Baiu is a Japanese name for rain and the rainy season that occurs in boreal early summer in the western North Pacific (WNP). This work proposes a methodology to determine the Baiu onset and closing dates using only an atmospheric parameter, the equivalent potential temperature (θ_e) in the lower troposphere, to reveal the interannual variability of Baiu season. First, the location of the Baiu front is determined by the maximum of the negative meridional gradient of θ_e, which identifies the zonal boundary of air masses. Then, the Baiu onset and closing dates are fixed on the basis of the northward movement of the Baiu front and development of the southern air mass in the WNP.
The mean onset date was estimated to be June 2 near Japan, which is comparable to the date published by the Japan Meteorological Agency (JMA) for South Kyushu, although the correlation was not significant between the two interannual variations. The meridional shift of the monsoon southwesterlies was concurrent with early or late onset dates. The mean closing date was July 21, about 1 week earlier than the JMA date for North Tohoku, but the correlation was significant between the two interannual variations. The closing date varied with an anomalous meridional dipole in atmospheric circulation, largely extending from the eastern Eurasian continent to the central North Pacific.
The correlation was insignificant between the interannual variations of the onset and closing dates, although both had dominant 2–4-year periodicity. There was also no significant correlation with typical large-scale interannual variations (i.e., the El Niño/Southern Oscillation, the tropospheric biennial oscillation of the Asian monsoon, and the North Atlantic Oscillation). Hence, these three interannual variations do not appear to be direct causes of the modulation of the onset and closing dates of the Baiu season near Japan.