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.


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