August 31, 2004
Japan Agency for Marine-Earth Science and Technology

Interannual variability of Baiu in Japan uncovered
- A mechanism of air-sea interaction in the Western Pacific affecting
the interannual variability of the Baiu front -


Tomohiko Tomita (Faculty of science, Kumamoto University), Takao Yoshikane and Program Director, Tetsuzo Yasunari (Hydrospheric Atmospheric Research Center, Nagoya University) of Hydrological Cycle Research Program of the Frontier Research Center for Global Change (FRCGC), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), demonstrated from the last 25-year global precipitation data based on meteorological satellites (Note 1) that the activity of the Baiu front has a significant 2-year cycle near Japan (Figure 1) and is closely-linked to the 2-year cycle variability of the sea surface temperature (SST) off the east of the Philippines. The activation of the Baiu front was also found out in the 1990s from the analyzed data (Figure 2).

The paper is under publishing and will appear in a few months on Journal of Climate of the American Meteorological Society.


The rainfall in Baiu season is prerequisite for summer water resources in Japan, while it causes severe natural disasters. It is therefore of importance to reveal the characteristics and mechanisms of the interannual variations of the Baiu front. This work found out the cyclic features of the Baiu front on interannual time scales and examined the air-sea interaction associated with the variability, focusing on the anomalous meridional circulations and the SST anomalies in the Western Pacific.


The activity of the Baiu front has a tendency that the strength alternates within a year or so, that is, the tendency of 2-year cycle. Heavy Baiu rainfall in last year and this year's heavy one is a typical case of this 2-year cycle (Reference 1). The interannual variability of Baiu, which is characterized by 2-year cycle, can be defined as the Biennial Oscillation (BO) mode. The polarity is set to be positive when the amount of precipitation is larger than normal near Japan.

The BO mode co-varies with SST, the convective activity, and the meridional vertical circulation off the east of the Philippines. In the case of positive BO, the SST off the Philippines is lower than normal, and the convection activity is anomalously weak. Therefore, the Baiu front can be active with the less downward flow near Japan through the meridional circulation. The opposite appears when the BO mode is negative. That is, when the SST off the Philippines is high and the convection is active, the Baiu activity is weak with the active downward flow near Japan(Figure 3, Reference 2).

The activation of the Baiu front in the 1990s seems to be related with the SST rising during winter-summer near Japan concurrent with the warming in Siberia and in Pacific.

This work clearly indicated that the linkage between the Asian monsoon and air-sea interaction during winter-summer in the western North Pacific is a key to up the predictability of the early summer climate in Japan represented by the activity of the Baiu front.


Japan Agency for Marine-Earth Science and Technology
Frontier Research Center for Global Change
Contact: Ms. Ota
Tel: +81 (0)45-778-5687 (direct)
Public Relations Office, General Affairs Department
Tel:+81 (0)46-867-9066

Note 1: Data used
1) Global Precipitation: NOAA's Climate Prediction Center Merged Analysis of Precipitation (CMAP)
2) Atmospheric Parameters: From the NOAA's National Centers for Environmental Prediction (NCEP) and National Center for Atmospheric Research (NCAR) reanalysis
3) Sea Surface Temperature (SST): Data edited by the United Kingdom Meteorological Office (UKMO)
*NOAA: National Oceanic and Atmospheric Administration in U.S.A