Variations of Baiu Precipitation over Japan in 1951-1980 and Large-scale Characteristics of Wet and Dry Baiu

[摘要] 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 (ΔU700 and ΔV700) and negatively with A1(t) of T. While ΔU700 is negatively correlated with A1(t) of T, the correlation between ΔV700 and ΔU700 and that between ΔU700 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 ΔV700, and2) negative A1(t) of T (in many cases concurrent with positive ΔU700, northeasterly flow from polar airmass to the north side of Baiu front, development of Baiu ridge and trough).

[学科分类] 大气科学