Sasaki and Ito ﻿
Progress in Earth and Planetary Science(2024) 11:15
https://doi.org/10.1186/s40645-024-00620-3

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open Access

Impact of the Kuroshio large meander
on local atmospheric circulationand precipitation in winterYoshi N. Sasaki1*   and Ryunosuke Ito1

Abstract
The Kuroshio, which flows to the south of Japan, typically takes two paths on decadal timescales; the straight pathand the large meander path, or the so-called Kuroshio large meander. This phenomenon is characterized by the presence of the cyclonic cold-core eddy located south of Japan, which leads to both negative and positive sea surface
temperature (SST) anomalies along the southeastern coast of Japan. To clarify the atmospheric response to these SST
anomalies in winter, we conducted a control experiment employing a regional atmospheric model with observed
SSTs and two sensitivity experiments in which the SST boundary conditions were substituted with those corresponding to the periods of for the Kuroshio large meander and the straight path. The differences in these two sensitivity experiments showed that the surface wind response to the SST anomalies associated with the Kuroshio large
meander was not only characterized by wind divergence over the cyclonic cold-core eddy, as reported previously
but also wind convergence along the southeastern coast of Japan. Interestingly, this wind anomaly blew into the positive SST anomaly along the east coast of Japan at around 36°N. Similar wind anomalies along the east coast of Japan
were observed in a reanalysis product. The results of the model simulation and the reanalysis product showedthat during the Kuroshio large meander period, the number of rainy days increased significantly over the warm
SST anomaly, while the precipitation and the number of rainy days decreased over the cyclonic cold-core eddy.
Moisture budget analysis revealed that the observed decrease in precipitation was attributed to the disparity
between the reduced evaporation and the anomalous horizontal moisture convergence in a region where the surface
winds were divergent. This moisture convergence was mainly induced by a decrease in specific humidity, implying
this change in specific humidity effectively mitigated the variation in precipitation.
Keywords Kuroshio, Large meander, Air-sea interaction, Precipitation, Regional atmospheric model*Correspondence:
Yoshi N. Sasaki
sasakiyo@sci.hokudai.ac.jp
1
Faculty of Science, Hokkaido University, Science 8th Bldg, 8‑3‑20, N10,
W8, Sapporo 060‑0810, Japan© The Author(s) 2024. 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 theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence 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 thislicence, visit http://creativecommons.org/licenses/by/4.0/.

Sasaki and Ito ﻿Progress in Earth and Planetary Science(2024) 11:15

1 Introduction
The Kuroshio, which is the western boundary currentof the subtropical gyre in the North Pacific, exhibits
two characteristic paths around the south of the Japanese mainland on decadal timescales (e.g., Kawabe 1980,
1995; Kida et al. 2015). One path is referred to as thestraight path, or non-large meander path (Fig. 1a), andthe other is the large-meander path (Fig. 1b). The mostrecent Kuroshio large meander event started in August
2017 (e.g., Qiu et al. 2023). These Kuroshio large meander events exert a significant influence on oceanic states,
such as oceanic temperature (e.g., Sugimoto et al. 2020)and coastal sea level (e.g., Kawabe 1985; Usui and Ogawa2022). In addition, changes in the parameters such as sea

Fig. 1 The wintertime (December–February) mean sea surface
height a in 2012 (straight path) and b in 2018 (large meander
path). The contour interval is 0.1 m. The data are downloaded
from the Copernicus Marine Service

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surface temperatures (SSTs) associated with the Kuroshio
large meander also impact the overlying atmosphere, as
will be mentioned in the next paragraph.
Along with the Gulf Stream region in the midlatitude
oceans, the Kuroshio region exerts a significant influence on the atmosphere (e.g., Nonaka and Xie 2003;
Sasaki et al. 2012; Parfitt and Seo 2018; Seo et al. 2023).
In particular, the warm SST associated with the Kuroshio and its variability markedly affects surface winds,
sea level pressure (SLP), and precipitation (e.g., Xie et al.
2002; Xu et al. 2011; Iizuka et al. 2013; Kuwano-Yoshida
and Minobe 2017; Sasaki and Yamada 2018; Hirata et al.
2021). Furthermore, the variability in the path of theKuroshio south of Japan on decadal timescales induces
marked SST fluctuations; the large meander causes
a positive SST anomaly along the southeastern coastof Japan and a large negative SST anomaly associatedwith a cold-core eddy (Fig. 2). Recently, Sugimoto et al.
(2020) reported that the development of the positive SST
anomaly is caused by the westward current associatedwith the Kuroshio. In the present study, the positive SST
anomaly along the southern coast of Japan is referred toas the warm SST tongue, and the negative SST anomaly
further south is referred to as the cool SST pool. Numerous studies have examined the atmospheric responses tochanges in the SST associated with the Kuroshio large
meander. For example, the path and intensity of theextratropical cyclones in winter were observed to changeover the Kuroshio large meander (e.g., Nakamura et al.
2012; Hayasaki et al. 2013; Tochimoto and Iizuka 2022).
However, relatively few studies have focused on the wintertime mean atmospheric response to the Kuroshio
large meander. Xu et al. (2010) investigated the atmospheric responses to the Kuroshio large meander eventfrom the summer of 2004 to the summer of 2005, especially in the winter of 2005. They observed that precipitation decreased over the cool SST pool and showed theexistence of a positive SLP anomaly over the cool SST
pool using regional atmospheric model experiments.
Murazaki et al. (2015) also used a regional atmosphericmodel to examine the atmospheric response to the Kuroshio large meander in winter, but the SST conditions usedin the model were estimated from the long-term integration of the regional ocean model. Based on their atmospheric model results, they reported that the frequency
of intense precipitation decreased over and around thenegative SST anomaly. Sugimoto et al (2021) reported
that the southeastern coastal region of Japan becomes
hot and humid in response to the effect of the warm
SST tongue on the atmosphere in summer. However, theatmospheric response of the mean winter state to thewarm SST tongue has not yet been reported. In addition,
as mentioned above, decreases in both precipitation andSasaki and Ito ﻿Progress in Earth and Planetary Science(2024) 11:15

Fig. 2 Difference in SST between the LM periods and the NLM
periods in winter from the OISST dataset. The contours indicate
the regions where the difference is statistically significant at the 95%
confidence level. The gray inset indicates the regions where the SSTof the WRF simulation was replaced. The LM and NLM periods
are defined in Sect. 2. The regions indicated by the blue box(33.5°N–34.5°N, 136.5°E–138.5°E) and indicated by the red box(32.5°N–33.5°N, 136°E–138°E) are used to define the Kuroshio large
meander index (see Sect. 2)

the frequency of intense precipitation in winter over thesouth of Japan have been reported, but the causes of these
decreases have not been discussed from the viewpoint of
the moisture budget. Given that these previous studies
focused mainly on the atmospheric response around thecyclonic eddy, the horizontal and vertical extents of these
atmospheric responses to the cool SST pool and thewarm SST tongue remain unclear. Therefore, the purposeof this study was to clarify the wintertime atmospheric
response to the cool SST pool and the warm SST tongue.
The present study is organized as follows. In Sect. 2,
we describe the regional atmospheric model and the settings used to investigate the atmospheric response to the
Kuroshio large meander. The model results of the atmospheric responses are described in Sect. 3. We focusedon surface wind changes and changes in precipitation in
Sects. 3.1 and 3.2, respectively. A summary and discussion are presented in Sect. 4.

2 Data and model
We employed the Weather Research and Forecasting(WRF: Skamarock et al. 2019) model, which solves theequations over terrain-following hydrostatic-pressure
coordinates. The model domain covers East Asia to thewestern North Pacific, and a geographical scope that is
considerably larger than the spatial scale of the Kuroshio large meander (Fig. 3). The model grid consists ofPage 3 of 13

240 meridional × 420 zonal grid points with a horizontal
resolution of 20 km and comprises 37 σ-levels. For convective parameterization and boundary layers processes,
we used a modified version of the Kain–Fritsch scheme(Kain and Fritsch 1990), the Yonsei University planetary
boundary layer scheme (Hong et al. 2006). The schemes
employed for longwave and shortwave radiations are theRapid Radiative Transfer Model (Mlawer et al. 1997) and
the Goddard shortwave scheme (Chou and Suarez 1994),
respectively. Initial and lateral boundary conditions werethe 6-hourly product of the National Centers for Environmental Prediction Climate Forecast System Reanalysis(NCEP-CFSR) product (Saha et al. 2010). The SST condition is the daily product of version 2 of the optimum
interpolation SST (OISST; Reynolds et al. 2007) provided
by the National Oceanic and Atmospheric Administration (NOAA), which has a 0.25° × 0.25° horizontal resolution and incorporates both in situ and Advanced Very
High-Resolution Radiometer SST data. The simulation
period using these forcing data is from November 30 toMarch 1 from 2001 to 2010. This run is referred to as the
CTL run. The model output is at 3-hourly intervals, andthe analysis focuses on its wintertime mean (December
to February).
In addition to the CTL run, two sensitivity runs were
performed to clarify the atmospheric responses to
the SST variability associated with the Kuroshio large
meander. In these sensitivity runs, the SST boundary
conditions within the area indicated by the gray box in
Fig. 2 were substituted with the daily SST composites
corresponding to the large meander (LM run) and nonlarge meander (NLM run) periods of the OISST dataset. To select these periods, we employed the time seriesof the southernmost latitude of the Kuroshio south ofJapan provided by the Japan Meteorological Agency(JMA) based on the ship observations from 1982 to2019. However, as this time series contains several missing values, we also used the time series of the coastal sea
level difference between Kushimoto and Uragami, also
provided by JMA. First, we calculated the wintertime
mean of these two time series. After normalization bystandard deviation and subtraction of the mean value,
we estimated the mean values of these two time series.
Based on this time series, the smallest 10 events (1982,
1983, 1984, 1987, 1988, 1990, 1991, 2005, 2018, and2019) and the largest 10 events (1992, 1993, 1994, 1995,
2002, 2003, 2006, 2007, 2008, and 2012) were identified as the Kuroshio large meander and the non-large
meander periods, respectively. Using these classifications, we estimated the daily composite of SST and used
these as the boundary forcing for the WRF simulationin the area indicated by the gray box in Fig. 2. Figure 2

Sasaki and Ito ﻿Progress in Earth and Planetary Science(2024) 11:15

Fig. 3 The domain and its topography of the WRF modelshows the SST differences between the LM periods andthe NLM periods. The statistical significance of these
SST differences was estimated using Student’s t test,
assuming that each winter season is independent. Threelarge SST anomalies are aligned in a north–south direction to the south of Japan. As mentioned in the Introduction, a warm SST tongue to the south of Japan wasreported by Sugimoto et al. (2020). The amplitude of
the negative SST anomaly associated with the cool SST
pool is several times larger than that of the positive SST
anomaly of the warm SST tongue. Except for these SST
anomalies, several statistically significant SST signals
were observed, such as the negative SST anomaly in the
Kuroshio Extension region, but these anomalies were
not included in our simulation.
To compare the simulated atmospheric responses
to observations, we used the ECMWF Reanalysis v5(ERA5) product for winter from 1982 to 2019 (Hersbach et al. 2020). To obtain atmospheric responses to the
Kuroshio large meander using this dataset, we define theKuroshio large meander index as the difference in the
SST of the OISST dataset averaged over the region indicated by the blue box (33.5°N–34.5°N, 136.5°E–138.5°E)
minus the red box (32.5°N–33.5°N, 136°E–138°E) in
Figs. 2 and 7, where both time series were normalized
before the subtraction. The former region and the latterregion correspond to the warm SST tongue and the cool
SST pool, respectively. We performed correlation andregression analyses using this index and the ERA5 dataset. An advantage of using this index is that the role of
the SST in the warm SST tongue and the cool SST pool
regions could be investigated, separately. The correlation coefficients between this index and the time seriesof the southernmost latitude of the Kuroshio, the coastal
sea level difference between Kushimoto and Uragami
provided by JMA, and the aforementioned mean valuesof the two time series were − 0.73, − 0.55, and − 0.59,
respectively.

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3 Results
Before presenting the results of the sensitivity experiments, we compared the climatology of the atmosphericstate around Japan in winter between the CTL run andthe ERA5 dataset (Fig. 4). The WRF simulation well
reproduced the mean northwesterly winds and SLP pattern in winter, that is, high and low SLPs in the west andeast of Japan, respectively. In addition, the model efficiently simulated the mean precipitation distribution,such as the large precipitation along the western coastof Japan and over the Kuroshio Extension, although itslightly underestimated them. Both ERA5 and WRF
showed that the large precipitation over the Kuroshio
Extension extended to the Kuroshio south of Japan.
Thus, these results show that the WRF simulation effectively reproduced the wintertime mean atmospheric statearound Japan.

Fig. 4 Winter mean precipitation (color), sea level pressure (contour)
and 10-m wind (vector) averaged from the years 2001 to 2010. a ERA5
reanalysis data, and b the CTL run of the WRF modelSasaki and Ito ﻿Progress in Earth and Planetary Science(2024) 11:15

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3.1 Wind response

In this subsection, we focused on the surface windresponse to the Kuroshio large meander. To this end, we
showed the differences between the LM runs minus NLM
runs. The simulated changes of the surface wind and its
divergence showed the surface wind divergence over thecool SST pool (Fig. 5). This wind divergence was collocated with the positive SLP anomaly (not shown). Thesewind divergence and positive SLP anomaly responses
over the cool SST pool have been reported previously(Xu et al. 2010; Murazaki et al. 2015). They also showedthat the relation between the negative SST anomaly andthe positive SLP anomaly was consistent with the pressure adjustment mechanism (e.g., Lindzen and Nigam
1987) with thermal advection by the prevailing northeasterly (Small et al. 2003; Xu et al. 2010). On the other
hand, the surface wind convergences were located over
the two positive SST anomalies associated with the
Kuroshio large meander (Figs. 2, 5b). In particular, theamplitude of the wind convergence over the warm SST
tongue was comparable to the wind divergence over thecool SST pool. These two surface wind convergences arelikely associated with the positive SST anomalies throughthe pressure adjustment mechanism. These wind convergences were not explicitly described in previous studies,
although Murazaki et al. (2015) showed similar convergence to the south of Japan.
A closer look at the statistically significant windchanges showed that the air outflow over the cool SST
pool was prominent (Fig. 5a). That is, the southerly andnortheasterly wind anomalies were located to the northand south of the cool SST pool, respectively. Interestingly, the northeasterly surface wind anomaly converging to the warm SST tongue extended far to the east of
the Japanese coast at around 36°N. This spatial extent of
the surface wind response cannot be simply explainedby the pressure adjustment mechanism, because the
SST anomaly does not extend there. This northeasterly
anomaly extended vertically from the surface to 950 hPa(not shown). Note that this northeasterly wind anomaly
is collocated with the reduction in meridional wind variance (not shown). This result might imply some relation
between this northeasterly wind anomaly and weather
disturbances, such as extratropical cyclones.
How far does the vertical structure of the surface windconvergence and divergence over the SST anomalies
extend? Figure 6b shows the simulated vertical velocity change along 138°E. A statistically significant downwelling wind anomaly was located over the cool SST pooland extended to 700 hPa. This downwelling wind anomaly was consistent with the surface wind divergence inthis region (Fig. 5). In contrast, the upward wind anomaly

Fig. 5 Difference of a 10-m wind and b the corresponding
divergence over the ocean between the LM runs minus NLM runs. Ina, the black (gray) vectors indicate the regions where the differenceof zonal or (and) meridional wind is (not) statistically significant
at the 95% confidence level. Red and blue contours indicate the SST
difference of + 0.5 °C and–0.5 °C, respectively, between the LM runs
minus NLM runs (see Fig. 2). In b, the contour indicates the regions
where the difference is statistically significant at the 95% confidencelevel

was observed over the warm SST tongue, although its
vertical extension was shallower than the downwelling.
These vertical extents in the upward/downward winds
were shallower than those over the Gulf Stream in winter, which have been reported to reach the upper troposphere (Minobe et al. 2010).

Sasaki and Ito ﻿Progress in Earth and Planetary Science(2024) 11:15

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Fig. 7 Regression coefficients of the 10-m wind as derived
from the ERA5 dataset onto the Kuroshio large meander index
from 1982 to 2019. This index is defined by the differencein the observed SST averaged over the region indicated by the bluebox (33.5°N–34.5°N, 136.5°E–138.5°E) minus the region indicatedby the red box (32.5°N–33.5°N, 136°E–138°E). The black (gray) vectors
indicate the regions where the corresponding correlation coefficientsfor either of zonal or (and) meridional wind are (not) statisticallysignificant at the 95% confidence level

Fig. 6 a Regression coefficients of upward pressure velocity
along 138°E based on the ERA5 dataset onto the Kuroshio
large meander index from 1982 to 2019. The contour interval
is 0.005 Pa ­s−1. b Difference of the vertical velocity of the model
along 138°E between the LM runs minus the NLM runs. The
contour interval is 0.2 cm ­s−1. In a, b, gray shading indicates regions
where the corresponding correlation coefficients and the difference
are statistically significant at the 95% confidence level, respectively

To confirm whether a similar atmospheric response
could be observed, we performed correlation and regression analyses using the Kuroshio large meander index(see Sect. 2) and the ERA5 product from 1982 to 2019.
To emphasize atmospheric signals with spatial scales
similar to the Kuroshio large meander, the area averageover 25°–45°N, 120°–150°E was subtracted from zonal
and meridional wind data before these calculations. The
surface wind change in the ERA5 product confirmed
the divergence over the cool SST pool and convergenceover the warm SST tongue (Fig. 7). Associated with thesewind convergence and divergence dynamics, the verticalvelocity anomaly extended to 700 hPa (Fig. 6a), which
was consistent with the simulation result. In addition, thenortheasterly wind anomaly east of the Japanese coast
was also observed. In contrast, the wind convergenceover the positive SST anomaly to the south of the cool
SST pool was not prominent in the ERA5 dataset, likely
because the positive SST anomaly of the ERA5 productin this region is weaker than that of the OISST dataset(not shown).
It is important to clarify the roles of the warm SST
tongue and the cool SST pool in inducing the northeasterly wind anomaly. To this end, we separately estimatedthe regression coefficients of the surface wind using the
SST averaged over the warm SST tongue and the cool
SST pool regions. The results showed that the northeasterly wind was correlated significantly with the SST averaged over the warm SST tongue region, but not with the
SST averaged over the cool SST pool region (Fig. 8). The
warm SST tongue was thus mainly related to northeasterly and easterly wind anomalies (Fig. 8a) that blows over
the warm SST tongue. These wind anomalies can be seenin the wind anomalies associated with the Kuroshio large
meander index (Fig. 7). On the other hand, the SST variability in the cold SST pool region was related to the local

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the moisture budget is as follows (e.g., Seager et al. 2010;
Kuwano-Yoshida et al. 2010):

pT∂qdp/ρw g = −∂tpT∇ · (qu)dp/ρw g + E − P, (1)pspswhere q is specific humidity, u is the horizontal windvector, p is pressure, ps is surface pressure, E is evaporation, P is precipitation, g is gravitational acceleration,and ρw is water density. Here, we set pT as 500 hPa andignore the moisture above this pressure level. Due tothe small magnitude of the left hand side of Eq. (1) (notshown), we restricted our analysis to the three terms on
the right hand side, that is, horizontal moisture convergence, evaporation, and precipitation, using the 3-hourly
model outputs. For each term, we present the difference
between the LM and NLM runs (Fig. 9). For example, thedifference in the horizontal moisture convergence term
can be written as follows:pT∇ · (qLM uLM )dpLM +−psFig. 8 As for Fig. 7, but for the regression coefficients of the 10-m
wind as derived from the ERA5 dataset onto the SST of the OISST
dataset averaged a over the region indicated by the blue box in Fig. 7
and b over the region indicated by the red box in Fig. 7. The signof the vectors of b is reversed for ease of comparison

divergence wind. These results suggest that the warm
SST tongue plays an important role in the anomalous
northeasterly wind along the east of the Japanese coastaround 36°N.
3.2 Precipitation response

In this subsection, we examined the precipitation
response to the SST anomalies associated with the Kuroshio large meander. In addition, we clarified the moisturebudget of the precipitation change. The basic equation ofpT∇ · (qNLM uNLM )dpNLM
pswhere the subscripts LM and NLM denote the valuablesin the LM run and the NLM run, respectively.
Since the only difference between the LM and NLM
runs was the SST boundary conditions, the evaporation change was considered as the forcing. Thus, we firstexamined the difference in evaporation between the LM
run minus the NLM run (Fig. 9b). The finding showedthat evaporation decreased over the cool SST pool, but
increased over the warm SST tongue and to the southof the cool SST pool during the LM periods, which wasconsistent with the SST anomalies. Like the SST anomalies, the evaporation anomaly over the cool SST pool was
several times larger than that over the warm SST tongue.
Compared to the evaporation changes, the precipitation response has different characteristic spatial patterns (Fig. 9a). The negative precipitation anomaly wasseen over the cool SST pool, consistent with the previous studies (Xu et al. 2010; Murazaki et al. 2015). Thisis also consistent with the expectation that the cool SST
pool would reduce the evaporation in that region andis accompanied by the presence of surface wind divergence aloft (Fig. 5b). However, the amplitude of the negative precipitation anomaly over the cool SST pool was
smaller than the corresponding changes in evaporation.
In addition, the geographical locations at which the peakchanges occurred also differed between precipitation andevaporation. Conversely, a positive precipitation anomaly
was observed to the south of the cool SST pool, but notSasaki and Ito ﻿Progress in Earth and Planetary Science(2024) 11:15

Page 8 of 13◂ Fig. 9 Difference in a precipitation, b evaporation and c horizontal

moisture convergence over the ocean between the LM minus NLM
runs. Solid and dashed contours indicate the SST differenceof + 0.5 °C and–0.5 °C, respectively, between the LM runs minus NLM
runs (see Fig. 2). In a, the gray contours indicate the regions
where the corresponding correlation coefficients are statisticallysignificant at the 95% confidence level

over the warm SST tongue, even though evaporation isenhanced in that region. These results indicate that the
response of precipitation to changes in evaporation is notstraightforward.
The imbalances between the changes in precipitation and evaporation resulted from changes in thehorizontal moisture convergence (Fig. 9c). Interestingly, the positive moisture convergence anomaly was
located over the cool SST pool, which contrasted withthe surface wind divergence observed in the region(Fig. 5). The amplitude of this moisture convergence
was comparable to that of the precipitation change, but
the sign is opposite. Since the change in evaporation isconsidered as the forcing mechanism, the decrease inevaporation was offset by the sum of the decrease inprecipitation and the horizontal moisture convergence.
Similarly, horizontal moisture divergence was locatedover the warm SST tongue, which is also inconsistent with the surface wind convergence in that region(Fig. 5b). As a result of this horizontal moisture divergence, the increase in precipitation is limited (Fig. 9a).
Conversely, the horizontal moisture convergence wasalmost zero over the positive evaporation anomaly
located to the south of the cool SST pool. In this region,
the enhanced evaporation associated with the positive
SST anomaly was balanced by enhanced rainfall. Therefore, the dynamics of moisture convergence are considered to be important for the precipitation response to
the Kuroshio large meander.
Are there any similarities in the imbalances between
the precipitation and evaporation changes over the cool
SST pool and the warm SST tongue as observed in the
ERA5 dataset? The ERA5 dataset also displayed negative anomalies in both precipitation and evaporation
over the cool SST pool (Fig. 10). The amplitude of theevaporation anomaly over the cool SST pool was larger
than that of the precipitation anomaly, which is consistent with the WRF simulation results (Fig. 9). Thesefindings imply that horizontal moisture convergenceover the cool SST pool is likely necessary to balance themoisture budget, although we did not examine this termSasaki and Ito ﻿Progress in Earth and Planetary Science(2024) 11:15

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in the context of the ERA5 dataset. Furthermore, evaporation was also enhanced over the warm SST tongue,
but precipitation did not increase in the region (Fig. 10).
These findings are consistent with the results of the
WRF simulation.
To better clarify the role of horizontal moisture convergence and divergence in the WRF simulation, the moisture convergence term can be separated into two terms,
as follows:−pT∇ · (qu)dp = −psFig. 10 Regression coefficients of a precipitation and b
evaporation in the ERA5 dataset onto the Kuroshio large
meander index from 1982 to 2019. Contours indicate the regions
where the corresponding correlation coefficients are statisticallysignificant at the 95% confidence level

−

pT
ps

−

pT
ps

uLM · ∇qLM dpLM +

pT

pT
ps

u · ∇qdp −pTq∇ · udp, (2)pswhere ρwg in the denominator is omitted for simplicity(see Eq. 1). The first term and the second term on theright hand side are referred to as the moisture convergence resulting from horizontal moisture advection andthe moisture convergence attributed to the wind convergence, respectively. Using this separation, we found thatthe change in the moisture convergence resulting fromhorizontal moisture advection (Fig. 11b) was opposite toand larger than the moisture convergence attributed to
the wind convergence over the cool SST pool (Fig. 11a).
Thus, the moisture convergence resulting from horizontal moisture advection is considered to play an important role in the horizontal moisture convergence in that
region. The negative anomaly in the moisture convergence attributed to the wind convergence over the cool
SST pool (Fig. 11a) is consistent with the surface winddivergence in that region (Fig. 5b). Conversely, the difference observed in the moisture convergence resulting
from horizontal moisture advection is negative over thewarm SST tongue (Fig. 11b), whereas the difference inmoisture convergence attributed to wind convergence issmall (Fig. 11a). Therefore, alternations in the moisture
convergence resulting from horizontal moisture advection attenuate changes in precipitation.
To further clarify the role of the moisture convergence
resulting from horizontal moisture advection, the term
can be separated as follows:uNLM · ∇qNLM dpNLM ≈ps(uLM − uNLM ) · ∇qLM dpLM −pTpsuLM · ∇(qLM − qNLM )dpLM +

pT
ps

(3)
(uLM − uNLM ) · ∇(qLM − qNLM )dpLM

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Fig. 12 As for Fig. 9, but showing the difference in moisture
convergence due to horizontal moisture advection causedby the difference in specific humidity between the LM run
and the NLM runs

SST pool and the negative difference over the warm SST
tongue. These findings suggest that the Kuroshio large
meander influences the horizontal moisture convergence
mainly via changes in the horizontal gradient of thespecific humidity in the lower troposphere (Fig. 13). A

Fig. 11 As for Fig. 9, but showing the difference in a the moisture
convergence attributed to wind convergence and b the moisture
convergence resulting from horizontal moisture advection

In this equation, the left hand side is not equal to theright hand side, but the residual is small. We checked thethree terms on the right hand side and found that thesecond term is considerably larger than the first and thirdterms (not shown). Even if the wind field of the second
term is replaced with the wind of the NLM run, the result
is robust. This second term is moisture convergence dueto the horizontal moisture advection caused by changes
in the specific humidity. As shown in Fig. 12, the change
in moisture convergence due to the horizontal moistureadvection caused by changes in the specific humidity issimilar to the changes associated with the moisture convergence resulting from horizontal moisture advection(Fig. 11b), namely, the positive difference over the coolFig. 13 As for Fig. 9, but showing the difference in specifichumidity (color; g ­kg−1) and the mean wind (vector) at 900 hPa.
Solid and dashed contours indicate the SST difference of + 0.5 °C
and − 0.5 °C, respectively, between the LM runs minus NLM runs

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decreased over the cool SST pool. Murazaki et al. (2015)
reported a decrease in the frequency of intense precipitation (> 1 mm /3 h) over and around the cool SST
pool based on their model results, and here we found a
decrease in the weak precipitation over the cool SST
pool or an increase in the frequency of the rainfall over
the warm SST tongue. When we employed a threshold of 0.5 mm ­day−1 to define a rainy day, a similar spatial pattern was obtained. However, when a thresholdof 5.0 mm ­day−1 was applied, the area of the positive
anomaly over the warm SST tongue decreased (notshown), implying that only the frequency of weak precipitation increased over the warm SST tongue. Thesefindings appear to be consistent with the observation thatthe amount of precipitation in that region is relatively
unchanged (Fig. 9a). As in the case of the surface windshown in Fig. 8, we calculated the correlation coefficientsof the number of rainy days from the ERA5 product using
the SST averaged over the warm SST tongue and the cool
SST pool, separately. The results show that the SST in thewarm SST tongue region (in the cool SST pool region)is correlated with the positive (negative) anomaly of thenumber of rainy days in that region (not shown). Overthe warm SST tongue, the enhanced evaporation (Fig. 9b)
and sensible heat release, which induce the verticallyunstable condition, and the upwind associated with thesurface wind convergence are favorable for the enhanced
precipitation (Fig. 14). However, because the anomalous
horizontal moisture divergence acts to reduce the moisture, the number of weak rainy days increased over thewarm SST tongue (Fig. 9a).

Fig. 14 a Regression coefficients for the number of rainy daysin the ERA5 dataset onto the Kuroshio large meander index
from 1982 to 2019. b Difference in the number of rainy days
between the LM runs minus NLM runs. In a, b, contours indicate
regions where the corresponding correlation coefficientsand the difference are statistically significant at the 95% confidence
level, respectively. Rainy days were defined using a 1.0 mm ­day−1
thresholddecrease (increase) in the evaporation over the cool SST
pool (warm SST tongue) induces the negative (positive)
specific humidity anomaly in the lower troposphere.
It is worth noting that the amount of precipitation
was not increased, but rather, the number of rainy days(> 1.0 mm ­day−1) was increased significantly over thewarm SST tongue in both the WRF and ERA5 results(Fig. 14). Similarly, the number of rainy days was

4 Summary and discussion
The wintertime atmospheric responses to the Kuroshio
large meander to the south of Japan, especially the cool
SST pool and the warm SST tongue (Fig. 2), were investigated using the ERA5 dataset and a set of WRF simulations. The results of sensitivity experiments conductedusing the WRF showed that the surface wind responses
to the SST anomalies associated with the Kuroshio large
meander were not solely characterized by the wind divergence over the cool SST pool, as has been reported previously, but also by the wind convergence over the warm
SST tongue (Fig. 5). These wind divergence and convergence were accompanied by downward and upward windreaching 700 and 800 hPa, respectively (Fig. 6a). Interestingly, during the Kuroshio large meander periods, anortheasterly wind anomaly blew into the positive SST
anomaly was observed along the east coast of Japan ataround 36°N (Fig. 5). Analyses using the ERA5 productcorroborated the presence of similar northeasterly wind
anomalies (Fig. 7) and underscored the importance of theSasaki and Ito ﻿Progress in Earth and Planetary Science(2024) 11:15

warm SST tongue in contributing to these wind anomalies (Fig. 8).
The response of precipitation to the Kuroshio large
meander was also prominent. During the period of
the Kuroshio large meander, the number of rainy days
significantly increased and decreased over the warm
SST tongue and the cool SST pool (Fig. 14), respectively, which was consistent with both the WRF simulation and the reanalysis product. However, changes in
the quantity of precipitation were only observed overthe cool SST pool (Fig. 9a). The moisture budget analysis revealed that the decrease in precipitation over thecool SST pool resulted from the interplay between thedecrease in the evaporation (Fig. 9b) and the anomalous
horizontal moisture convergence (Fig. 9c). Interestingly,
the anomalous horizontal moisture convergence (divergence) observed over the cool SST pool (the warm SST
tongue) was mainly induced by a decrease in the specifichumidity (Figs. 12, 13), despite concurrent surface winddivergence (convergence) (Fig. 9c). These changes acted
to mitigate changes in precipitation.
In this study, we found that the surface wind responseto the Kuroshio large meander extends along the eastof the Japanese coast (Figs. 5, 7). This gives rise to aninteresting question concerning the oceanic responseto this anomalous wind. For example, these surface
wind anomalies have the potential to induce positive
sea level anomalies along the Japanese coast via Ekman
advection. These sea level anomalies could then propagate clockwise along the Japanese coast and potentiallyinfluence the Kuroshio to the south of Japan. To ourknowledge, no studies have reported on the potential existence of such a feedback loop in this region. A
regional atmosphere–ocean coupled model would provide helpful insights into this topic, particularly sincecoastal sea level variability around Japan is primarily
driven by other processes, such as westward propagating Rossby waves (Sasaki et al. 2014) or alterations to
the path of the Kuroshio to the south of Japan (Usui andOgawa 2022).
Abbreviations
JMA	Japan Meteorological Agency
LM	Large-meander
NCEP-CFSR	National Centers for Environmental Prediction Climate Forecast
System Reanalysis
NLM	Non-large meander
NOAA	National Oceanic and Atmospheric Administration
OISST	Optimum interpolation sea surface temperature
SLP	Sea level pressure
SST	Sea surface temperature
WRF	Weather research and forecasting model
Acknowledgements
We thank Prof. S. Minobe and Dr. F. Ogawa for their comments on our study.
We also thank two anonymous reviewers for their constructive comments on
the manuscript.

Page 12 of 13

Author contributions
YNS and RI proposed the topic and designed the study. RI carried out themodel experiments. YNS and RI analyzed the simulation and reanalysis data.
YNS wrote the first draft of the manuscript. All authors read and approved thefinal manuscript.
Funding
This work was supported by JSPS KAKENHI Grant Number 19K03960 and
19H05704.
Availability of data and materials
The JMA data were obtained from https://​www.​data.​jma.​go.​jp/​kaiyou/​data/​
shind​an/b_​2/​kuros​hio_​stream/​kuros​hio_​stream.​html. The ERA5 product was
obtained from https://​cds.​clima​te.​coper​nicus.​eu/​cdsapp#​!/​datas​et/​reana​
lysis-​era5-​press​ure-​levels-​month​ly-​means?​tab=​overv​iew and https://​cds.​clima​
te.​coper​nicus.​eu/​cdsapp#​!/​datas​et/​reana​lysis-​era5-​single-​levels?​tab=​overv​
iew. The OISST product was obtained from https://​www.​ncei.​noaa.​gov/​data/​
sea-​surfa​ce-​tempe​rature-​optim​um-​inter​polat​ion/​v2/​access/. The NCEP-CFSR
product was obtained from https://​www.​ncei.​noaa.​gov/​data/​clima​te-​forec​
ast-​system/​access/. The outputs from the WRF model are available from Yoshi
N. Sasaki on reasonable request.

Declarations
Competing interests
The authors declare that they have no competing interests.
Received: 18 October 2023 Accepted: 15 March 2024

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