A Climate Perspective of the Quasi-Stationary Front in Southwestern China: Structure, Variation and Impact

Abstract Yunnan-Guizhou quasi-stationary front (YGQSF) is a unique weather phenomenon that frequently occurs during winter half year over the Yunnan-Guizhou Plateau in southwestern China. Most of previous studies analyzed it only with synoptic cases. This study investigates the structure, variation, and impact of YGQSF from a climate perspective, using long-term high-resolution atmospheric reanalysis and high-density station records for 1981-2016. An objective method quantifying YGQSF is proposed and three indexes are defined to measure the intensity, frequency, and location of YGQSF, respectively, with the horizontal gradient of air potential temperature at a terrain-following level of sigma 0.995. With these indexes, climatological structure, subseasonal variability as well as climatic impact of YGQSF are comprehensively examined. In climatology, YGQSF as a north-south-oriented low-level front is found to occur the most frequently during January-February-March (JFM), determined predominately by the coldness from the east of the front. The structure of YGQSF identified essentially reflects an obstruction of high-terrain Yunnan (the western part of the plateau) to the low-level cold air mass, which makes near-surface cold northeasterly winds cease westward intruding and veer upward over relatively low-terrain Guizhou, transporting moisture upward and forming low clouds. A sharp climate contrast is thus formed between two sides of YGQSF: cold, sunless, and continuously rainy Guizhou versus warm and sunny Yunnan. Furthermore, YGQSF features significant subseasonal variations with periods at around 30d and 60d largely in its intensity. Anomalously strong YGQSF events which are caused 75% by the cold anomaly from the east but less than 17% by the warm anomaly from the west yield different anomalous structures, but consistently amplify the sharp climate contrast between Yunnan and Guizhou.