Suzuki and Tatebe Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-020-00381-9(2020) 7:67

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open AccessFuture dynamic sea level change in thewestern subtropical North Pacificassociated with ocean heat uptake andheat redistribution by ocean circulation
under global warming
Tatsuo Suzuki*

and Hiroaki Tatebe

Abstract
In the present study, the relative importance of ocean heat uptake and heat redistribution on future sea levelchanges in the western North Pacific has been reconciled based on a set of climate model experiments in whichanomalous surface fluxes of wind stress, heat, and freshwater in a warmed climate are separately given to those
fluxes in a pre-industrial control simulation. Our findings suggest that the basin-wide ocean heat uptake andresultant heat accumulation by the climatological-mean advection are required to explain the future dynamic sea
level (DSL) rise in the western subtropical North Pacific caused by the thermal expansion of subtropical mode water(STMW). At the same time, it has been recognized that the localized heat uptake in association with the wintertime
mixed-layer formation around the Kuroshio Extension can be solely attributed to the future STMW change. The
thermally induced component is a dominant contribution to the future DSL rise in the western subtropical North
Pacific compared to the contributions of wind-induced and halosteric components, which, especially the former,
have been reported as a dominant factor resulting from a linear response of the ocean to the northward shift andstrengthening of the mid-latitude westerly over the North Pacific in a warmed climate.
Keywords: Sea level, Global warming, Heat uptake, North Pacific

1 Introduction
The global ocean stores more than 90% of the heat energy accumulated in the climate system of the Earth during recent decades (e.g., Levitus et al. 2012; Church et al.
2013a, b). Whereas the ocean heat uptake works to reduce abrupt global warming, the thermal expansion ofseawater causes sea level rise, which has been recognized
as a severe societal issue due to future global warming(Willis and Church 2012). The ongoing global mean sea
level rise will continue in the future few centuries depending on future greenhouse gas emissions, even when
* Correspondence: tsuzuki@jamstec.go.jp
Japan Agency for Marine-Earth Science and Technology, 3173-25
Showa-machi, Kanazawa-ku, Yokohama, Kanagawa 236-0001, Japanthe greenhouse gas concentration is stabilized (Church
et al. 2013a, b). In addition, regional sea level changes
will significantly deviate from global mean values. For
coastal and small island countries, the territorial sea level
rise can be substantially more important than the global
mean (Cazenave and Cozannet 2014). The complete regional sea level changes are characterized by many processes involving the ocean, the atmosphere, thegeosphere, and the cryosphere. Dynamical ocean response, which can be represented in global atmosphereocean general circulation models (AOGCMs), is an essential factor.
The dynamic sea level (DSL) is defined as the deviation from the ocean geoid (e.g., Zhang et al. 2014;© 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
appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate ifchanges were made. The images or other third party material in this article are included in the article's Creative Commonslicence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commonslicence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain
permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

Suzuki and Tatebe Progress in Earth and Planetary Science(2020) 7:67

Griffies et al. 2016). The geographical pattern of the projected DSL change under global warming can be attributed to changes in ocean density and circulation (e.g.,
Lowe and Gregory 2006; Suzuki and Ishii 2011). In thepresent study, we focus on the heat uptake and the DSLchange in the western subtropical North Pacific. The
DSL changes in the western North Pacific are characterized by a northward shift of the Kuroshio Extension (KE)path and strengthening of the Kuroshio and the KE recirculation gyre. These characteristic patterns are observedin the multi-model ensemble mean of the Coupled Model
Intercomparison Project (CMIP) models (e.g., Sueyoshi
and Yasuda 2012; Zhang et al. 2014; Yin 2012; Church
et al. 2013a, b; Terada and Minobe 2018). Sueyoshi and
Yasuda (2012) showed that models with a northward shiftof the KE exhibit a poleward shift of the Aleutian Low.
Zhang et al. (2014) also inferred that the poleward displacement of the mid-latitude westerly wind causes thepoleward expansion of the subtropical gyre in the upper
ocean. This is consistent with downscaling experiments
using the ocean circulation model by Liu et al. (2016).
However, changes in the surface wind stress alone cannot
fully explain the strengthening of the KE recirculation.
Analyzing CMIP5 models, Terada and Minobe (2018)suggested that the strengthening of the KE under global
warming can be caused by enhanced meridional pressure
gradient associated with thermal expansion of subtropicalmode water (STMW) and corresponding steric sea level
rise in the KE recirculation. Two earlier studies using different climate models suggested that the ocean heat uptake due to the global warming causes thermosteric sea
level rise in this region (Lowe and Gregory 2006; Suzuki
and Ishii 2011). They estimated the effect of heat flux as
the residual of overall change under global warming
minus the contribution of changes in the wind stress andthe freshwater flux. Therefore, influences of the surface
heat flux and resultant ocean heat uptake on future sea
level changes are not directly quantified. Furthermore,
Terada and Minobe (2018) inferred that the decrease of
STMW density is induced by the downward heat flux
anomaly related to the northward shift of the KE, which is
caused by wind stress change. However, the relationshipbetween the ocean heat uptake and the DSL rise in the
Kuroshio and the KE recirculation gyre remains unclear.
In the present study, we use the sixth version of the Modelfor Interdisciplinary Research on Climate (hereinafter,
MIROC6) to investigate the future DSL change in the western subtropical North Pacific. In order to clarify the causesof regional variations, we conducted idealized experiments
following Gregory et al. (2016). Furthermore, we focus on
the contributions of the ocean heat uptake and heat redistribution by ocean circulation on the DSL change in order tounderstand the physical nature of the relative sea level rise in
the KE recirculation.

Page 2 of 10

2 Methods
The climate model used in the present study, MIROC6,
has been cooperatively developed by the Japanese modeling community (Tatebe et al. 2019). The atmosphericcomponent has a horizontal resolution of a T85 spectral
truncation with 81 vertical levels. The ocean component
has a tri-polar horizontal coordinate system with a nominal resolution of 1° with 62 vertical levels. The preindustrial control experiment (piControl) reasonably
simulated the climatological values of the oceanic environment corresponding to major ocean gyres. Because oflimited computational resources, we also used
MIROC5.2, which has a lower resolution atmosphericcomponent (T42 spectral truncation with 40 verticallevels) with different physical parameterizations, whereas
the ocean component of this model is the same as in
MIROC6 (Tatebe et al. 2018). MIROC5.2 has a lower
computational cost than MIROC6, allowing more ensemble experiments to be performed.
We conducted a series of idealized model experiments
for detailed analysis of the DSL change and the ocean
heat uptake in the North Pacific, following the procedure provided by the Flux-Anomaly-Forced Model Intercomparison Project (FAFMIP; Gregory et al. 2016). In
the FAFMIP experiments, a prescribed set of surface flux
perturbations (shown in the upper panels of Fig. 1) is
additionally given to the ocean component of the climate
model during a pre-industrial control simulation. These
perturbations are defined as the difference between the
climatological monthly time mean of years 61–80 of the
1% per year CO2 concentration increase experiments of
CMIP5 (hereinafter, 1pct CO2 experiment) when the atmospheric CO2 concentration becomes approximately
double and the climatological monthly time mean of thecorresponding 20 years of piControl in the CMIP5-MME(multi-model ensemble mean; Gregory et al. 2016).
Anomalous ocean heat uptake into the North Pacific(Fig. 1b) is approximately 145 TW, approximately 20%of the global ocean heat uptake. In the North Pacific, thefeatures of the prescribed flux perturbations of CMIP5MME (Fig. 1 a though c), such as the northward shift of
the Aleutian Low, are substantially the same as the fluxchanges observed in the 1pct CO2 experiments of
MIROC6 and MIROC5.2 (not shown).
The lower panels of Fig. 1 show the DSL changes for the
CMIP5-MME, MIROC6, and MIROC5.2 in the 1pct CO2
experiment relative to each control experiment. The CMIP5MME shows the characteristics of the DSL distribution associated with the climatological North Pacific circulation (contours in Fig. 1d). We can see the remarkable meridionalslope of the mean DSL between 32° N and 38° N, which corresponds to the KE. A similar meridional slope is also foundin MIROC6 (Fig. 1e) and MIROC5.2 (Fig. 1f), although theslope is much steeper. In the 1pct CO2 experiment, the DSL

Suzuki and Tatebe Progress in Earth and Planetary Science(2020) 7:67

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Fig. 1 Imposed surface flux perturbations in the North Pacific for a wind stress (arrows), b downward heat flux, and c downward water flux.
These values are anomalies computed from the time mean of years 61–80 in the 1pct CO2 experiment relative to climatology in piControl. In a,colors indicate the stream function of the Sverdrup transport. The lower panels show the dynamic sea level changes of d the CMIP5 multi-model
ensemble, e MIROC6, and f MIROC5.2. Time-averages are taken in years 61–80 of the 1pct CO2 experiments for CMIP5-MME and in years 51–90
for MIROC6 and MIROC5.2. Note that the basin average in the North Pacific is subtracted from the dynamic sea level change at each grid.
Contours indicate piControl climatology. Dashed red and solid blue rectangles in e indicate the Kuroshio Extension recirculation (20° N–35° N,
140° E–160° E) and the Kuroshio Extension (33° N–37° N, 140° E–150° E) defined in the present study, respectively

rises along the KE, and south of the KE is commonly seen in
CMIP5-MME, MIROC6, and MIROC5.2. In order to investigate what physical processes caused these changes, we focus
the KE and KE recirculation regions defined as the solid blue
(33° N–37° N, 140° E–150° E) and dashed red (20° N–35° N,
140° E–160° E) rectangles in Fig. 1e.
The designated climate experiment in which only the surface wind-stress perturbations are given additionally to thepre-industrial control simulation of MIROC6 is referred toas the faf-stress experiment. Similarly, the climate model experiment in which the surface heat (freshwater) flux perturbation is given is referred to as the faf-heat (faf-water)
experiment. In the faf-heat experiment, the net heat flux into
the ocean component is Q = QSST + F, where QSST is the net
surface heat flux computed in the climate model, and F is
the prescribed heat flux perturbation (Fig. 1b). Note thatchanges in the sea surface temperature (SST) due to thegiven surface heat flux perturbation (F) could induce resultant surface heat flux to compensate for the SST change, reducing the total heat input. In order to avoid this negative
feedback, two additional tracers of added heat (TA) and redistributed heat (TR) are introduced in the faf-heat experiment(Bouttes et al. 2014; Method B in Gregory et al. 2016). Here,TA, the initial values of which are globally zero, is prognosed
as an ocean tracer forced by F, and TR, the initial conditionof which is taken from the ocean temperature of piControl,
is also prognosed as an ocean tracer forced by QSST. The

surface heat flux QSST is estimated using the SST containedin TR. The ocean potential temperature, θ, which is the sumof TA and TR, is integrated with Q. That is to say that thesurface heat flux perturbation, F, works to change the ocean
temperature and density, and thus the thermosteric height
and ocean circulation. In the passive-heat experiment, TA isalso installed in piControl as a passive tracer and integrated
with F. Each FAFMIP experiment is performed with a single
ensemble simulation in MIROC6 and with three-member
ensemble simulations in MIROC5.2. Although 70 year-longintegration is required in the FAFMIP protocol, in MIROC6,
we extended the integration period to 90 years in order toobserve the DSL response to the external forcing, separate
from internal variabilities.
Several diagnostic variables are shown in the present
study. In order to evaluate the effect of wind stresschanges on the basin-scale circulation field, we compute
the Sverdrup transport from the wind stress changes. The
stream function was derived by integrating the Sverdruptransport from the east coast at each latitude, as follows:ψ ð xÞ ¼ −

1
β

Z xe

∇τdx

ð1Þ

x

where ψ is the stream function of the Sverdrup transport, β is the meridional differentiation of Coriolis

(2020) 7:67

Suzuki and Tatebe Progress in Earth and Planetary Scienceparameter, xe is the longitude of the east coast, and τ is
the wind-stress vector.
In order to investigate the DSL change due to the heat
input into the ocean, we have estimated thermosteric sea
level change as follows:

Δηθ ¼

Z H
0

α  Δθdz ¼

Z H
0

α  ΔT R dz þ

 T A dz

Z H
0αPage 4 of 10

ΔηTA, which does not affect the DSL or ocean
circulation.
Note that we have subtracted the North Pacific basin
average from the raw sea level changes at each grid point
in order to determine the spatial distribution in thepresent study.

3 Results and discussion
ð2Þ

where α is the coefficient of the thermal expansion, Δθ
is the potential temperature difference relative to the climatology of the piControl, and H is ocean depth. In the
faf-heat experiment, the thermosteric sea level change(Δηθ) is separated into the components due to the redistributed heat change (ΔηTR) and the components due tothe added heat (ΔηTA), as shown in the right-hand sideof Eq. (2). In the passive-heat experiment, we estimate

3.1 Regional sea level response to surface flux
perturbations

In order to find the spatial pattern in the North Pacific,
we illustrate the regional changes in DSL as deviations
from the basin average in the upper panels of Fig. 2. In
the faf-stress experiment, MIROC6 shows a DSL rise in
the KE region due to a northward shift of the KE (Fig.
2a). This response is in good agreement with the Sverdrup transport caused by the additional wind stress perturbations (Fig. 1a). The wind-stress perturbation isrelated to the northward shift of the Aleutian Low, as

Fig. 2 Dynamic sea level changes (cm) for years 51–70 relative to piControl for a faf-stress, b faf-heat, and c faf-stress experiments of MIROC6.
The contours in the upper panels indicate the climatological dynamic sea level in piControl. Regions with signals at a 90% confidence level are
hatched. The middle panels show the heat flux change ΔQSST (Wm−2) for years 1–70 relative to piControl for d faf-stress, e faf-heat, and f fafstress experiments of MIROC6. The contour indicates the climatological net heat flux into the ocean in piControl. The lower panels (g–i) are sameas the upper panels, but for the ensemble mean of three members of MIROC5.2. Areas in which the signs of the three members do not agree
were masked out

Suzuki and Tatebe Progress in Earth and Planetary Science(2020) 7:67

mentioned by Sueyoshi and Yasuda (2012). Although abasin-scale DSL rise in the subtropical North Pacific is foundin CMIP5-MME (Fig. 1d), this is not the case for the fafstress experiment. Between 20° N and 30° N, the changes in
the wind stress tend to suppress the DSL rise (Fig. 1a). The
faf-stress experiment also shows that the DSL is depressed in
the KE recirculation region to the south of 30° N.
The faf-heat experiment shows a significant increasein DSL in the recirculation region of the KE, where
STMW is distributed (Fig. 2b). The spatial pattern is notconsistent with the local heat input of the heat flux perturbation, which is relatively uniform in the subtropicalgyre (Fig. 1b). The physical mechanism of this DSL isdiscussed later herein. The DSL change in the faf-water
experiment (Fig. 2c) exhibits a basin-wide pattern similar
to that in the Pacific Decadal Oscillation (e.g., Mantua
et al. 1997; Yasuda and Sakurai 2006). However, thisspatial pattern disappears in the following two decades.
This may indicate that the warming signals and internalvariability in the climate system are not sufficiently distinguished. The characteristic spatial pattern of the DSLchanges seen in the faf-stress and the faf-heat experiments of MIROC6 are also found in the MIROC5.2 (Fig.
2g and h). In the faf-water experiment, however,
MIROC5.2 shows a DSL rise in the KE recirculation,
which differs from MIROC6.
The freshwater perturbations applied for the faf-water
experiment show that the freshwater evaporates in thesubtropical region and falls in the form of rain in thesubarctic region (Fig. 1c). These changes are consistent
with strengthening of the hydrological cycle over thetwentieth and early twenty-first centuries (Durack et al.
2013). Although this could cause freshening of the
STMWs and the DSL rise in the KE recirculation, theresponse is not robust across the models. Therefore, theresults of the faf-water experiment will not be discussed
further in the present study.
In the FAFMIP experiments, we used a climate model thatconsiders air-sea coupled feedback processes that can occur
in response to the SST changes. Therefore, the sum of theregional DSL changes resulting from individual experiments
of faf-stress, faf-heat, and faf-water is not equal to that in the
1pct CO2 experiment. The surface heat flux (Q) changes correspond to the SST changes (especially due to the changes in
TR for the faf-heat experiment). In the faf-stress experiment,
the northward movement of the KE with a large amount ofheat release causes anomalous heat absorption to the southof the KE and heat release to the north (Fig. 2d). In the fafheat experiment, the heat release in the KE is increased relative to piControl (Fig. 2e). This is consistent with the enhancement of the subtropical gyre circulation, including theKE, and its heat transport. On the other hand, the net ocean
heat absorption into the entire North Pacific is approximately
the same in these experiments.

Page 5 of 10

In order to investigate the time evolution of these DSLchanges, we show the time series of the area-averaged
DSL in the KE region (33° N–37° N, 140° E–150° E) andthe KE recirculation region (20° N–35° N, 140° E–160°
E) in order to determine the meridional shift of the KE
and the strength of the recirculation gyre of the KE, respectively (upper panels in Fig. 3). The gray band in the
figure indicates the width of the interannual variations
in piControl. The range is estimated as ± Z S, where S is
the interannual standard deviation in piControl, and Z(1.96) is the 95th percentile of the normal distribution.
In the KE recirculation region, the faf-heat experimentexhibits a significant DSL rise, more than the amplitudeof internal variability (red line in Fig. 3a). This signal appears clearly after about the 30th year and persists thereafter (dotted red line in Fig. 3a). Focusing on the KE,
there are no significant changes beyond the width of interannual variations in piControl. However, looking at
the difference in the mean over a longer time scale fromthe 20th year to the 90th year, the DSL of the faf-stressexperiment becomes higher than in piControl by approximately 3 cm (dotted green line in Fig. 3b). This difference between the faf-stress and piControl isstatistically significant at the 90% confidence level in thetime mean over decades, as indicated by the hatchedarea in Fig. 2a. Similar DSL changes in the KE and the
KE recirculation are observed in the ensemble mean of
the three members of MIROC5.2 (Fig. 3e and f).
In order to understand the mechanisms of these DSLchanges, we investigate the stream function of the Sverdrup transport derived from the wind stress change (including the wind stress perturbation for the faf-stressexperiment). In the KE region, the variation in the DSL
follows the changes in the Sverdrup transport stream
function (Fig. 3d), suggesting that the wind stress change
causes the northward shift of the KE. The correlation coefficient of the DSL change is approximately 0.76 with a
2-year lag. The time lag could be explained by the response time of ocean circulation to the wind stress change
through the baroclinic Rossby wave propagation (Yasudaand Sakurai 2006). The DSL rise in the KE recirculationconsists of the thermosteric sea level change (red line in
Fig. 3c). On the other hand, the DSL change associatedwith the wind-driven Sverdrup transport stream functionin this region is much smaller than that associated withthe thermosteric component for time mean over decades(Fig. 3c). We describe the regional sea level growth causedby the heat flux perturbation in the next section.
3.2 Heat accumulation in the Kuroshio Extension
recirculation gyre

The thermosteric sea level change Δηθ and its components ΔηTR and ΔηTA in the faf-heat experiment and thethermosteric sea level change due to TA (ΔηTA) in theSuzuki and Tatebe Progress in Earth and Planetary Science(2020) 7:67

Page 6 of 10

Fig. 3 Time series of DSL in a the KE recirculation (20° N–35° N, 140° E–160° E) and b KE (33° N–37° N, 140° E–150° E) for faf-stress experiment(green lines), faf-heat experiment (red lines), and faf-water experiment (blue lines). The red dotted line in a shows the time average of the faf-heatexperiment between years 30 and 90 (6.5 cm). The green dotted line in b indicates the time average of the faf-stress experiment between years
20 and 90 (2.7 cm). The shaded areas in the upper panels indicate the range of piControl. c The DSL change (black line), the thermosteric sea
level change (red line), and the Sverdrup transport stream function (dotted sky-blue line) in the KE recirculation for the faf-heat experiment. d Thechanges in the DSL (black line) and the Sverdrup transport stream function (dotted sky-blue line) in the KE for the faf-stress experiment. e, f Same
as a and b, respectively, but for the ensemble mean of MIROC5.2. The red dotted line in e shows the time average of the faf-heat experiment
between years 30 and 70 (6.1 cm). The green dotted line in f indicates the time average of the faf-stress experiment between years 20 and
70 (2.1 cm)

passive-heat experiment are shown in Fig. 4. In order tofocus on these spatial distributions, we show the deviations from the North Pacific basin average. The DSL risein the KE recirculation can be explained by the additional heat associated with global warming, suggesting
the warming of STMW. However, the contribution of

Suzuki and Tatebe Progress in Earth and Planetary Science(2020) 7:67

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Fig. 4 a The thermosteric sea level change Δηθ (cm), b thermosteric sea level change due to TR (ΔηTR), and c thermosteric sea level change due
to TA (ΔηTA) for years 51–70 in the faf-heat experiment of MIROC6, and d the thermosteric sea level change due to TA (ΔηTA) for years 51–70 in
the passive-heat experiment of MIROC6. Note that deviations from the basin average in the North Pacific are depicted in the figures. Areas withsignals at a 90% confidence level are hatched in a. Note that the basin average in the North Pacific is subtracted from the thermosteric sea levelchange at each grid. Contours indicate the climatological DSL in piControl

the TR field is negative in this area (Fig. 4b). As a result,
the thermosteric sea level rise (Fig. 4a) is smaller thanthat estimated from the TA field (Fig. 4c).
The thermosteric sea level due to the TA in the fafheat experiment is in good agreement with that in thepassive-heat experiment (Fig. 4d). We show the timemarch of TA along 160° E for the faf-heat and thepassive-heat experiments in Fig. 5. Both added heattracers are subducted into the ocean subsurface to thesouth of 30° N and accumulated in the KE recirculation
region, where STMW is distributed. The passive tracer
does not affect the oceanic circulation. Therefore, these
agreements indicate that heat transport by climatologicalmean currents largely determine the thermosteric rise in
the Kuroshio recirculation under global warming.
We conducted the four additional faf-heat experimentswith modified heat flux perturbations using MIROC5.2
in order to specify which areas of heat input contributeto the DSL rise in the KE recirculation. In these experiments, the experimental configurations are the same as

the faf-heat experiment using MIROC5.2, but the heat
flux perturbation is limited to the specific areas, namely,
the subtropical gyre region (4° N–38° N, 100° E–100°W), the subpolar gyre region (38° N–64° N, 100° E–100°W), the southern part of the subtropical gyre (4° N–20°
N, 130° E–130° W), and the areas excluding the Kuroshio and the KE recirculation region (20° N–35° N, 120°
E–160° E). The net surface heat flux perturbations into
the ocean in these regions are 96.2 TW, 37.8 TW, and33.3 TW, and the remaining value excluding 13.8 TW
from the whole heat flux, respectively. Each experimentis integrated for 70 years from the initial conditions
taken from piControl of MIROC5.2, which are the sameas that of the first member of the FAFMIP experiments
in MIROC5.2.
In these experiments, heat accumulation is observed in
the KE recirculation (i.e., STMW is warming), suggestinga thermosteric sea level rise, although the magnitude isdifferent (Fig. 6). Applying heat flux perturbation to theglobal ocean, the thermosteric sea level increase due toSuzuki and Tatebe Progress in Earth and Planetary Science(2020) 7:67

Page 8 of 10

Fig. 5 Time march of latitude-depth sections of the added heat tracer TA (°C) along 160° E for the first decade, the third decade, and the fifth
decade for the faf-heat experiment (upper panels) and the passive-heat experiment (lower panels). Contours indicate the potential density (kgm−3) in piControl

the additional heat tracer TA (ΔηTA) is 25.06 cm (sum of
the basin average and the deviation in the KE recirculation in Fig. 6) on the average of the final two decades of
the 70 year-long integration in the KE recirculation region (20° N–35° N, 140° E–160° E). The added heat

input in the subtropical and subpolar regions contributes
to the increases in ΔηTA in the KE recirculation of approximately 16.84 cm and 3.46 cm, respectively. The heat
input in the subtropical gyre explains approximately
67.2% of the total sea level rise due to the added heat TA

Fig. 6 Thermosteric sea level change (cm) due to the added heat TA (ΔηTA) in the faf-heat experiment and in the case when the area to which
the application of heat flux is limited to b the subtropical gyre region (4° N–38° N, 100° E–100° W), c the subpolar gyre region (38° N–64° N, 100°
E–100° W), d the southern part of the subtropical gyre (4° N–20° N, 130° E–130° W), and e the area excluding the Kuroshio and the Kuroshio
Extension recirculation (20° N–35° N, 120° E–160° E) for MIROC5.2. Note that the basin average in the North Pacific is subtracted at each grid. The
numbers on each figure show the basin average of the North Pacific (NP mean) and the area average of the deviation in the Kuroshio Extension
recirculation (KR mean). Contours indicate the climatological DSL in piControl. The solid boxes indicate the area given by heat flux perturbation(b, c), and the dotted box indicates the area excluding the heat flux perturbation (e)Suzuki and Tatebe Progress in Earth and Planetary Science(2020) 7:67

in the KE recirculation region. Applying the heat flux
perturbation to only the southern part of the subtropicalgyre, the additional heat tracer increases by 6.11 cm,
explaining 24.4% of the total rise. We have also estimated the thermosteric sea level rise (Δηθ) including theeffects of TR in Table 1. In the faf-heat experiment, thethermosteric sea level rise Δηθ is 17.81 cm on averagefor the last two decades of the 70 year-long integrationin the KE recirculation region. The changes in the redistributed heat TR reduce the thermosteric sea level rise toapproximately two-thirds of the increase due to TA. Terada and Minobe (2018) found moderate correlation between the heat input to the south and southeast of Japanand the density decrease of STMW in the CMIP5
models. However, the additional faf-heat experiment that
excludes the flux perturbation in the Kuroshio and the
KE recirculation region (i.e., the south and southeast ofJapan) can also explain 89% of the thermosteric sea level
rise in the KE recirculation (Table 1). This means theheat absorption in this area does not fully explain thetemperature rise of STMW and the related sea level rise.
The results of the present study indicate that ocean heat
flux in the broader field of the subtropical gyre is essential for heat accumulation in STMW.

4 Conclusions
We have estimated the contribution of ocean heat uptake to
the DSL change in the western subtropical North Pacific following a set of climate model experiments designated in the
FAFMIP protocol. Note that, unlike in previous studies, thesurface heat flux changes under the global warming are explicitly given as a surface boundary condition.
The heat flux changes under global warming can lead toa continuous rise in sea level in the KE recirculation region due to the increase in temperature of STMW. The
process of heat transfer into the subsurface that causes the
Table 1 Thermosteric sea level change (Δηθ) and ΔηTA in the KE
recirculation (20° N–35° N, 140° E–160° E) on average for the
final two decades of the 70 year-long integration for (a) the fafheat experiment and the four additional faf-heat experiments
limiting the heat flux perturbation to the specific areas, (b) the
subtropical gyre region (4° N–38° N, 100° E–100° W), (c) the
subpolar gyre region (38° N–64° N, 100° E–100° W), (d) the
southern part of the subtropical gyre (4° N–20° N, 130° E–130°W), and (e) the areas excluding the Kuroshio and the KE
recirculation region (20° N–35° N, 120° E–160° E)
Δηθ (cm)

ΔηTA (cm)(a) faf-heat

17.81

100%

25.06

100%

(b) STG

10.82

61%

16.84

67%

(c) SPG

1.95

11%

3.46

14%

(d) STGS

3.62

20%

6.11

24%

(e) Excluding KER

15.82

89%

21.29

85%

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warming of STMW is primarily determined by the climatological mean flow. This means that the thermostericcontribution due to local heat input cannot sufficiently reproduce the DSL change. The change in wind stress during warming contributes to the northward shift of the KE,
but cannot be distinguished significantly from internalvariations. The northward shift causes the heat input
anomaly to the south and southeast of Japan (Fig. 2d), although the heat uptake is not sufficient to cause the DSL
rise in the recirculation region of the KE. This means thatthe total heat uptake in the whole subtropical gyre causes
the DSL rise in the KE recirculation.
Zhang et al. (2014) showed that a decrease in the DSL
could have occurred associated with the spin-down of
the subtropical gyre due to changes in the wind stressfield. These responses are also found in the faf-stress experiment. The KE recirculation region defined in thepresent study corresponds to the northern part of theNorth Pacific subtropical gyre. The DSL change in the
KE recirculation during global warming depends onwhether the response to wind stresses or the contribution of the ocean heat uptake is dominant. Assumingthat climatological mean currents determine the redistribution of surface heat input under the global warming,the model uncertainty in the future DSL change, a partof which is caused by thermosteric processes, could arise
from differences in the representation of ocean climatology among climate models. In the future, we intend tocompare the analysis of the present study with the results of the models participating in FAFMIP, to investigate the causes of variations in sea level change for eachfactor, and to clarify the uncertainty in future sea levelchange projections.
Acknowledgements
Model simulations were performed on the Earth Simulator at JAMSTEC. The
authors would like to thank Shoshiro Minobe, Yoshiki Komuro, and Masahiro
Watababe for their fruitful comments. The authors are much indebted to Koji
Ogochi, Teruyuki Nishimura, Hiroaki Kanai, and Hiroshi Koyama for their longterm support in areas related to model development, source code
optimization, and server administration.
Authors’ contributions
TS proposed the topic, designed the study, conducted the modelexperiments presented in the paper, and wrote a large part of the paper. HT
developed the model and supported the idealized experiments. TS and HT
continuously discussed the results. All authors have read and approved thefinal manuscript.
Funding
This research was supported by the Integrated Research Program for
Advancing Climate Models (TOUGOU program) of the Ministry of Education,
Culture, Sports, Science and Technology (MEXT), Japan (JPMXD0717935457).
Availability of data and materials
The primary dataset supporting the conclusions of this article is available
from the Earth System Grid Federation (ESGF). Contact the author for
additional data sets.
Competing interests
The authors declare that they have no competing interest.

Suzuki and Tatebe Progress in Earth and Planetary Science(2020) 7:67

Received: 3 March 2020 Accepted: 16 October 2020

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