Local-scale circulation associated with enhanced poleward moisture transport for Meiyu-Baiu heavy rainfall over western Japan

Heavy rainfall events in western Japan during early July have become more frequent, yet the underlying mechanism behind this trend during the late stage of the Meiyu-Baiu rainy season remains unclear. Our long-term analysis of short-duration events revealed that a quasi-stationary Rossby wave train enhances the poleward transport of moisture from the western Pacific, contributing to the frequent occurrence of heavy rainfall events over western Japan. The local-scale circulation over the East China Sea plays a substantial role in producing this quasi-stationary Rossby wave train, which is closely linked to enhanced deep convection over the Kuroshio warm current, characterized by a distinct sea surface temperature (SST) front. The coarse resolution of both the model and SST data may hinder the ability of climate simulations to capture the local-scale circulation, underscoring the importance of quasi-stationary atmospheric circulation for a better understanding of heavy rainfall events through poleward moisture transport.


Introduction
Extreme rainfall events caused by atmospheric rivers (ARs) are expected to be enhanced by increased atmospheric moisture due to climate change (e.g.Payne et al 2020).However, the location and duration of AR landfalls, which are typically less than a week, are strongly influenced by atmospheric circulation, resulting in uneven increases in water vapour and localized heavy rainfall (Hu et al 2017).While the role of atmospheric circulation in ARs is crucial, especially in the transition regions between the subtropics and the mid-latitudes, the response of atmospheric circulation to climate change is highly uncertain and inconclusive, unlike the expected thermodynamic responses.For instance, the summertime circulation over the North Pacific is influenced by direct radiative forcing and oceanic warming, leading to a tug-ofwar on the North Pacific subtropical high response to global warming (Shaw and Voigt 2015).
East Asia is one of the regions in the world highly impacted by ARs (e.g.Payne et al 2020), particularly during the East Asian summer monsoon, also known as the Meiyu-Baiu rainy season (e.g.Kamae et al 2017, Park et al 2021).The Meiyu-Baiu rainfall is supplied by moisture transport from the subtropics along the western ridge of the North Pacific subtropical high (e.g.Sampe and Xie 2010).Over the recent 40 years, western Japan has experienced the largest increase in Meiyu-Baiu precipitation in East Asia (Matsumura and Horinouchi 2023), leading to frequent heavy rainfall events that have caused severe natural disasters with devastating socioeconomic impacts, such as the 2018 event that caused 237 fatalities (EOCJ 2019).Notably, many of these heavy rainfall events, including the 2017 Northern Kyusyu (5-6 July; Tsuji et al 2020), July 2018 (5-8 July; Sekizawa et al 2019, Takemura et al 2019), and July 2020 (3-14 July; Zhao et al 2021, Kawano and Kawamura 2022) events, have consistently occurred in early July, which is the late stage of the Meiyu-Baiu season in western Japan.However, the reason why such events occur frequently in western Japan during early July remains unclear.Given the uneven variability in water vapour over time and space for ARs, the lack of understanding of the long-term changes in the quasi-stationary component producing ARrelated precipitation, which lasts less than a week, could be a contributing factor, rather than the stationary component like the North Pacific subtropical high.
To explore the frequent occurrence of heavy rainfall events in early July over western Japan, we investigated decadal changes in short-duration events during the Meiyu-Baiu season.Our approach differs from conventional trend analysis based on monthly or seasonal means.We reveal that local-scale circulation over the Kuroshio sea surface temperature (SST) front plays a substantial role in producing quasistationary Rossby waves in early July, which contribute to the frequent occurrence of heavy rainfall events over western Japan by enhancing poleward moisture transport from the western Pacific.Our study highlights the importance of considering local-scale and quasi-stationary atmospheric changes to better understand AR-related extreme rainfall events.

Data and methods
To capture the fine structure of AR-related precipitation, atmospheric data were obtained from the ERA5 reanalysis (Hersbach et al 2020) with a 0.25 • × 0.25 • horizontal grid based on 6-hourly data, except for convective precipitation (3-hourly data).We used daily precipitation data based on rain gauge measurements over land from the Climate Prediction Center (CPC; Chen et al 2008) with a 0.5 This study focuses on decadal precipitation changes during the Meiyu-Baiu period (June-July) for 40 years, from 1982 to 2021, limited by OISST's satellite estimates.We compare the periods between 1982-2001 and 2002-2021, as the characteristic of East Asian precipitation variability on an interannual timescale has shifted from variations of frontal intensity to those of cumulus convective activity in the mid-2000s (Matsumura and Horinouchi 2023).Although trend analysis is generally based on monthly or seasonal means, in this study, we use daily or 5 day mean data (pentad from 1 June) to discuss AR-related precipitation with short duration (less than a week).We confirmed the robustness of decadal changes in the 5 day mean data for early July (6-10 July), regardless of excluding years with heavy rainfall events such as 2018 and 2020 (not shown).The significance test is a standard two-tailed t-test with degrees of freedom based on the number of years. We

Enhanced poleward IVT and quasi-stationary circulation
While western Japan has experienced the most significant increase in East Asian summer precipitation in the last 40 years (Matsumura and Horinouchi 2023), the changes seem to be concentrated during the late stage of the Meiyu-Baiu season.variability at 500 hPa in early July, coinciding with a maximum of precipitable water (almost 50 mm) (figure S1), consistent with the observed changes in precipitation.
The ERA5-based IVT also reveals that the largest increase in IVT occurs in early July, compared to late June (figure 1(b)).There is a robust linear relationship between changes in Meiyu-Baiu precipitation and IVT throughout the Meiyu-Baiu season, not just in early July (figure S2).The Meiyu-Baiu rainband is climatologically formed along the westerly jet, resulting in a larger eastward IVT than northward IVT during the Meiyu-Baiu season over western Japan (figures 1(c) and (d)).In the whole period from June to July, the mean ratio of eastward to total IVT over western Japan is estimated to be around 60%, while the northward IVT contributes only about 30% to the total IVT, increasing with the seasonal progression of Meiyu-Baiu rainfall.However, there is no linear relationship between changes in daily precipitation and eastward IVT (figure S2), although the eastward IVT contributes to increased precipitation during the conventional peak in late June (figures 1(a) and (c)).Instead, the changes in northward IVT highly correlate with those in precipitation above the 99% confidence level (figure S2), accounting for the changes in the total IVT in early July (figures 1(b) and (d)).These results indicate that while the eastward IVT dominates moisture transport during the Meiyu-Baiu season in the mean state, the increased precipitation over western Japan is primarily attributed to the increased poleward IVT, particularly in early July, which contributes to the frequent occurrence of heavy rainfall events over western Japan.
To understand the reasons behind the significant increase in precipitation and poleward IVT over western Japan in early July, we examine 5 day mean IVT in the difference between the recent and earlier decades.During 6-10 July, IVT experiences an extreme increase of 200 kg m −1 s −1 over southern Japan, including the southwest Islands, in the recent decades, which is 1.5 times higher than that in the earlier decades (figure 2(a)).This extreme increase in IVT is the result of the poleward transport from the subtropical Pacific by intensifying cyclonic circulation over the East China Sea (ECS) and anticyclonic circulation off the Pacific coast of Japan (figure 2(b)), leading to increased precipitation over western Japan.Conversely, decreased IVT over China is attributed to an increase in southwestward moisture flux that brings cool and dry air toward south China with higher climatological θ e .The uneven IVT changes are associated with a wave train originating around from the Yellow Sea, where Rossby waves propagate eastward along the westerly jet and intensify anticyclonic circulation over northern Japan (figure 2(c)), favouring for the development of the western Pacific subtropical high (e.g.Matsumura and Horinouchi 2016).The wave activity is most pronounced in the mid-troposphere rather than the upper troposphere, consistent with a previous study on interannual variability (Matsumura and Horinouchi 2023).The sea level pressure (SLP) also has a wave train tilted northward with height (figure 2(d)), reflecting a tilt of the jet axis during the Meiyu-Baiu season.However, the centre of the weakened SLP is located southeastward compared to that of the intensified mid-tropospheric cyclonic anomaly over the Yellow Sea, reminiscent of the characteristics of a Rossby wave response to enhanced heating.
Figure 2(e) shows latitude-vertical section of specific humidity over the weakened SLP centre.The specific humidity increases the most in the lower troposphere, especially at 800-700 hPa, with pronounced moistening over the ocean surface between 25 • and 30 • N, where mid-tropospheric divergence is evident, consistent with the Rossby wave train at 500 hPa (figure 2(c)).Compared to the ocean surface moistening and mid-tropospheric divergence, the lower-tropospheric moistening shifts poleward around 30 • N, as a result of poleward moisture transport (figure 2(b)).While these results represent a characteristic of cumulus convection, the absence of a poleward shift of lower-tropospheric moistening in summertime stationary circulation (Matsumura and Horinouchi 2023) emphasizes the importance of quasi-stationary circulation for poleward moisture transport.Indeed, the quasi-stationary Rossby wave train lasts only for about a week.For the previous 5 d from 1 to 5 July, the Meiyu-Baiu frontal structures, which is also a quasi-stationary system, are apparent, with a deeper trough over the Korean Peninsula and strong convergence by lower-level southerlies around 35 • N, leading to north-south gradients in specific humidity (figure S3).During 6-10 July, however, enhanced cumulus convection over the ECS, which differs from the Meiyu-Baiu frontal structures, acts as a forcing for the Rossby wave train (figure 2), which weakens during 11-15 July, shifting the stronger IVT equatorward, far from western Japan (figure S4).Interestingly, while IVT increases over the entire ECS (figure 2(a)), the intensified ascent is confined to a narrow band between 125 • and 130 • E (figure 2(f)), even though vertical velocity remains upward in the recent two decades (solid contours) along the Meiyu-Baiu rainband.This might be attributed to the difference in the development of cumulus convection between the tropical and subtropical or mid-latitudes oceans.

Local-scale circulation over the Kuroshio SST front
Compared with tropical SST, subtropical SST are typically more susceptible to atmospheric forcing.One notable example is the western Pacific subtropical high, which exerts a strong influence on the subtropical SST (e.g.Xie et al 2009).However, during the heavy rainfall event over western Japan in July 2018, the pronounced enhancement of poleward low-level moisture transport was found to accompany enhanced oceanic evaporation around the Kuroshio current and active convection over the southern ECS (Sekizawa et al 2019, Takemura et al 2019).Similarly, in the July 2020 heavy rainfall event, increased moisture over the Kuroshio current in the ECS was transported to western Japan, further enhancing the heavy rainfall (Kawano and Kawamura 2022).Consequently, the subtropical sea plays a substantial role as a potential moisture source for midlatitude regions (Zhao et al 2021).Here we examine the atmospheric changes over the ECS, with a specific focus on its crucial role in driving quasi-stationary circulation for poleward moisture transport.
Figure 3 shows longitude-time section of SST, surface latent heat flux, surface wind, and vertical velocity at 500 hPa averaged over 27 • -29 • N in the ECS.In the Meiyu-Baiu season, the Kuroshio current exhibits a warm SST tongue structure ranging from 26 • C to 28 • C, which is warmer than the SST at the same latitude in the ECS and the western Pacific (e.g.Sasaki et al 2012).In the recent decades, the SST front has become even stronger from June to mid-July, with higher SST than in the earlier decades.This sharp Kuroshio SST front is accompanied by an upward surface latent heat flux of 80 W m −2 , which increases over the SST front from 1 July in the recent decades, spreading broadly over the east-west sides in mid-July (figure 3(b)).The surface wind speed (contours) shows a distinct relationship with the SST front, with high wind speed over the warm tongue from June to mid-July (figure 3(c)).Interestingly, coincident with the increased surface latent heat flux, the cyclonic circulation (vectors) intensifies with a centre over the SST front by mid-July, leading to the largest increase in wind speed to the east of the SST front (around 130 • -135 • E), consistent with the centre of the weakened SLP over the ECS (figure 2(d)).Ascent motions at 500 hPa also intensify along the SST front in the recent two decades (figure 3(d)), reflecting the zonally spreading Meiyu-Baiu rainband by late June, which climatologically marks the end of the rainy season in this region due to the northward migration of the rainband along the westerly jet.However, from then until mid-July, the intensified ascent motions over the SST front become more evident.Indeed, as the Meiyu-Baiu rainband moves northward along the jet with seasonal progress, the contribution of convective precipitation to total precipitation becomes pronounced over the SST front (not shown).These findings on decadal changes agree with previous studies based on high-resolution satellite observations and favouring the development of cyclonic-like circulation (figure S3).In the peak phase from 6 to 10 July, SST warming is suppressed over and to the south of the SST front, due to the intensified cyclonic circulation and increased surface latent heat flux, which in turn, results in contrasting SST changes between the east (high) and west (low) of the western Pacific during 11-15 July (figure S6).These results imply an interactive coupling between the atmosphere and ocean in the ECS and the western Pacific.As a result, the quasi-stationary Rossby wave train only last for about a week.
To confirm the increased precipitation over the Kuroshio SST front, we show the June-July mean GPCP precipitation with a horizontal resolution of 2.5 • grids in figure 4(a).While the Meiyu-Baiu rainband forms zonally along the westerly jet, GPCP precipitation shows the largest increase in the southern ECS, extending northeastward over the SST front of 27 • C and 28 • C.This is consistent with ERA5based convective precipitation, which also shows a substantial increase along the SST front (figure 4(b)).These changes in both GPCP and ERA5 precipitation suggest that Meiyu-Baiu precipitation over the SST front is not only climatologically organized (Sasaki et al 2012, Xu et al 2018) but also evident in decadal changes, although there is also a possibility that tropical cyclones affect the long-term mean state (e.g.Arakane and Hsu 2021).Meiyu-Baiu precipitation strongly depends on the Asian jet (Horinouchi et al 2019), which has weakened over the past 40 years (Matsumura and Horinouchi 2023).However, the most significant changes in Meiyu-Baiu precipitation occur over the SST front, not along the jet.This suggests that local SST forcing is likely to contribute to the increased Meiyu-Baiu precipitation through local-scale atmospheric circulation rather than large-scale atmospheric circulation, although further research needs to clarify the impact of SST threshold on convection over SST front (e.g.Kim et al 2023).Our findings closely align with the mechanism identified by Liu et al (2021), who demonstrated that mesoscale SST forcing, associated with oceanic fronts and eddies along the Kuroshio Extension, can exert a remote influence on landfalling ARs and the resulting heavy precipitation along the west coast of North America.
Despite the observed increase in precipitation along the Kuroshio SST front, the 50-member mean of the ECHAM5 model with 0.75 • grids shows little change in precipitation over the ECS (figure 4(c)).This might be because the model lacks the ability to simulate the climatological precipitation (black contours) using 1 • grids-based SST (red contours) with a much smoother SST front than OISST with 0.25 • grids (figures 4(a) and (b)).Indeed, in comparison with 2 • grids-based SST, precipitation tends to increase along the SST front (figure 4(d)), consistent with the result of regional climate models with higher horizontal resolutions of 20 km (Sasaki et al 2012) and 25 km (Xu et al 2011).Note that even the High-Resolution Model Intercomparison Project is based on HadISST2 with 1 • grids equivalent (Haarsma et al 2016).Such higher-resolution models have demonstrated an enhanced capability to simulate atmospheric moisture transport from the ocean to land (e.g.Demory et al 2014, Liu et al 2021).However, modelling Meiyu-Baiu precipitation is complicated, as it depends not only on SST forcing but also on cloud microphysics and large-scale atmospheric circulation.In particular, the westerly jet, which is critical for Meiyu-Baiu precipitation, is well reproduced by the recent weakening in the ECHAM5 model (figure S7), supporting that the increased precipitation over the SST front is unlikely to be related to large-scale atmospheric circulation.

Conclusions
Our study aimed to understand the reasons behind the frequent occurrence of heavy rainfall events in western Japan during early July by analysing the decadal changes in short-duration events during the Meiyu-Baiu season.We found that the localscale circulation over the Kuroshio SST front in the ECS plays a substantial role in producing a quasistationary Rossby wave train in early July, which enhances poleward moisture transport from the western Pacific and contributes to heavy rainfall events over western Japan.The location and timing of changes in the quasi-stationary circulation are closely associated with enhanced deep convection over the Kuroshio SST front.Clarifying the changes in quasistationary circulation that contribute to AR-related precipitation is challenging, even for the stationary components, which have a highly uncertain and inconclusive response to climate change.Our study highlights the importance of focusing on the quasistationary component to better understand AR landfall location and duration.

Figure 1 .
Figure 1.Time series of 5-day running mean of (a) daily CPC precipitation and (b) IVT averaged over western Japan (see figure 2(d)) for 1982-2001 (blue) and 2002-2021 (red).Black line in (a) denotes the ratio of standard deviation on interannual variability in 2002-2021-1982-2001.Latitudes-time section of (c) eastward and (d) northward IVT difference between the averaged periods 2002-2021 and 1982-2001 over 130 • -135 • E. Dotted contours indicate the 95% significant level and solid contours indicate the ratio of eastward and northward IVT to total IVT for 2002-2021 (contours starting at 50% and increasing in intervals of 10) respectively.

Figure 3 .
Figure 3. (a) Longitude-time section of 5-day running mean of SST averaged over 27 • -29 • N in the ECS for 2002-2021 (solid red) and 1982-2001 (dashed red).Black contours denote mean zonal SST gradient ( • C per degree longitude) for 2002-2021 (solid: positive, dotted: negative) and shadings indicate its difference between the two periods.As in (a), but for (b) surface latent heat flux, (c) scalar wind speed and wind vector at 10 m, and (d) upward vertical p velocity at 500 hPa.Hatching and cross hatching in (b)-(d) denotes the 90% and 95% confidence level, respectively.Solid contours denote mean latent heat flux 80 W m −2 in (b), mean scalar wind speed (contour interval is 0.5 m s −1 ) in (c), and mean vertical p velocity (contour interval is 0.02 Pa s −1 ) in (d) for 2002-2021.

Figure 4 .
Figure 4. Precipitation difference between the averaged periods 2002-2021 and 1982-2001 for June-July means in (a) GPCP and (b) convective precipitation based on ERA5.(c) As in (a), but for precipitation based on ECHAM5 simulations forced with Hurrell SST in the periods of 2002-2020 and 1982-2001.(d) As in (c), but for precipitation difference between ECHAM5 simulations forced with Hurrell SST and ERSSTv5 for 2002-2020.Black contours indicate mean precipitation (contour interval is 2 mm d −1 ) and red contours indicate mean SST 27 and 28 • C (OISST, 0.25 • girds) for 2002-2020 in (a) and (b), and Hurrell SST (1 • girds) and ERSSTv5 (2 • girds) used in the simulations for 2002-2020 in (c) and (d).
regional climate model experiments (Xu et al 2011, Sasaki et al 2012) that demonstrate the Kuroshio SST front anchors the narrow band of convective precipitation and ascent motion by enhancing upward latent heat flux.To clarify the impact of the SST front on deep convection, we follow previous studies (Xu et al 2011, Sasaki et al 2012) attributing it to changes in temperature and moisture in the marine atmospheric boundary layer.From 6 to 10 July, ERA5based convective precipitation, convective instability, and surface warming and moistening are all enhanced over the warm SST tongue (figure S5), confirming the modelling results (Xu et al 2011, Sasaki et al 2012).These lower-atmospheric changes suggest that the enhanced upward surface latent heat flux along the SST front can contribute to deep convection.However, the atmospheric changes over the SST front only last for about a week in early July.The SST front undergoes warming and sharpening from June to early July in the recent decades (figure 3(a)).During 1-5 July after the northward migration of the Meiyu-Baiu rainband, however, there is an increase in both the SST and surface latent heat flux, especially in the SST warm tongue of 28 • C (figure S6), which corresponds to the threshold for deep convection (e.g.Graham and Barnett 1987, Fu et al 1994), likely