Study On Structure And Variation Characteristics Of South Branch Trough Based On Objective Identification Of Synoptic Scale

The active days of the South Branch Trough during winter of 1981-2018(from December to February of the next year)were identified by the objective identification method of the low pressure trough in this paper.Based on the identified active days of the South Branch Trough,the three-dimensional structure and typical background circulation characteristics of its activities were studied.From the point of combination of synoptic and climatological,a new index of the South Branch Trough was established,and the characteristics of multi-time scale variability of weather and climate in the South Branch Trough were analyzed.The main conclusions are as follows:(1)The objective identification method was used to identify a total of 3,429 days in winter from 1981 to 2018,which accounted for 2,247 days,accounting for 65.5% of the total days.(2)The southern branch trough is mainly manifested as the middle and lower tropospheric trough,with baroclinic characteristics,and the signal are most obvious in700 h Pa in the vertical direction,and the signal is gradually weakened above 500 h Pa.Compared with previous studies on climatology mean state Angle,the signal strength of the south branch trough based on synoptic average recognition is stronger and the vertical extension height is higher.Although the south branch trough is mainly manifested in the middle and lower troposphere trough,it is not a simple local low pressure trough,and its activities are closely related to the south branch westerly flow in the upstream,the East Asian trough and the subtropical high system in the downstream area,and the upper subtropical jet.(3)On active days of the south branch trough,the typical circulation field showed as follows: there was a significant low pressure trough signal in the north of BOB in the subtropical region,and the subtropical high broke near the Bay of Bengal,and it was distributed in block shape;The lower trough near the Ural Mountains is stronger,which is conducive to the accumulation of cold air and southward deepening of the southern trough,while the strength of the East Asian trough is weaker.On active days of the nonsouth branch trough,there was an obvious trough of low pressure in the northern Arabian Sea,the subtropical high did not break in the BOB,and the West Pacific subtropical high was distributed in a zonal manner.(4)In this paper,a new index of the South Branch Trough was constructed from the perspective of synoptic and climatological combination by taking the characteristics of wind and pressure field into account as a parameter.The index can better reveal the relationship between the South Branch Trough and the circulation field and the precipitation field.The new index reveals that the south branch trough has a significant period of quasi-4-year interannual variation.