Features And Mechanisms Of The Summer Land High Over Asian Mid-latitude Areas

Land high at mid-latitudes over Asian is investigated in terms of observed data;NCEP/NCAR daily reanalysis data, monthly SST data and CCM3.6 model, including geographydistribution and time variation. Then land high around Baikal is examined as a case, which occursmost frequently since the climate chop at the end of 1970s. Later, the causes of the abnormaloccurrence are studied by diagnosis and numerical test, involving in inner dynamics and exteriorforcing. Finally, the influence of the land high on China summer climate is discussed. The resultssuggest:There are two types of land highs (DG+ & DG-) with related large scale circulations beingopposed. Since 1979, the land highs over (35-55°N; 90-105°E) area increase greatly, especially inJuly and August. Land highs maintaining over 3 days are the majority, among which the landhighs with closed center are the minority.There are mainly three paths for wave energy propagating during the maintaining of landhigh. And the sea area near Eurasia both in Atlantic and Pacific may be wave resource. The waveenergy propagates along the main paths with ridge south to Baikal obtaining energy from thezonal flows by the momentum and heat transportation of wave. Thereby the land high is able tomaintain.The warmer SSTs near coastal sea of Atlantic and Pacific are one of the dominant factors forthe more frequently occurring land high near Baikal. There exist two principal patterns for SST(Model& Mode2) and atmosphere (Amodel& Amode2) of Northern Hemisphere. Amodel isinfluenced mainly by SSTA and the other is principally the influencing-factor of the SSTA at midlatitudes. SSTs in key areas of Model are warming greatly since 1979 consisted with variation ofthe land high, being important factor for the more frequently occurring land high near Baikal.Different SSTA in tropical (especially in Pacific) is corresponding to the two land highsrespectively.The land high will weaken Asian summer monsoon with northerly wind departure over east China. However, the influence on climate of the two type land high pattems is different on theirlarge-scale settings. Related to the decadal variation of the land high, the annual relationshipbetween rainfall of North China and related circulation are changed. The Subtropical high of westPacific is unable to arrive at the North China during more frequently occurring land high time,and rainfall over North China is closely related to western weather system at mid-high latitudes.Otherwise, opposite circumstance will happen with the rainfall principally influenced by theSubtropical high.