Long-term variations of the Baiu frontal zone and midsummer weather in Japan

A comparison of midsummer (August 1 to 10) daily sunshine duration averaged over 10 years for the periods 1986 similar to 1995 and 1959 similar to 1968 indicates that it dropped by as much as 2 hr day(-1) in some observational sites in central Japan. The diurnal temperature range and water vapor pressure in midsummer also decreased. A long-term examination of the Baiu frontal zone location suggests that its slower northward movement in recent years affected the midsummer weather. Furthermore, the sea level pressure increased in northern Japan, while the equivalent potential temperature in the lower atmosphere dropped in central Japan. The change in the northward movement of the Baiu frontal zone is likely to be associated with the strengthening of the polar airmass around Japan.