluded that the Kuroshio
characteristics of temperature and salinity are robust to
the projected climate change, but the Oyashio water
properties are very sensitive to the emission scenarios.
Although the change of Oyashio TS profiles in RCP8.5
varies with models, all models predict that Oyashio
temperature will be over 10 °C in the end of the twentyfirst century.

Comparison of two front detection methods

April–May mean latitudes of the Oyashio and Kuroshio
fronts detected in two methods for all FORP-NP10
products are shown in Fig. 7. The mean latitudes of thefronts in historical and RCP runs, difference between
TS-based and temp-based front latitudes, and differenceof latitudes between historical and RCP runs are summarized in Table 1. Both the Oyashio and Kuroshio
fronts under RCP8.5 scenario tend to shift northward in
the temperature-based estimation and the TS-based

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Fig. 7 Time series of the April–May mean Oyashio and Kuroshio fronts latitude from RCP2.6 and RCP8.5 scenarios of four models. Black (gray)curves are TS-based (temperature-based) latitude of historical simulation from 1982 to 2005. Blue (light blue) curves are TS-based (temperaturebased) latitude of RCP2.6 simulation from 2006 to 2100. Red (orange) curves are TS-based (temperature-based) latitude of RCP8.5 simulation from2006 to 2100

estimation. Notable point is that the shifts by thetemperature-based estimation are remarkably larger than
the shifts estimated by the TS-based detection method(Fig. 7).

The mean latitudes of the fronts in historical and RCPruns, difference between TS-based and temp-based front
latitudes, and difference of latitudes between historicaland RCP runs are summarized in Table 1. Both Oyashio

Nishikawa et al. Progress in Earth and Planetary Science(2020) 7:29

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Table 1 Table caption
Difference between future and historical
OY temp OY TS Diff TS-temp KR temp KR TS Diff TS-temp OY temp
MRI-CGCM3

RCP2.6

RCP8.5OY TS

KR temp

KR TS

early 21st 39.66

39.92

0.26

34.86

34.37

-0.49

0.26

-0.03

0.27

0.08

late 21st

39.90

40.16

0.26

35.01

34.45

-0.56

0.50

0.21

0.42

0.16

early 21st 39.66

39.89

0.23

34.82

34.41

-0.41

0.26

-0.06

0.23

0.12

late 21st

40.96

39.98

-0.98

35.93

34.55

-1.38

1.56

0.03

1.34

0.26

39.40

39.95

0.55

34.59

34.29

-0.30

-

-

-

-

early 21st 42.19

42.07

-0.12

36.11

35.14

-0.97

0.94

0.79

0.33

0.29

late 21st

41.72

-1.15

36.63

35.13

-1.50

1.62

0.44

0.85

0.28

HISTORIALMIROC5

RCP2.6

RCP8.5early 21st 42.43

41.94

-0.49

36.04

35.16

-0.88

1.18

0.66

0.26

0.31

late 21st

42.96

41.94

-1.02

38.28

35.74

-2.54

1.71

0.66

2.50

0.89

HISTORIALIPSL-CM5A-MR RCP2.6

RCP8.5

41.25

41.28

0.03

35.78

34.85

-0.93

-

-

-

-

early 21st 40.84

41.22

0.38

35.76

34.84

-0.92

0.44

0.80

0.26

-0.15

late 21st

40.95

41.04

0.09

35.99

34.81

-1.18

0.55

0.62

0.49

-0.18

early 21st 40.63

40.93

0.30

35.68

34.88

-0.80

0.23

0.51

0.18

-0.11

late 21st

42.40

41.14

-1.26

37.45

35.33

-2.12

2.00

0.72

1.95

0.34

40.40

40.42

0.02

35.50

34.99

-0.51

-

-

-

-

HISTORIALGFDL-ESM2M

RCP2.6

RCP8.5HISTORIAL

42.87

early 21st 41.85

41.71

-0.14

36.27

35.25

-1.02

0.41

0.83

0.41

0.33

late 21st

41.36

-0.08

36.10

35.43

-0.67

0.00

0.48

0.24

0.51

41.44

early 21st 42.07

41.48

-0.59

36.60

35.36

-1.24

0.63

0.60

0.74

0.44

late 21st

42.44

41.80

-0.64

37.13

35.29

-1.84

1.00

0.92

1.27

0.37

41.44

40.88

-0.56

35.86

34.92

-0.94

-

-

-

-

and Kuroshio fronts shift northward in most of thecases. On average of four models, the Oyashio front latitude in the late twenty-first century (2051–2100 mean)is 0.67° (temperature-based)/0.44° (TS-based) larger thanhistorical mean in RCP2.6 and 1.57° (temperaturebased)/0.58° (TS-based) in RCP8.5. For the Kuroshio
front, 0.50° (temperature-based)/0.19° (TS-based) larger
than historical mean in RCP2.6 and 1.77° (temperaturebased)/0.46° (TS-based) in RCP8.5. The northward shifts
are larger in RCP8.5 than that in RCP2.6. At the samedefinition of fronts, there is no significant difference between northing of the Oyashio fronts and the Kuroshio
fronts. The degree of northward shift is larger intemperature-based estimation than in TS-based estimation (see the difference between historical and RCP runsin Table 1). Mean latitudes of the Oyashio and Kuroshio
fronts tend to be higher in temperature-based estimation
than in TS-based estimation (see the difference between
temperature-based and TS-based in Table 1).
Since the temperature-based and the TS-based fronts
latitudes are significantly different in MIROC5 RCP8.5
case, we plotted surface ocean state from MIROC5
RCP8.5 case in May 2080 in Fig. 8. According to thetemperature-based front detection method, the Oyashio
front (5 °C at 100 m) is located in the coastal area of
Hokkaido and Kuril Islands and the offshore Kuroshio

front (14.5 °C at 200 m) is located above 40° N (Fig. 8a).
However, according to the surface velocity field (Fig. 8d,e), the current continues from the EKC turns to eastnear 155° E and 44° N, which seems to be corresponding
to the Oyashio current. The 100 m depth temperature inthat region is about 9 °C (Fig. 8b). From Fig. 8e, theKuroshio main stream is obviously located around 32–
37° N and the corresponding 200 m temperature isabout 17 °C (Fig. 8c). On the other hand, thetemperature-based Kuroshio front is located around 40–
41° N, along which 200 m depth salinity is about 34.2
PSU (Fig. 8f). The salinity is obviously too low for the
Kuroshio waters.

Discussion
We estimated transitions of the Oyashio and Kuroshio
waters TS profiles and respective front positions under
the influence of the projected climate change using thehigh-resolution regional ocean future projection FORPNP10. The fronts transitions evaluated by two front definition methods, conventional temperature-based method
and newly proposed TS-based method, are compared.
While the Kuroshio water TS profile is relatively stable inall cases, the Oyashio water TS profile experiences large
shift toward high temperature and salinity rapidly in all
RCP8.5 cases. Both the Oyashio and Kuroshio fronts tend

Nishikawa et al. Progress in Earth and Planetary Science(2020) 7:29

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Fig. 8 a Distribution of the TS based Oyashio water (red shade) and TS-based Kuroshio water (blue shade) in May 2080 from RCP8.5 scenario of
MIROC5. The isotherm of 5 °C represents the temperature-based Oyashio front and the isotherm of 14.5 °C represents the temperature-based
Kuroshio front. b 100 m and c 200 m temperature distribution. d Surface current vector. Color represents current direction. e Surface absolute
velocity. f 200 m salinity distribution

to shift northward in the temperature-based estimationand the TS-based estimation. But the notable point is thatthe shifts by the temperature-based estimation are remarkably larger than the shifts by the TS-based estimation. In the Oyashio front detection case, temperaturechange in Oyashio water causes the difference estimates
depending on a choice of the detection methods in
RCP8.5 cases, too. Due to the Oyashio warming, the Oyashio front looks moving southward from tracking the isotherm of 5°°C at 100 m, which is the conventional
definition of the Oyashio front position.
Since the high-temperature Oyashio waters penetrate
into the Kuroshio-Oyashio transition zone, isotherm of
14.5 °C at 200 m, which is the conventional definition of
the Kuroshio front position, appears in the far northfrom the actual Kuroshio front position as shown in Fig.
8. Thus, the temperature-based front detection method
misses the actual front positions under the projected climate change due to global warming impacts. The TSbased estimation also suggests that the Oyashio andKuroshio fronts shift northward in all cases, but the latitudinal changes are lower than 1°. For that reason, we
recommend the TS-based definition for discussing the
Oyashio and Kuroshio front transitions under the projected climate change using FORP-NP10 products. Beingaware of temperature and salinity change of the Oyashio
and Kuroshio water properties is a key to follow thetransitions of the Oyashio and Kuroshio fronts and eventually transitions of water properties of KOTZ under the
projected climate change.
As summarized in the previous section, this northward
shift tendency is anticipated prior to the ocean downscaling experiments from analysis of the STSF zero latitude based on the CMIP5 wind stress forcing. However,this tendency is noticeable only in the specific set of theforcing, MIROC5 and IPSL-CM5A-MR under RCP8.5
scenario (Fig. 5). Among the FORP-NP10 products, thelargest northward shift of about 6° in the Kuroshio front
can be found in the MIROC5 and IPSL-CM5A-MR
under RCP8.5 scenario based on the temperature-based

Nishikawa et al. Progress in Earth and Planetary Science(2020) 7:29

estimation. However, the corresponding Kuroshio fronts
estimated by the TS-based detection method do notshow significant northward shift compared to the othermodels, MRI-CGCM3 and GFDL-ESM2M. In the caseof the Oyashio front, its significant northward shift ofabout 4° can be seen in MIROC5, IPSL-CM5A-MR, and
MRI-CGCM3 under RCP8.5 scenario and minor northward shift of about 2° in MIROC5 under RCP8.5 scenario based on temperature-based estimation. Thispattern is not consistent with the poleward shift of the
Oyashio front anticipated from the analysis of the STSM
zero latitude in Fig. 5b on which MRI-CGCM does notshow significant poleward shift. Further, such clear connection between wind stress and the latitudinal shift of
the fronts is missing in the TS-based estimation. Theseobservations tell us that the latitudinal shift of the Kuroshio and Oyashio fronts can be explained partly by thewind stress shift, but transitions in water mass properties
have large impact in determining the Kuroshio and the
Oyashio fronts under the projected climate change.
According to the TS-based transitions in fronts positions,the latitudinal change of the Oyashio and Kuroshio fronts
will not be so drastic as is estimated from the conventional
approach. It is possible to say that the impacts of the climatechange on the fish migration and fishing grounds location
are small from these results. However, we alert that the global warming may have serious impacts on the distribution
and stock of fish in the Oyashio and Kuroshio region. At thepresent time, many fish do the seasonal migration between
the Kuroshio and Oyashio region. If their migration ecology
depends on temperature, the Oyashio warming will influenceon it. Our results are from April to May. Since the Oyashio
water has a seasonal cycle in intensity and its distribution(Sekine 1988), the degree of temperature and salinity change
in other season may be different from spring. However, we
expect that the same change occurs in other seasons. This isa subject for future analysis. We are also concerned about
the Oyashio productivity. The nutrient amount in the Oyashio water will decrease. The rich nutrients in the Oyashio
water are supplied by the vertical mixing in the upstream
Oyashio region. The drastic warming of the Oyashio water
will possibly change the nutrient cycle. It is necessary to investigate why the Oyashio water changes, for example,
whether the OSMW or the EKC contributes to the Oyashio
warming, and how the Oyashio warming will have an impact
on the ecology in the Northwestern Pacific in future.

Conclusions
We show three important results in this study. First, thefront latitude of the Oyashio and Kuroshio will shiftnorthward. Second, Oyashio TS profile will drasticallychange. Third, the fronts positions vary depending on
the detecting method, temperature-based or TS-based.
The northward shift of the Kuroshio front has alreadyPage 11 of 12

been reported by previous studies. We confirm their results by using the first eddy-resolving CMIP5 model.
And this is the first study that suggests the possibility of
the Oyashio front northward shift. Also, the Kuroshio
and Oyashio TS profiles in the future are firstly estimated in this study. The third results are related to thesecond one. Our study suggested that since the drastic
TS profile change, the conventional front definition
based on isotherm distribution will probably be uselessin the future. There are no negligible information forstudies about impact assessment of global warming.
Authors’ contributions
SN, HI, TW, and YI developed the ocean climate projection products. HN
analyzed the model output and wrote the first draft manuscript. SN, HI, andTW provided ideas for the study and helped in interpretation of the model
output. SN and TW collaborated with the corresponding author in theconstruction of the manuscript through several drafts. All co-authors participated in discussions about the results and commented on the manuscript.
All authors read and approved the final manuscript.
Funding
HN, SN, HI, TW, and YI are supported by the Social Implementation Programon Climate Change Adaptation Technology (SI-CAT) Grant Number
JPMXD0715667163 from the Ministry of Education, Culture, Sports, Scienceand Technology (MEXT). HN is supported by JSPS KAKENHI Grant number
18K14516. TW is supported by the Nansen Center (NERSC) basic funding.
Availability of data and materials
The FORA-WNP and FORP data set used in this study is available from theauthors upon request. The request URL of FORA-WNP is http://synthesis.jamstec.go.jp/FORA/e/.
Competing interests
The authors declare that they have no competing interests.
Author details
1
Japan Agency for Marine-Earth Science and Technology, 3173-25
Showa-machi, Kanazawa-ku, Yokohama, Kanagawa 236-0001, Japan. 2The
Nansen Center, Thormøhlens gate 47, 5006 Bergen, Norway. 3Bjerknes Centre
for Climate Research, Jahnebakken 5, 5007 Bergen, Norway.
Received: 27 December 2019 Accepted: 27 May 2020

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