Variations of Baiu precipitation over Japan in 1951-1980 and large-scale characteristics of wet and Dry Baiu

Abstract

Variations of precipitation in June and July over Japan (Baiu precipitation) are studied for a 30-year period of 1951-1980 with special emphasize on the occurrence of wet (large precipitation) and dry (small precipitation) Baiu season. EOF analysis is aplied to monthly total precipitation (R), monthly total sunshine-duration (S), monthly mean surface temperature (T) in June and July. In spite of sporadic and localized nature of Baiu precipitation, ~50% of the R-variance is accounted by the 1st EOF component. The 1st spatial function (B1(x)) of R shows localized maximum value over southwestern Japan while B1(x) of T shows localized maximum value over northeastern Japan. Any significant trend and periodic long-term variations are not seen in the 1st EOF’s time variation function (A1(t)) of R. The relations among A1(t) of R, S, T and large-scale conditions in the East Asia are examined. A1(t) of R is positively correlated with anomaly of U and V-component of 700mb wind in the northern periphery of the Pacific anticyclone (∆U700 and ∆V700) and negatively with A1(t) of T. While ∆U700 is negatively correlated with A1(t) of T, the correlation between ∆V700 and ∆U700 and that between ∆U700 and A1(t) of T are very small. From the analysis, following two independent large-scale conditions favorable for wet Baiu are pointed out; 1) positive ∆V700, and 2) negative A1(t) of T (in many cases concurrent with positive ∆U700, northeasterly flow from polar airmass to the north side of Baiu front, development of Baiu ridge and trough).