An underlying factor of increasing early winter precipitation in the Hokuriku region of Japan in recent decades

Using a reanalysis dataset and large‐ensemble simulation results, this study examines a possible factor of increasing trend in early winter precipitation in recent decades in the Hokuriku region of Japan. Monthly precipitation in December has a significant increasing trend after the early 1990s, which is different from those in January and February. The increasing precipitation in December is related to that in the sea surface upward latent heat flux due to intensified winter monsoon circulation and warming sea surface temperatures (SSTs) over the Sea of Japan. December averaged SSTs show a trend pattern in recent decades that is similar to the negative phase of the interdecadal Pacific oscillation (IPO), accompanied by positive trends from the eastern Indian Ocean to the western tropical Pacific. The enhanced trend of convection over the Bay of Bengal is seen; suggesting a combined effect of climatologically high SSTs and IPO‐related warmed SSTs over the region. Trends in recent decades of an upper‐level wavy pattern from South Asia to near Japan along the subtropical jet associated with enhanced convection near the Bay of Bengal and the related pressure drop from Japan to the north are seen, which contribute to intensified winter monsoon circulation.


| INTRODUCTION
Winter monsoon near Japan is characterized by a northwesterly wind.The northwesterly wind, which is originally cold and dry, significantly absorbs sensible and latent heats over the Sea of Japan, and eventually causes large amounts of precipitation on the Sea of Japan side of the country.
In recent winters, serious traffic disruptions due to unprecedentedly heavy snowfalls have occurred frequently, as exemplified by the case of December 2020 in and around the Hokuriku region of Japan, which locates on the Sea of Japan side of the country.Many previous studies (e.g., Honda et al., 2009;Outten & Esau, 2012;Tang et al., 2013) indicated influences of reduced Arctic sea-ice minima on anomalous coolness of Eurasia and the resultant intensified winter monsoon circulation over East Asia, reminiscent of the relationship with the heavy snowfalls the in recent winters.Kawase et al. (2022) showed the impacts of air temperature and sea surface temperature (SST) changes due to historical global warming since the pre-industrial period on a heavy snowfall in mid December of 2020 based on their sensitivity experiments.These global warming impacts are expected to be common in cases of heavy snowfall in the entire winter season, whereas it is noticeable that unprecedented heavy snowfall is particularly observed in early winter.Using observational and reanalysis datasets, Yasunaga and Tomochika (2017) suggested that the enhanced winter monsoon circulation during recent decades primarily causes the precipitation increase in early winter.Although they also examined the statistical relationship between monthly precipitation and SST, statistical analysis based on a longer period is desirable to clarify the relationship to warming SST over the Sea of Japan.Although Yasunaga et al. (2019) further suggests a close relationship between early winter precipitation over the Sea of Japan side and tropical precipitation over the eastern Indian Ocean, the underlying factor of unique characteristics in early winter and its significance has not been elucidated.Using observational dataset, Takahashi (2021) examined long-term trends in winter snowfall and precipitation amount on the Sea of Japan side and indicated the decreasing trends in the Hokuriku region.The trends not only for winter but also each month in winter should be examined because the oceanographic and atmospheric climatology change interseason.In fact, Sakai et al. (2010), Zhao et al. (2022) and Ma et al. (2022) showed different influences of El Niño Southern Oscillation (ENSO) on winter monsoon circulation over East Asia between in the early and late season, motivating us to examine monthto-month differences in long-term trends of precipitation over the Sea of Japan side.
Based on this background, this study examines a factor of the trends in recent decades of enhanced winter monsoon circulation related to the increasing trend in precipitation in early winter in the Hokuriku region of Japan by using a reanalysis dataset and large ensemble simulation results.This line of approach is important for understanding the mechanism of the characteristic winter climate near Japan and reducing the socioeconomic impact of the unprecedented heavy snowfall.

| DATA AND METHOD
Regional averaged monthly precipitation in the Hokuriku region, which covers 9 observational stations (see red circles in Figure 1) in Niigata, Toyama, Ishikawa, and Fukui prefectures of Japan for December-January-February (DJF) from 1946 to 2022, was used to describe interannual timeseries of precipitation in the Hokuriku region of Japan.Herein, precipitation ratios refer to ratios to the climatological average from 1991 to 2020.
To analyze the atmospheric circulation associated with the precipitation trend of recent decades, monthly averaged data of the Japanese 55-year reanalysis (JRA-55; Kobayashi et al., 2015)  used.To infer convective activities in the tropics, we used monthly averaged data of outgoing longwave radiation (OLR) in the DJF period from 1979 to 2022 provided by the National Oceanic and Atmospheric Administration.To analyze oceanographic conditions, monthly averaged data of the Centennial In Situ Observation-Based Estimates of the Variability of SSTs and Marine Meteorological Variables (COBE-SST2; Hirahara et al., 2014) in December from 1958 to 2022 were also used.To examine whether correlation and regression coefficients are statistically significant or not, we performed a two-sided Student's t-test with degree of freedom of N -2.Hereafter N represents a sample size (i.e., number of years).Furthermore, to evaluate statistical significance for linear trend of interannual timeseries, we performed tests according to the Mann-Kendall rank statistics (Kendall, 1938; hereafter referred to as the M-K test), which is a representative non-parametric test applicable to timeseries data out of the normal distribution.
To assess the relationship of enhanced winter monsoon circulation near Japan with the long-term trend and interdecadal variability, large-ensemble simulation results of the Database for Policy Decision-Making for Future Climate Change (d4PDF) (Mizuta et al., 2017) are used.The d4PDF is performed by the Meteorological Research Institute Atmospheric General Circulation Model version 3.2 (MRI-AGCM3.2) (Mizuta et al., 2012), having 100 ensemble members from 1951 to 2010 for the historical simulations (HIST) and the non-warming simulations (NonW).The HIST was conducted by using the observed SST and sea ice based on COBE-SST2 and the historical external forcing of greenhouse gases, ozone, and aerosols.The ensemble experiments were conducted with random initial and SST perturbations.NonW removes the warming trend in the observed SST with sea ice adjusted to be consistent with the given SST and the pre-industrial level of external forcing.The d4PDF is described in detail in Mizuta et al. (2017).

| TREND OVER THE SEA OF JAPAN DURING RECENT DECADES
Figure 2 describes interannual timeseries of the monthly precipitation ratio in the Hokuriku region of Japan for DJF.The long-term variability of precipitation presents different characteristics in the 3 months.Although there are large interannual variabilities, the precipitation in December shows a decreasing trend before the late 1980s, while the trend is increasing again after the early 1990s (Figure 2a).This is consistent with the interdecadal variability of wintertime heavy snowfall cases, as indicated by Yamazaki et al. (2019).In other months, there are no precipitation trends similar to that in December, and the long-term precipitation trends in January and February are decreasing (Figure 2b,c), particularly in February.Aside from the late winter features (Figure 2c), it is noteworthy that the precipitation in early winter has shown an increasing trend in recent decades (Figure 2a).Hereafter, this study focuses on characteristics of December.
Figure 3 represents interannual timeseries of surface latent heat flux, northwesterly component of 10 m wind, and SSTs in December, and SSTs in November averaged over the southwestern part in the Sea of Japan (130-135 E, 35-40 N; red dashed rectangle in Figure 1; hereafter referred to simply as the Sea of Japan) from 1958 to 2022.The region is immediately upstream of the Hokuriku region in the climatological winter monsoon circulation (not shown) and shows local maxima of interannual variability in latent heat flux, which is described later, on the Sea of Japan (shading in Figure 1).The upward latent heat flux significantly increased after the early 1990s (Figure 3a; with a confidence level of 99% based on the M-K test), showing a significant relationship with the significantly enhanced trend of northwesterly winter monsoon circulation (Figure 3b; with a confidence level of 95% based on the M-K test) with a high correlation coefficient of +0.82 and that of +0.80 even if the linear trend is removed during the period from 1991 to 2022 (N = 32), which is consistent with results of Yasunaga and Tomochika (2017).Hereafter this study focuses on the recent trend in December when the significant trends are seen, and the strength of the monsoon circulation was measured using a northwesterly wind of 10 m (Figure 3b).Note that the northwesterly wind over the Sea of Japan and the monsoon index (Hanawa et al., 1989), which is defined as the difference in sea level pressure between two grid points near Irkutsk, Russia (105 E, 52.5 N) and near Nemuro (145 E, 43.75 N), have a significant relationship with the correlation coefficient of +0.85 during the period from 1958 to 2022 (N = 65), indicating that the northwesterly wind and a pressure drop over the northeastern part of Japan are proper indices of the winter monsoon circulation intensity.The enhanced trend of monsoon circulation is closely related to the increasing precipitation (e.g., Arai & Yasunaga, 2019;Yamashita et al., 2012).
SSTs over the Sea of Japan also show significant warming trends in December and November (Figure 3c,d) with confidence levels of 95% and 99% based on the M-K tests, respectively.During the period from 1991 to 2022 (N = 32), the latent heat flux has no correlation with the SST in December with the correlation coefficient of +0.13 (Figure 3a,c) because of the SST cooling effect due to the intensified winter monsoon circulation.The low correlation between monthly-averaged latent heat flux and SST is consistent with Takahashi and Idenaga (2013), who showed a time-dependent impact of SST over the Sea of Japan on precipitation on the Sea of Japan side.Meanwhile, the latent heat flux in December indicates a significant relationship with the SST in November before the winter monsoon circulation intensifies, with a correlation coefficient of +0.59.This indicates that winter monsoon circulation in the condition of warming SST taken over from autumn contributes to the increasing latent heat flux in early winter.Hereafter, this study focuses on mechanisms of the trend in recent decades of enhanced winter monsoon circulation over the Sea of Japan as a primary factor of increasing precipitation.

| LARGE-SCALE CIRCULATION ASSOCIATED WITH THE ENHANCED WINTER MONSOON
This section examines the large-scale circulation associated with the enhanced trend in recent decades of winter  patterns including those in the mid-latitude Pacific are similar to the negative phase of the Interdecadal Pacific Oscillation (IPO; e.g., Folland et al., 1999;Henley et al., 2015;Power et al., 1999), corresponding to the observed persistent negative IPO phase since the 2000s.In association with the warming trend of the SST, a trend of enhanced convection is centered in and around the Bay of Bengal (Figure 4b), which induces significantly negative trends in sea level pressure, as the Matsuno-Gill response, in and around the tropical Indian Ocean (Figure 4d).The enhanced trend of convection near the Bay of Bengal further promotes a wavy pattern of the geopotential height trend from South Asia to the seas east of Japan along the subtropical jet in the upper troposphere (Figure 4c).The wavy height pattern extends from further upstream and can be traced back to near the North Atlantic, where warming SST trends are significantly seen (Figure 4a).These features are consistent with previous studies (e.g., Chen et al., 2016Chen et al., , 2020;;Song et al., 2022;Wu & Chen, 2020;Zhao et al., 2019), who indicated influences of the anomalous SST in the North Atlantic to East Asian climate via teleconnection in the upper troposphere.However, the upstream pattern in Figure 4c may be mainly due to internal atmospheric variability as described later based on Figures 5 and 6.Negative geopotential height trends to the west of Japan, which are accompanied by an upper-level wavy pattern (Figure 4c), are consistent with a significant pressure drop from Japan to the north (Figure 4d).Although the negative height trends are small because of dominant positive trends in the surrounding region, the negative trends are larger and significant by removing the global average of heights (not shown).This indicates that the upper-level trough leads to a quasistationary lower-level pressure minimum in its eastward direction and enhanced winter monsoon circulation over the Sea of Japan triggered by extratropical cyclone development.In the condition of strong baroclinicity near Japan, stationary waves have a westward tilted vertical structure, consistent with Maeda et al. (2021), who analyzed the modulation of planetary waves associated with the Eurasian pattern and showed the westward tilting wave structure.The pressure drop near the Sea of Okhotsk far from Japan (Figure 4d) may be associated with warming SST trend over the region (Figure 4a).The relationship between the pressure drop and enhanced winter monsoon circulation over the Sea of Japan is supported by the aforementioned relationship between 10-m northwesterly wind over the region and the monsoon index (Hanawa et al., 1989).The 200-hPa geopotential height regressed onto precipitation ratio in the Hokuriku region shows negative anomalies from Japan to its west (not shown), consistent with the corresponding relationship between the upper-level enhanced trough and the increasing precipitation trend in the Hokuriku region.The aforementioned remote impact from tropical convection is consistent with Tsukijihara and Kawamura (2022), who indicated a relationship of northward moving extratropical cyclone activity along the Kuroshio current with enhanced convection over the Bay of Bengal and the related downstream Rossby wave propagation.
Positive trends of OLR in the Northern Hemisphere high-latitudes (Figure 4b) correspond to those of geopotential height (Figure 4c), suggesting a reflection of the Arctic warming.The trends in recent decades in the Arctic warming and intensified winter monsoon circulation are consistent with previous studies (e.g., Honda et al., 2009;Outten & Esau, 2012;Tang et al., 2013).Although the enhanced trend of winter monsoon circulation is generally related with the SST cooling over the Sea of Japan (Takahashi & Idenaga, 2013), there is a clear warming SST trend over the sea (Figure 4a).The possible mechanism of the warming SST trend has been proposed by previous studies as an increasing trend in the Tsushima current flow (Kida et al., 2021), the Kuroshio axis variability south of Japan, and the associated coastally trapped waves into the Japan Sea (Taguchi et al., 2022).
Although the aforementioned results suggest a link between the enhanced early winter monsoon circulation over the Sea of Japan in recent decades and interdecadal SST variability, there is a concern that the trend in recent decades and the significance may be disturbed by large internal atmospheric variability that is not related to the interdecadal variability and historical global warming.To address this concern, Figures 5 and 6 show the linear trend fields of SST, precipitation, 200 hPa geopotential height, and sea level pressure in December from 1991 to 2010 derived from the 100-member ensemble mean of HIST and NonW in d4PDF.The large ensemble mean fields show atmospheric circulation without the internal atmospheric variabilities.Note that the period from which the linear trend is derived is different from that in the reanalysis because the available period of d4PDF is until 2010.Both simulation results indicate an SST trend pattern in recent decades that is similar to the negative phase of the IPO (Figures 5a and 6a) and the associated enhancement of convection centered over the Bay of Bengal (Figures 5b and 6b).Further, the geopotential height at 200 hPa shows an upper-tropospheric wavy pattern from South Asia to the seas east of Japan (Figures 5c and  6c).It is noteworthy that the wavy pattern upstream of South Asia is not seen, indicating an influence of internal atmospheric variability on the upstream wavy pattern in the reanalysis (Figure 4c).The wavy patterns accompany geopotential heights relatively decreasing and negative trends for HIST and NonW simulations (Figures 5c and  6c, respectively), contributing to pressure drops near Japan to the north (Figures 5d and 6d).As in the reanalysis, the negative trends for HIST and NonW simulations are clearer by removing the global average of heights (not shown).A difference in positions of negative anomaly centers for between analysis (Figure 4c) and d4PDF (Figures 5c and 6c) may be associated with upstream wavy pattern over Eurasia seen in the analysis.The difference further suggests that wavy pattern upstream of South Asia, which is clearly seen in Figure 4c, is primarily attributable to an internal atmospheric variability.Correlation coefficients between 10-m northwesterly wind over the Sea of Japan and Hanawa et al. (1989)'s monsoon index during a period from 1951 to 2010 (N = 60) for ensemble mean in HIST and NonW simulations are +0.34 and +0.23 with confidence levels of 95% and 90%, respectively.These characteristics of trends in recent decades are consistent with those for the reanalysis (Figure 4), which indicates the existence of the enhanced winter monsoon circulation in December without the internal atmospheric variability.The trend characteristics are generally in common with both HIST and NonW simulations, demonstrating that trends of the recent decades can be explained not by historical global warming but by the interdecadal variability, as seen as the negative IPO-like SST trend pattern (Figures 4a, 5a  SST trends in the equatorial Pacific (Figures 4a,5a,6a) also show the La Niña-like zonal pattern.Previous studies (Ma et al. 2022;Sakai et al., 2010;Zhao et al. 2022) showed ENSO has stronger impacts on winter monsoon circulation over East Asia in early winter compared to late winter.Although SST pattern associated with the La Niña is not independent with that with the IPO, the aforementioned results suggest that the La Niña-like SST trends may also be associated with the enhanced trend in recent decades of winter monsoon circulation over the Sea of Japan.

| POSSIBLE FACTOR OF ENHANCED CONVECTION IN EARLY WINTER
Although the trend of tropical SSTs in December of recent decades described in Section 4 was also commonly seen in January and February, the trends of enhanced convection centered over the Bay of Bengal and the related upper-tropospheric wavy pattern and pressure drops near Japan were not seen in the months (not shown).This suggests that whether enhanced convection centered over the Bay of Bengal is seen is an underlying factor of the intraseasonal difference in the trend in recent decades of enhanced winter monsoon circulation over the Sea of Japan.
To assess the possible relationship between the enhanced convection centered over the Bay of Bengal seen only in December and the high SST over the region accompanied by the seasonal progression, a scatter diagram of monthly SST and OLR averaged over the Bay of Bengal (80-120 E, 5-15 N) for each month in DJF from 1979 to 2022 is shown in Figure 7a.SSTs in December (red circles) are climatologically higher than those in January and February (blue and green circles) in association with the seasonal progression of SST lasting from autumn.Consistent with the higher SST in December, OLRs in the month are lower than those in the other 2 months, which indicates a climatologically favorable condition for enhanced convection near the Bay of Bengal in December.Figure 7b-d show 200 hPa geopotential height fields regressed onto the OLR averaged over the Bay of Bengal (80-120 E, 5-15 N) for reanalysis and precipitation over the region for HIST and NonW simulations.All of the regressed heights clearly exhibit uppertropospheric wavy patterns from South Asia to the seas east of Japan along the subtropical jet, causing a favorable condition for pressure drops from near Japan to the north, also consistent with the result of Tsukijihara and Kawamura (2022).This result indicates that climatologically higher SSTs and recent anomalously high SSTs in association with the interdecadal variability in December are expected to make a favorable condition for enhanced convection near the Bay of Bengal, which links with the enhanced winter monsoon circulation over the Sea of Japan through the uppertropospheric teleconnection.The increasing trends of precipitation in the Hokuriku region of Japan in early winter after the early 1990s are, therefore, suggested to be promoted by a combined effect of the climatologically high SSTs and IPO-related warmed SSTs near the Bay of Bengal.

| CONCLUSION
This study examined a mechanism of increasing trend of recent decades in December precipitation in the Hokuriku region of Japan using JRA-55 reanalysis dataset and d4PDF large-ensemble simulation results.Monthly precipitation in December in the Hokuriku region of Japan showed a significant increasing trend after the early 1990s, but a similar precipitation change was not seen in January and February.The increasing precipitation trend was associated primarily with increases in sea surface upward latent heat flux over the Sea of Japan due to enhanced winter monsoon circulation, and secondly with a warming SST trend over the region.In recent decades, December's averaged tropical SST showed a trend pattern similar to the negative phase of the IPO.The enhanced trend of convection was seen near the Bay of Bengal in association with warming SST trends over the region accompanied by the IPO-like SST trends.The enhanced trends of tropical convection further excited trends of an upper-tropospheric wavy pattern from South Asia to near Japan along the subtropical jet, and the related pressure drop from Japan to the north, corresponding to enhanced winter monsoon circulation.The enhanced trends of convection near the Bay of Bengal, a unique feature for December, were suggested to be promoted by the combined effect of the climatologically high SSTs lasting from autumn and negative IPO-related warming SST trends over the region.
Although this study focused on the recent trends of enhanced winter monsoon circulation associated with interdecadal variability of SST and atmospheric circulation, the trend in recent decades of warming SST over the Sea of Japan in late autumn and its impacts on the early winter monsoon will be an additional perspective to be investigated.Further investigation is needed to elucidate the detailed process of the SST warming trend over the Sea of Japan in recent decades based on previous studies (Kida et al., 2021;Taguchi et al., 2022).
in DJF from 1958 to 2022 were F I G U R E 1 Observation stations in the Hokuriku region of Japan (red circles) and a defined region to calculate area averages over the Sea of Japan (130-135 E, 35-40 N, red dashed rectangle).Shading indicates the standard deviation of the monthly averaged sea surface latent heat flux (unit: W m À2 ) in December during a period from 1991 to 2022.The figure on the right shows an enlarged map in and around the Hokuriku region and the observation stations (red circles).

F
I G U R E 2 Interannual timeseries of monthly precipitation ratios in the Hokuriku region of Japan (gray bars) for (a) December, (b) January, and (c) February during a period from 1946 to 2022.Blue lines denote the 11-year running mean.monsoon circulation over the Sea of Japan (Figures 3b).
Figure 4 represents linear trend fields of SST, OLR, 200 hPa geopotential height and sea level pressure in December from 1991 to 2022 derived from the reanalysis.In recent decades, the SST shows cooling trend in the central to eastern equatorial Pacific and a significant warming trend from the eastern Indian Ocean to the western tropical Pacific (Figure4a).These SST trend F I G U R E 3 Interannual timeseries of (a) the surface latent heat flux (unit: W m À2 ), (b) northwesterly component of 10 m wind (unit: m s À1 ), (c) SSTs (unit: C) for December averaged over the Sea of Japan (130-135 E, 35-40 N) from 1958 to 2022.(d) The same as in (c), but for November.Red lines denote linear regression during a period from 1991 to 2022, which are statistically significant at more than 95% confidence level based on the Mann-Kendall test.

F
I G U R E 4 Linear trend fields of (a) SST (unit: C/10 year), (b) OLR (unit: W m À2 /10 year), (c) 200 hPa geopotential height (unit: m/10 year), and (d) sea level pressure (unit: hPa/10 year) in December from 1991 to 2022 based on reanalysis data.Single and double hatches denote the linear trend of 90% and 95% confidence levels based on the Mann-Kendall test, respectively.

F
I G U R E 5 Linear trend fields of (a) SST (unit: C/10 year), (b) precipitation (unit: mm d À1 /10 year), (c) 200 hPa geopotential height (unit: m/10 year), and (d) sea level pressure (unit: hPa/10 year) in December from 1991 to 2010 based on the ensemble mean of HIST simulations in d4PDF.Single and double hatches denote the linear trend of 90% and 95% confidence levels based on the Mann-Kendall test, respectively.
, and 6a).Note that trends in the HIST simulation results are clearly positive in a wide area in association with the historical global warming in recent decades.

F
I G U R E 6 The same as in Figure 5, but for the ensemble mean of NonW simulations in d4PDF.F I G U R E 7 (a) A scatter diagram of the monthly SST (unit: C) and OLR (unit: W m À2 ) averaged over the Bay of Bengal (80-120 E, 5-15 N) for each month in DJF from 1979 to 2022.Red, blue, and green circles denote December, January, and February, respectively.(b-d) show 200 hPa geopotential height (unit: m) regressed onto (b) OLR (unit: W m À2 ) averaged over the Bay of Bengal based on reanalysis data and precipitation (mm d À1 ) over the region based on ensemble members of (c) HIST and (d) NonW simulations.Green vectors in (b-d) denote 200 hPa wave activity fluxes (Takaya & Nakamura, 2001) derived from the regressed geopotential height and the climatological average (from 1991 to 2020) of zonal and meridional winds.Single and double hatches in (b-d) denote the regression of 90% and 95% confidence levels, respectively.