Effects of blocking anticyclones in Eurasia in the rainy season (Meiyu/Baiu season)

Abstract

Distribution of BAs in Eurasia during the Meiyu/Baiu season and some possible influences of BAs on the behavior of the MB frontal zone were examined through statistical analyses. Then detailed analyses were made for a case in late June 1982. BAs appear with higher frequency in the regions of 31-60°E, 91-110°E and 131-150°E in the latitudes of 50-70°N. The maximum frequency is frond in the region of 131-150°E. The appearance frequency of BAs in region of 50-70°N/81-120°E (Region B) showed a positive significant correlation with the persistency of the Baiu period in Japan. On the other hand, the appearance frequency of BAs in the region of 50-70°N/121-160°E (Region C) showed a positive correlation with both the persistency of the Meiyu period and the rainfall amount during that period in Central China. A statistical analysis suggests that the existence of cold air in the lower layer in 32.5-35°N/112.5-117.5°E (corresponding to the area just to the north of the Meiyu front), largely associated with a BA in Region C, is an important factor in activating the Meiyu front in Central China, and in enhancing the lowlevel southerly wind component in South China. A case study for June 1982 showed that the cold air intrusion toward Central China in the situation of Type C occurs mainly in the lower layer, and is accompanied by the development of a baroclinic instability wave around 50°N/100-110°E. The formation of the Type C situation is a favorable condition for cold air coming southward to the Meiyu frontal zone. On the other hand, there was a Rossby wave-like (height) anomaly pattern propagating from the Okhotsk Sea to the tropics through Japan shown by the presence of a BA in Region C in June. Although forcing processes have not been clarified yet, a case study in late June 1982 suggests that the wave was amplified around the Lake Baikal to the Okhotsk Sea area. The propagation might result in a negative anomaly of geopotential height at 500 mb level to the east of Japan, a southward shift of the Baiu front and weakening (or southward shift of) the sub-tropical high in nearby regions. The present paper shows the different effects of the Type C stiuation on climatological features of the Meiyu front from those of the Baiu front.