Analysis on the climate circulation patterns for abnormal cold-wet and warm-dry winters in Southwest China

Based on data collected at 409 surface meteorological observation stations in Southwest China (SWC) and reanalysis data from NCEP/NCAR and NOAA from 1981 to 2022, this study analyzes climate change characteristics and formation mechanisms of abnormal cold-wet/warm-dry winter climate patterns in SWC. The results show that cold-wet and warm-dry are the most common climate configurations in SWC during winter. Since the 1980s, the entire region shows a significant increase trend of warm-dry winters. A stronger (weaker) Baikal lake high and a weaker (stronger) Qinghai-Xizang plateau high in the upper and middle troposphere as well as a stronger (weaker) Siberian high in the middle and lower troposphere synergistically form a quasi-barotropic "north positive south negative" ("south positive north negative") circulation pattern, which is the key atmospheric circulation background for the cold-wet (warm-dry) winter in SWC. Sea surface temperatures (SSTs) over the area to the south of Aleutian Islands, regions in the southern and northern sides of the eastern equatorial Pacific and the region to the southwest of Australia during autumn are key precursor signals of the circulation background. Higher (lower) SSTs over the area to the south of Aleutian Islands can enhance (weaken) the Azores high continuously from autumn to winter and then strengthen (weaken) the Baikal lake high and the Siberian high with eastward propagation of Rossby waves from the enhanced (reduced) Azores high. Lower (higher) SSTs over the regions in the southern and northern sides of the eastern equatorial Pacific can weaken (strengthen) geopotential height over the low latitudes of the Northeast Pacific and the North Atlantic in winter, which will then trigger positive (negative) abnormal Rossby waves sources that spread eastward by Rossby waves, resulting in lower (higher) Qinghai-Xizang plateau high. Moreover, higher (lower) SSTs in the region to the southwest of Australia can enhance cross-equatorial southerlies (northerlies) in upper levels and subsequently strengthen (weaken) the Middle East jet, leading to lower (higher) Qinghai-Xizang plateau high.