The Interannual Variability Of The Southern Branch Trough And Its Relationship With The Winter Climate In Southern My Country

This paper makes use of ERA–interim reanalysis data and bases on objectiveidentification technology of trough to discern activities of south branch trough in winter for 1979 to 2015,on this basis,a south branch trough intensity index is defined,then correlation analysis,composite analysis,wavelet analysis,wave activity flux and other methods are used,to study interannual variation characters of south branch trough and factors that affect interannual variation rate of south branch trough,and relationships between strong(weak)year of south branch trough and general atmospheric circulation,as well as climate of south China in winter are further analyzed.Main conclusions are as follows:(1)By using an objective identification method,activities of south branch trough could be well identified,the most frequent active area is located near 90°—92°E south of the middle of center Tibetan Plateau(TP).The south branch troughindex can better reflect the relationship between the 700 hPa geopotential height change and the strength of the south branch trough,and at the same time it has the best ability to reflect the detour flow of westerly jet of south branch of subtropics.There are quasi–4–year and quasi–14–year cycles in intensity of the southbranch trough,and there are significant interdecadal changes in these two cycles,that is,the quasi–four–year cycle is most notable around 1985,and the quasi–14–year cycle is significant around 1996.(2)The 200–hPa westerly jet over the south branch trough and the atmosphericheat source in the area north of the Arabian Sea of the upstream of south branch trough have important influences on the interannual variability of the strength of the south branch trough.When upper westerly jet increases(decreases),detour flow caused by obstructing of highland plateau topography to lower west wind also increases(decreases),intensity of south branch trough increases(decreases).Variation of south branch trough intensity is close related to wave energy propagating along subtropical westerly jet,wave energy of strong year of south branch trough mainly comes from inner westerlies propagating along axis of jet stream and the Bay of Bengal;wave energy of weak year of south branch trough mainly comes from Bay of Bengal and the eastern of it.In addition,atmospheric heat sources in the area north of the Arabian Sea will cause cyclonic convergence across the TP and its surrounding low–level flow fields.On the south side of the plateau,this cyclonic circulation superimposes on large – scale westerly winds,making itsouth branch trough have developed significantly.(3)The strength of the south branch trough in winter plays an important role in the cold air activity and the winter water vapor transport in China.In abnormal strong(weak)year of south branch trough,north air stream comes from high latitude is abnormally strong(weak),influence of cold air is notably strong(weak),subtropical anticyclone is weak(strong),which is beneficial(not beneficial)to south burst of cold air.In abnormal strong year of south branch trough,there is a water vapor band near 20°N originating from the Bay of Bengal that bypassed south side of plateau and reaches our country,which is a main water vapor channel for southwest China and southern China,in the meantime,moisture convergence in southwest area is remarkable;In abnormal weak year of south branch trough,transfer of water vapor of south branch trough to southwest and southern of China via the Bay of Bengal is weak,in the meantime,moisture convergence in southwest and southern of China and the Philippines sea area is weak.(4)The strength of the south branch trough plays an important impact on China in winter.When the south branch trough is anomalously strong,from the southwest to south China have more precipitation,and the positive center with much precipitation is located in the northeast of Guangdong Province to Fujian Province.At the same time,temperatures in most parts of the country are abnormally low,and temperatures in the middle and lower reaches of the Yangtze River and in South China are the most significant.When the south branch trough is abnormally weak,from the southwest to south China has less precipitation,and the negative anomaly with less precipitation is located in central Guangdong and Guangxi.At the same time,most areas in the country are high,especially the temperature anomalies in East China and southwest regions are most significant.