SOLA, 2011, Vol. 7, 005−008, doi:10.2151/sola.2011-002

Long-Term Changes of Seasonal Progress in Baiu Rainfall
Using 109 Years (1901−2009) Daily Station Data
Hirokazu Endo
Meteorological Research Institute, Tsukuba, Japan

Abstract
This study investigates long-term changes in Baiu rainfall in
Eastern and Western Japan using daily precipitation records at 37
stations for the years 1901 through 2009, focusing on its seasonal
progress. This period is much longer than various data analyzed
in previous observational studies. In the early Baiu season (early
to mid June), significant long-term decreasing trends are observedin Eastern and Western Japan, accompanying large inter-decadal
variation in the former half of the 20th century. In the late Baiu
season (mid to late July), in contrast, significant long-term
increasing trends are observed on the Japan Sea side of Easternand Western Japan. No significant trends are recognized either inthe mid Baiu season (late June to early July) or in the entire Baiu
season (June to July) over all regions. It is interesting to note thatthe observed tendency of delayed Baiu withdrawal in the last 109
years, when global warming has been in progress, is similar to its
future changes projected by climate models.

1. Introduction
The rainy season in early summer over East Asia, which is
generally called the Baiu season in Japan, is brought by northwardmigration of a narrow rain band with a stationary frontal zone
from May to July. In Japan, a clear climatological rainfall peak isobserved in the Baiu season. By the end of July, the frontal zone
reaches northern Japan and becomes unclear, while concurrently
the midsummer season starts over many parts of Japan. The transition is characterized by an increase of sunshine duration and temperature and a decrease of precipitation (Ninomiya and Murakami
1987; Matsumoto 2002).
According to observational studies, the withdrawal of the Baiu
season over Japan has been delayed and climate in the midsummer
season following the Baiu season has been unstable in the latter
half of the 20th century. Yamakawa (1988) investigated prevailingsurface pressure patterns using the daily surface weather charts forthe period 1941−1985 and found that the withdrawal of the Baiu
season has become later after the late 1970s. Sato and Takahashi(2001) compared sunshine durations from 1 to 10 August between
two periods of 1959−1968 and 1986−1995 over Japan and foundthat sunshine duration for the later period was significantly lesscompared with that for the earlier period. They concluded that thechange is caused by the slower northward movement of the Baiu
frontal zone than before. Inoue and Matsumoto (2003) investigated sunshine duration for the periods 1951−2000 over Japan andfound that the withdrawal of the Baiu season has been delayedsince the 1980s in central and western Japan. Inoue and Matsumoto (2006) compared the two periods of 1961−1979 and 1980−
1999 regarding precipitation over East Asia, as well as sunshine
duration and temperature over Japan. They found that northward
movement of the Meiyu-Baiu frontal zone becomes slower in thelater period and that midsummer (16 July to 15 August) with cooltemperatures and little sunshine has frequently appeared in central
Japan since the 1980s.
Corresponding author: Hirokazu Endo, Climate Research Department,
Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki,305-0052, Japan. E-mail: hendo@mri-jma.go.jp. ©2011, the Meteorological Society of Japan.

The data analyzed in the previous observational studies wererestricted within the latter half of the 20th century. This is of
insufficient length in order to discuss whether these observed
trends are forced by external influences such as anthropogenic
activities because inter-decadal natural climate variability cannot
be discounted in a 50-year-or-shorter record (Wang et al. 2007).
In addition, the trends indicated by the previous studies are notnecessarily tested statistically enough. Therefore, it is necessary
to investigate long-term trends of the seasonal delay using much
longer observed data and using statistical tests.
Historical daily precipitation data at all stations of the Japan
Meteorological Agency (JMA) were recently digitalized, and JMA
conducted a quality check of the data at 51 stations where theuniformity of observation data has been maintained since 1901
(Fujibe et al. 2006). Using this precipitation dataset, which has
much longer temporal coverage than the various data analyzed
in previous studies, we studied long-term changes of the Baiu
rainfall during the last 109 years in Japan, focusing on its seasonal
progress. Stations in the Tohoku region (northernmost part ofthe main island of Japan), where the Baiu season exists, are notincluded in this study because the Baiu season over this region ismarked by a minimum sunshine duration, not by a rainfall peak
(Ninomiya and Murakami 1987).

2. Data
The daily precipitation data at 37 selected JMA stations, which
are in Eastern Japan and Western Japan, were analyzed for theperiod 1901 to 2009. The stations were divided into four regionsof the Japan Sea side of Eastern Japan (EJ), the Pacific side of
Eastern Japan (EP), the Japan Sea side of Western Japan (WJ) and
the Pacific side of Western Japan (WP), as shown in Fig. 1. See
the details of the divisions in JMA (2005a). We also defined combined regions of the Japan Sea side of Eastern and Western Japan(EJ+WJ), the Pacific side of Eastern and Western Japan (EP+WP)
and Eastern and Western Japan (E+W). The total number of stations included in these regions is 3, 15, 8, 11, 11, 26 and 37 for EJ,
EP, WJ, WP, EJ+WJ, EP+WP and E+W, respectively. Details of

Fig. 1. Location of stations analyzed in this study. Regional divisions are:Japan Sea side of Eastern Japan (EJ) (red circle), Pacific side of EasternJapan (EP) (blue circle), Japan Sea side of Western Japan (WJ) (red dot),
and Pacific side of Western Japan (WP) (blue dot). Shade indicates topography (unit: m).

Endo, Long-Term Changes of Seasonal Progress of Baiu Rainfall in 109 Years (1901−2009) Daily Datathe stations analyzed are listed in supplement Table S1.
In preprocessing, historical daily data is averaged for 10-day
or 11-day periods at each station (e.g., 1 to 10 (early) July, 11
to 20 (mid) July and 21 to 31 (late) July in 2002). The averaged
data was calculated only when more than 80% of daily data hadnormal or quasi-normal flags (JMA 2005b) in each 10-day or
11-day period. There are hardly any missing records in the dataset,as also shown in Table 1 in Fujibe et al. (2006). For regional averages, normalized data at each station (i.e., the ratio relative to theaverages over the base period (1901−1930)) are used in order toaverage with the same weight on each station data, except for Fig.
2.

3. Results
Figure 2 shows 10-day mean precipitation from late May
to mid August in the first 30 years (1901−1930) and in the last
30 years (1980−2009) of the analyzed period. Much rainfall is observed in the period from June to July over all regions, especiallyover Western Japan (WJ and WP). The rainy period in WesternJapan slightly precedes that in Eastern Japan (EP and WP) because
the rainy season is brought by the northward migration of a rainband over East Asia (Ninomiya and Murakami 1987). Rainfall
peaks are observed in late June for EP, WJ and WP and in earlyJuly for EJ in the first period.
We can see changes in the climatological rainfall from the first
period to the last period. In many regions, precipitation decreases
in early to mid June, while precipitation increases in mid to lateJuly. It seems that the rainy season somewhat shifts later from thefirst period to the last period, especially over EJ and WJ. The time
for rainfall peaks did not change except for EJ, where the peaktiming shifted to mid July.
Figure 3 shows regional mean long-term trends from 1901
to 2009 by 10-day intervals. The trends are derived by applying
the linear regression to the regional averages of the normalized
precipitation at each station. Precipitation has decreasing trends
for early to mid June (i.e., before the Baiu rainfall peak) in many
regions. Decreasing trends at a rate of approximately 25% per
century are observed in all regions in mid June. On the other hand,
precipitation has increasing trends for mid to late July (i.e., after
the Baiu rainfall peak) in many regions. Remarkable increasing
trends at a rate of over 50% per century are observed in WJ in mid

to late July and in EJ in mid July. It is also noted that precipitation
decreases in early August in all regions.
Long-term trends indicate different features depending on
the stage of the Baiu season and on the region (Fig. 2 and Fig. 3).
Next, we analyze long-term trends and variations in four periods
related to the stage, consisting of the early Baiu season (Period 1:
1 to 20 June), the mid Baiu season (Period 2: 21 June to 10 July),
the late Baiu season (Period 3: 11 to 31 July) and the entire Baiu
season (Period 4: 1 June to 31 July). To estimate the robustnessof the long-term trend, two types of statistical significance tests
are applied. One is based on the linear regression model using theleast-square (LS) method and the other is based on the nonparametric Mann-Kendall (MK) rank statistic (Kendall 1938).
Table 1 shows long-term linear trends of precipitation for
1901−2009 in each region and in each period and their statisticalconfidence levels using the MK method and the LS method. In
Period 1, decreasing trends are found in all regions with ranges
from −17% (WP) to −23% (WJ). The trend in E+W (−20%) issignificant at the 90% confidence level in both tests, although thetrends in other local regions do not reach the 90% level in either
test. In Period 3, on the other hand, quite large increasing trends
are found on the Japan Sea side (EJ (+54%), WJ (+54%) andEJ+WJ (+54%)) with significance at the 90% or 95% level, while
increasing trends on the Pacific side (EP (+3%), WP (+8%) and
EP+WP (+5%)) are small. In particular, confidence levels of thetrends in EJ+WJ reach the 95% level in both tests. Interestingly,
trends in Period 2 and Period 4 are not clear, although those in
the regions on the Japan Sea side are relatively large but are notstatistically significant. It should be noted again that statisticallysignificant long-term trends are observed in the early and latestages of the Baiu season, but not in the middle stage of the Baiu

Fig. 3. Linear trends (% per 100 years) of regional mean precipitation
ratios for 1901 through 2009.
Table 1. Linear trend (% per 100 years) of regional mean precipitation
ratio for 1901 through 2009 and its confidence level determined using theMann-Kendall (MK) method and the least-square (LS) method. One (two)asterisk denotes statistical significance at the 90% (95%) confidence level.
Period 1: 1 to 20 June. Period 2: 21 June to 10 July. Period 3: 11 to 31
July. Period 4: 1 June to 31 July
RegionPeriod 1

Period 2

Period 3

Period 4

Trend MK LS Trend MK LS Trend MK LS Trend MK LS

Fig. 2. Regionally averaged 10-day mean precipitation (mm day−1) fromlate May to mid August in the first 30 years (1901−1930: white bars) andin the last 30 years (1980−2009: black bars). (a) EJ, (b) EP, (c) WJ, (d)WP. Key: I = 1 to 10, II = 11 to 20, III = 21 to 30 (or 31) days of eachmonth.

EJ
EP
WJ
WP

−22
−19
−23
−17

8
−2
16
−5

54
3
54
8

*

*

*

**

EJ+WJ −23
EP+WP −18
E+W
−20

14
−3
2

54
 5
20

** **

*

*

13
−7
12
−6
12
−6
−1

SOLA, 2011, Vol. 7, 005−008, doi:10.2151/sola.2011-002
nor the entire Baiu season.
Figure 4 shows the time series of regional mean precipitation
ratio relative to the 1901−1930 average in Period 1, Period 2 andPeriod 3. In this figure, results of EJ+WJ and EP+WP are given
because features in the long-term trends are similar between EJ
and WJ and between EP and WP (Fig. 3 and Table 1). In Period
1 (Fig. 4a), precipitation has been decreasing with large year-toyear variations in EJ+WJ and EP+WP. The 11-year running mean
precipitation in both combined regions fluctuates in a similar way.
Precipitation has clearly decreasing trends in the latter half of theanalyzed period, while precipitation exhibits large inter-decadal
variability in the former half of the analyzed period. In Period

3 (Fig. 4c), precipitation has been increasing with large year-toyear variations in EJ+WJ. The 11-year running mean precipitation
increases abruptly in EJ+WJ around the 1940s and a large difference from that in EP+WP is observed after the 1960s. It is also
noted that heavy rainfall years (1957, 1971, 1972, 1982, 1983,
1987, 2003 and 2006) exceeding 250% are observed only in thelatter half of the analyzed period in EJ+WJ. In other words, interannual variability of rainfall has been greatly increasing in the late
Baiu season in EJ+WJ. Standard deviations of the variability inthe region in the late Baiu season are 66%, 77%, and 96% for theperiod 1901−1940, 1941−1980 and 1981−2009, respectively. In
Period 2 (Fig. 4b), long-term trends are not clear, as also shownin Table 1, but with relatively large decadal variability showing
decreased rainfall around the 1940s and increased rainfall aroundthe 1950s and the 1960s.
Figure 5 shows spatial distribution of the trends and its statistical significance based on the MK test. In Period 1 (Fig. 5a),decreasing trends are observed over almost all of the stations.
Stations with significant decreasing trends are distributed overcentral Japan and the Japan Sea side of western Japan. Twelve stations have significant decreasing trends at the 90% level. In Period
3 (Fig. 5b), both decreasing and increasing trends are observed
although stations with increasing trends are dominant. Stations
with large increasing trends are mainly distributed on the Japan
Sea side of Japan. Six stations have significant increasing trends
at the 90% level. In Period 2 (not shown), no station has trends at
the 90% level. In Period 4 (not shown), two stations (Sakai and
Hamada) have increasing trends, while one station (Tsu) has adecreasing trend at the 90% level.

4. Summary and discussions
We investigated long-term changes in Baiu rainfall in Easternand Western Japan using daily precipitation records at 37 stations
for the period 1901 to 2009, focusing on its seasonal progress. In
the early Baiu season (early to mid June), significant decreasing
trends with a 90% confidence level are observed in Eastern andWestern Japan, accompanying large inter-decadal variation in theformer half of the 20th century. In the late Baiu season (mid tolate July), in contrast, significant increasing trends with a 95%
confidence level are observed on the Japan Sea side of Easternand Western Japan, while clear trends are not found on the Pacificside. No significant trends are recognized either in the mid Baiu
season (late June to early July) or in the entire Baiu season (Juneto July) over all regions. In addition, not only averaged rainfall
but also interannual variability of rainfall has been increasing on
the Japan Sea side in the late Baiu season.
This study reveals that precipitation has a long-term increasing trend with statistical significance on the Japan Sea side of
Eastern and Western Japan in the late Baiu season. This trend ofdelayed Baiu withdrawal is consistent with that shown by previous
studies analyzing observational data in the latter half of the 20th

Fig. 4. Time series of regional mean precipitation ratios in (a) Period 1, (b)Period 2 and (c) Period 3. Red line: EJ+WJ. Blue line: EP+WP. The thicklines indicate the 11-year running mean. The straight lines are obtained bylinear regression. The base period of the ratio is 1901 through 1930.

Fig. 5. Geographical distribution of linear trends of precipitation for 1901
through 2009 in (a) Period 1 and (b) Period 3. Red (blue) circles indicate
increasing (decreasing) trend. The size of each mark reflects the magnitudeof the trend. Filled circles denote statistical significance at the 90% confidence level by the MK test.

Endo, Long-Term Changes of Seasonal Progress of Baiu Rainfall in 109 Years (1901−2009) Daily Data

century (Yamakawa 1988; Sato and Takahashi 2001; Inoue andMatsumoto 2003; Inoue and Matsumoto 2006; Takahashi 2009).
Observational stations with the long-term trend in the late Baiu
season are mainly on the Japan Sea side. Why is the trend clearer
at stations on the Japan Sea side? According to Yamakawa (1988),
Sato and Takahashi (2001) and Inoue and Matsumoto (2006), theBaiu frontal zone is clearer and stays longer from the late Baiu
season to the following midsummer season in recent decades.
These trends would bring regions on the Japan Sea side more rainfall because the mean frontal zone is over the Japan Sea during
the season. In addition, Nagata and Mikami (2010) reported, basedon the HadSLP2 (Allan and Ansell 2006), that the North Pacificsubtropical high has extended southwestward in the 20th century.
It is likely that this long-term change alters moisture transport,
which might affect the observed long-term rainfall trend.
Latest future projections with climate model studies indicatethat global warming induced by increasing greenhouse gases willlead to increased Baiu rainfall and a delay of the Baiu withdrawal(Kimoto 2005; Kitoh and Uchiyama 2006; Kusunoki et al. 2006;
Kusunoki and Mizuta 2008). It is interesting to note that theobserved tendency of delayed Baiu withdrawal in the last 109
years, when global warming has been in progress, is similar to theprojected future changes.
This study also reveals that precipitation has a long-term decreasing trend with statistical significance over Eastern and Western Japan in the early Baiu season. Inoue and Matsumoto (2003)
showed a similar tendency based on observed sunshine duration
data for the period 1951−2000, although the trend was not so clear
in their results. This observed trend is also consistent with Kawase
et al. (2009), who indicated that, based on their pseudo global
warming downscaling method, the Baiu rainband in June (theformer half of the Baiu season) will shift southward in the future,
although results of Kusunoki et al. (2006) and Kusunoki and
Mizuta (2008) do not have such a clear tendency in June. Menon
et al. (2002) and Ramanathan et al. (2005) point out that historicalaerosols largely influence precipitation in South and East Asia. We
may need to keep in mind influences of aerosols when we discusspossible anthropogenic influence on the historical trends. Further
analysis is needed to reveal observed long-term climate changes
in the summer season, using sunshine duration and sea level pressure data at stations and long-term re-analysis data over the 20th
century.

Acknowledgements
We are thankful to Ms. Hitomi Saitoh of JMA for providing
the historical daily rainfall data. We appreciate valuable commentsfrom Dr. Tomoshige Inoue of Tsukuba University and Mr. Koji
Ishihara of MRI. We also appreciate the valuable comments fromDr. Tomoaki Ose and Dr. Akio Kitoh of MRI on the first draft. The
reviewers’ comments were very helpful in the revision.

Supplement
Table S-1 is a list of stations used in this study.

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