The Influence of the Eastern-Type and Western-Type Southern Branch Troughs on Precipitation in Winter and Its Structural Characteristics

Using 30 years of NCEP/NCAR reanalysis data from winter (January), along with daily precipitation data from the China Meteorological Administration (CMA), this study employed an objective recognition method to classify southern branch troughs (SBTs) into eastern and western types. Subsequently, an analysis was conducted to identify the structural characteristics of each type and evaluate their effects on precipitation. The findings reveal that the eastern and western SBTs are located at different coordinates at 500 hPa, each affecting distinct precipitation zones. Specifically, the eastern SBT influences regions east of 90° E, particularly Yunnan, while the western SBT primarily impacts areas west of 90° E, with a pronounced effect on southern Tibet. Dynamic analysis indicates that both types of SBTs manifest as baroclinic troughs, though the western type exhibits stronger dynamic conditions than the eastern type. Both SBTs types are characterised by warm, moist advection ahead of the trough and cold, dry advection behind it. The eastern SBT shows more favourable thermal conditions than the western SBTs. Water vapour associated with both types of SBTs is concentrated at the front of the troughs below 450 hPa, with convergence observed below 700 hPa. Additionally, the eastern SBTs generate more pronounced water vapour convergence compared to the western SBT. Notably, torrential precipitation related to the eastern SBTs is driven by water vapour from the outer circulation of the western Pacific subtropical high, with additional moisture from the Bay of Bengal. In contrast, the water vapour associated with the western SBT primarily originates from the Arabian Sea and is supplemented by moisture from the Bay of Bengal.