Circulation Characteristics and Precursory Signals of the Dominant Modes of Rainfall in the Rainy Season in Southwest China

This paper aims to explore the precursory signals of the dominant rainfall modes during the rainy season in Southwest China (SWC) through statistical analysis and numerical experiments. Utilizing rainfall observation data from 1981 to 2021, two distinct dominant modes of rainfall anomalies in the rainy season of SWC are identified, the associated atmospheric circulation and early atmospheric and oceanic signals are further investigated. The results reveal that the first mode, characterized by above-average rainfall across SWC, involves anticyclones over Lake Baikal, India, and South China Sea (SCS), as well as a cyclone over Okhotsk Sea, coinciding with a southward shift of the East Asian westerly jet, and vice versa. This pattern is mainly influenced by the early winter North Atlantic Oscillation (NAO). The winter NAO affects the density of Arctic sea ice and sea surface temperature (SST) anomalies over tropical Atlantic, leading to Eurasian teleconnections and circulation anomalies over India. The second mode exhibits a north-south dipole pattern, with more rainfall in the south and less in the north and vice versa. Weaker mid-high latitude cold air activities, accompanied by an anomalous anticyclonic circulation over Japan and neighboring regions, a cyclonic circulation over Philippines, and a weakened subtropical westerly jet, creates favorable conditions for more rainfall in the southern SWC and less in the north. Preceding negative SST anomalies in the Indian Ocean contribute to this dipole pattern by intensifying cyclonic circulation anomalies over the Philippines and northern East Asia, collaboratively transporting moisture to the southern SWC. The simulated impact of SST anomalies in India Ocean on dipole pattern precipitation, using the global general circulation model (CAM5.3), aligns with the diagnostic results, further confirming the role of Indian Ocean SST anomalies in influencing dipole pattern precipitation in SWC. This study provides valuable insights for improving rainy season predictions in SWC.