Interannual Variability of the Onset and Withdrawal Dates of the Baiu Season

Baiu is a local name indicating the rainfall or the rainy season in early summer in Japan. Since the dates of onset and withdrawal are determined empirically by staff in the Japan Meteorological Agency, more objective determination is desired for scientific research, particularly, for the seasonal prediction of Baiu. This work proposes a new definition using the gradient of equivalent potential temperature (∇ θe) for the onset, withdrawal, and the period of Baiu. In addition, the physical mechanisms of their interannual variability are discussed through case studies. By the northward shift of region with large ∇ θe, the average date of the Baiu onset is determined around May 23 near Japan, and the interannual variability is characterized by the periodicity of two to three years. The earliest onset date was recoded in 1998, while the latest ones were in 1981, 1986 and 1987. On the other hand, the average date of the Baiu withdrawal was around July 17 with the earliest date in 1997 and the latest one in 1982. The interannual variability of the withdrawal dates is manifested by the 3-yr and interdecadal variations. The results of case studies clearly show that the large-scale pressure system around the Baiu front, i.e., the subtropical Pacific high, the Okhotsk high to the north, and the thermal low over the Asian continent collaboratively control the formation and northward shift of large ∇ θe. That is, the dates of onset and withdrawal are modified in such a pressure system. For instance, the early onset is forced by both of the Pacific and Okhotsk highs straddling the Baiu front. Further discussions will be introduced in December.