Structural Characteristics Of The Southern Branch Trough In Late Winter And Late Spring And Its Possible Causes

The south branch trough(SBT)is a semi-permanent low pressure trough existing in the winter half year(October to May of the next year),in which January and May are the two peak periods of the south branch trough.Based on the NCEP/NCAR reanalysis data,which were collected four times a day,an objective identification method was used to establish the south branch channel database for the past 41 years(1979-2019).The spatiotemporal distribution characteristics of the southern trough in recent 41 years were analyzed by using various climatic statistical methods.Furthermore,the differences in dynamic and thermal structures of the typical southern trough in midwinter(January)and late spring(May)were analyzed respectively.Finally,the possible causes of the south branch trough in midwinter and late spring are preliminarily revealed by using the T-N wave flux and the full-type vertical vorticity trend equation.The main conclusions are as follows:(1)In the winter half year,the variation of the south branch trough showed a bimodal pattern in frequency,which was in January(midwinter)and May(late spring)respectively,while the intensity showed a single peak pattern,that is,the peak value was in January and the trough value was in May,but the duration of the south branch trough in May was longer.In terms of the frequent area of the south branch,the south branch is mainly located to the west in midwinter,and the south branch is mainly located to the east in late spring,that is,the east branch.(2)The dynamic and thermal structures of the south branch trough show different characteristics in midwinter and late spring.In the midwinter,the south branch of the trough is a barotropic structure,located north by west,with strong and deep vorticity,relatively strong upward movement in the trough area,low temperature and low humidity.At the end of spring,the south branch of the trough has certain baroclinic property,the location is south and east,relatively shallow,the upward movement of the trough area is relatively weak,high temperature and high humidity.(3)The formation of the south branch trough in midwinter is closely related to the dynamic effects of westerly jet,vorticity advection and T-N wave flux,while the formation of the south branch trough in late spring is mainly affected by the thermal effects of pseudo-equivalent potential temperature,specific humidity advection and precipitation.Quantitative diagnosis and analysis of a typical case of the south branch trough by using the full vorticity equation show that the conclusions are consistent.Furthermore,through the diagnosis of the strongest cases in the south branch trough,it is found that the location distribution of the dynamic term is more consistent with the trough area in midwinter,while the location distribution of the thermal term is more consistent with the trough area in late spring.