Baiu: the Rainy Season in Japan
Koichi Kurihara
Climate Prediction Division
Japan Meteorological Agency
1. Introduction
The Baiu is a cloudy and rainy period in early summer in Japan. The word
(bai-u) is written in the same chinese characters as the Mei-yu but with
different pronunciation. Thus the word is believed to come from China. All areas
except the northern-most island of Hokkaido have this rainy season that lasts
from early June to late July. In the Nansei islands region, southern-most part
of the Japanese Archipelago, the Baiu starts and ends about one month earlier.
2. Atmospheric features during the Baiu
A typical surface weather condition during the Baiu is as follows. A
stationary front is formed and meso-scale cyclones are occasionally generated on
them, bringing about rainy and cloudy weather at short intervals over Japan. To
the north of the front an anti-cyclone is often formed around the Sea of
Okhotsk. This causes cold and wet northeasterly into the Northern and the
Eastern Japan, especially along the Pacific Ocean side. On the other hand warm
and wet southerly flows toward the front are prevailing partly due to the
Pacific high in the western North Pacific and southwesterly monsoon flows from
Southeast Asia. Continuation of the stationary front along the Japanese
Archipelago is mainly due to the balance of these highs.
These surface features are associated with characteristic upper flow
patterns. Toward early summer position of the westerly jet reaches at the
latitude of the Tibetan plateau, and the jet tends to be separated to two
streams under the influence of topography. One at a higher latitude tends to
meander over the area between the eastern part of China and the Sea of Okhotsk
forming a blocking high around the Sea of Okhotsk. While air mass of this high
is warmer at higher level because of its origin, surface air is cooling due to
cold sea surface temperature of the Sea of Okhotsk, bringing about surface cool
and wet flows as mentioned above. The other flow, which remains south of the
Plateau and is located over Japan, is associated with formation of the Baiu
front. South of this westerly is located the subtropical high in the western
North Pacific, which is most evident at 500 hPa level. Toward the midsummer the
Tibetan high gradually develops and the jet at the lower latitude weakens,
disappears, and then replaced by the subtropical high over Japan and its
surrounding area. This brings the end of the rainy season with hot and humid
weather becoming dominant. It is noted that change of the weather between before
and start of the rainy season is generally less evident but weather often
changes dramatically from wet and cool to hot and sunny when the rainy season
ends in late July. This is associated with abrupt northward shift or
disappearance of the westerly jet over Japan.
3. Baiu climatology
The period of the Baiu has been operationally identified at the Japan
Meteorological Agency(JMA) for each year and for 11 areas. We try to identify a
transition period of about five days for start and end of the Baiu although by
nature it is not easy to identify the period definitely in some cases. On the
average of 30 years(1961-1990), for example, the Baiu starts around 11th of May
and ends around 23rd of July in Okinawa, southern-most part of Japan. The date
here represents the center of a transition period. In the main islands it starts
about one month later. In Tokyo it starts around 9th of June and ends around
20th of July. In Aomori, northern-most part of the Honshu Island, it starts
around 14th of June and ends around 26th of July. The period of the Baiu is
about 40 days on average. Precipitation for the period is 300 to 400 mm in the
Pacific Ocean side of the Eastern and the Western Japan and 500 to 600 mm in
Kyushu. This corresponds to about 20% to 30% of annual precipitation.
However year to year variations in the start/end of the Baiu, its period,
and precipitation during the Baiu are quite large. For example, the Baiu
continued for some eighty days in Kyushu Island in 1954, while it was only 11
days in an area of the Eastern Japan in 1958. In 1993 we could not identify the
period of the Baiu because wet and cloudy weather continued toward late August
in most part of Japan. This caused significant damage on socio-economic
activities, especially on agriculture. Actually the summer of 1993 was recorded
as the 'coldest' summer in 39 years since 1954 (JMA,1994).
4. Conclusion
While the Baiu has a very important role as one of major water resources,
its nature of interannual variability has significant impacts on our daily life
and on our socio-economic activities. Understanding of its mechanism of year to
year variations is one of the most important challenges for us. In our study it
was found that in the El Niño year the rainy season tends to last longer than
the average (Kurihara,1995). And that precipitation tends to be above normal,
especially in the Western Japan. Delay of the end of the rainy season seems
partly due to weaker than normal subtropical high in the western North Pacific.
As for the blocking activity which is another factor to the Baiu's variability,
interannual variability in the snow cover in the spring season in the Eurasian
Continent is closely associated with blocking activities during the Baiu (Kodera
and Chiba, 1989). These relationships need farther studies for better
understanding of and possible application to prediction of the interannual
variability of the Baiu.
References
Japan Meteorological Agency, 1994: Unusually Cold Temperature and Long
Rains of Summer, 1993 in Japan. Technical Report, No. 115, Japan Meteorological Agency, 231pp.
Kodera K. and M. Chiba, 1989: West Siberian Spring snow cover and East
Asian June 500mb height. Papers in Meteorology and Geophysics, 40, 51 -54.
K. Kurihara, 1995: East Asian Monsoon Circulation Anomalies Associated with the El Niño-Southern Oscillation. Proceedings, International Workshop on
the East Asian Monsoon, Seoul 1995, 45-56.
Dr. Koichi Kurihara
Climate Prediction Division
Japan Meteorological Agency
E-mail: ko_kuri@hq.kishou.go.jp