Blockings In Eurasia And Its Relation To The Precipitation In The Yangtze River Valley During Boreal Summer

Blockings over Eurasia are examined based on the NCEP/NCAR reanalyses of500hPa geopotential height and zonal wind for the period 1950-2004. Results showthat blockings in Eurasia have distinct seasonal variations and geographic diversities.In spring blockings occurred frequently over the Ural Mts. In summer blockingsoccurred frequently over four regions, namely the Ural Mrs, West of the Lake Baikal,East of the Lake Baikal, and the Sea of Okhotsk. Much fewer blockings occurred inautumn. In winter most blockings occurred over around the Ural Mts. The occurrencefrequency of blockings over the Lake Baikal in summer accounts for 69 percent of itsannual total. There is an intraseasonal variation of the occurrence frequency ofblockings in Eurasia with a significant period of 30-60d as well as eastwardpropagation of the blocking high centers in summer.Blockings in Eurasia and their relation to the precipitation over the YangtzeRiver Valley during boreal summer are studied by use of composite analysis. Resultsshow that the precipitation over the Yangtze River valley is deficient during theMeiyu season, when a blocking occurs over the Ural Mrs., while the precipitationover between the Huanghe River and the Yangtze River is much more than normal,when a blocking occurs over the West of the Lake Baikal. As a blocking occurs overthe East of the Lake Baikal, the precipitation over the North of the upper reaches ofthe Yangtze River increases, while the precipitation over the South of the YangtzeRiver decreases significantly. The positive anomalies of the precipitation over theYangtze River valley are strongly associated with the blocking occurring over the Seaof Okhotsk. When a single blocking occurs over the Ural Mts, the precipitation overthe Yangtze River valley will be significantly deficient. However, when twinblockings occur over the Ural Mrs. and East Asia, the precipitation over between theYangtze River valley and Huaihe River will be much more than normal. Further studyindicates that after the collapse of the single blocking over the Ural Mts, theprecipitation over the Yangtze River valley will be much more than normal duringboreal summer.In June and July the precipitation over the Yangtze River valley is much largerthan normal in the years with more twin blockings, especially in 1954, 1969, 1983and 1998. Most subtropical high ridges are situated further south than normal in theyears with more twin blockings in Eurasia, whilst the western edges of the subtropicalhigh are located uniformly further west than normal. The configuration of thecirculations in middle and low latitudes provides an appropriate condition for theintersection of the southwesterlies that bring the warm and moist air to the YangtzeRiver valley with the southward cold air from boreal middle and high latitudes, which therefore forms the strong precipitation over the Yangtze River valley and results inflood disasters by a large amount. Further study indicates that in the years thatblockings occurred frequently over East Asia in June and July, South Asian Highshifts eastward due to blockings which occurred frequently over the Ural Mts., andbecause of that the western edges of the subtropical high over the western Pacific arelocated west than normal.