Frequent extreme precipitation events have resulted in severe disasters to agriculture, the economy, and human health in recent decades. Widespread concerns about extreme precipitation have been aroused among the public, the government, and the research community. Researchers have conducted numerous studies on the mechanisms responsible for extreme precipitation events (Agel et al., 2018;Collow et al., 2016). Favorable local dynamic conditions (ascending motion) and instability are critical for the occurrence of extreme precipitation. Abundant moisture supply is another indispensable condition for extreme precipitation.In recent years, many studies have focused on moisture transport and sources for precipitation, including precipitation extremes. Two main approaches are widely used in studies on moisture transport. One is in the Eulerian view by calculating water vapor transport or by identifying and tracking the variation in atmospheric rivers (Sun & Wang, 2013;Tan et al., 2021). A deficiency of the Eulerian view is that it can neither track moisture from the sources to the precipitation area nor track moisture from the target region back to its sources. The other is in the Lagrangian view by tracking the moisture of particles (or air parcels) forward or backward using Lagrangian trajectory models (Sodemann et al., 2008;Sodemann & Zubler, 2010). The Lagrangian model can provide moisture variation processes from evaporation to precipitation along the transport trajectories. Several Lagrangian trajectory models are available, such as the Lagrangian particle dispersion model FLEXPART (

This study analyzes the synoptic‐scale circulation patterns favorable for regional extreme precipitation events over southwest China (SWC) from 1979 to 2018 occurring during the rainy seasons. The whole SWC is regionalized into two subregions, western SWC and eastern SWC, according to the spatial distribution of two extreme precipitation indices. Furthermore, the atmospheric patterns associated with the independent regional extreme precipitation events (REPEs) that occurred over the two subregions are categorized into two types using k‐means clustering. For type 1, a Rossby wave train originating from the Northeast Atlantic almost a week before REPEs leads to an anomalous low over the eastern Tibetan Plateau (TP), inducing upper‐troposphere divergence and ascending motion over SWC. Moreover, anomalous convective activities over India and the Bay of Bengal (the enhanced and westward‐extended western Pacific subtropical high) are critical precursors of REPEs over western (eastern) SWC. For type 2, an anomalous high is developed over the Ural Mountains at least a week prior to REPEs. The subsequent anomalous low over Lake Baikal and eastward‐shifted South Asian high are conducive to forming updrafts over SWC. An anomalous anticyclone at the middle‐to‐lower troposphere south of the TP is the main moisture contributor. Discrepancies in the zonal location of the Lake Baikal low and the strength of the Ural Mountain high, as well as the intensity of atmospheric circulation anomalies at low latitudes, are the major differences between the type 2 events occurring in western and eastern SWC.

The interannual variability in summer precipitation intraseasonal oscillation intensity over southwest China (SWC) is investigated in this study. The results indicate that the 7–20-day period dominates the intraseasonal variability in summer SWC precipitation. The leading mode of summer SWC precipitation 7–20-day oscillation intensity (SPOI) is a north-south dipole pattern with prominent interannual variability. The atmospheric circulation anomalies from both tropics and mid-high latitudes are responsible for the interannual variability in the dipole pattern. In the tropics, an enhanced local Hadley cell and an anomalous anticyclone over southern China and the Northwest Pacific contribute to the north-positive-south-negative pattern of SPOI by inducing moisture convergence/divergence over northern/southern SWC in the background state. In the mid-high latitudes, the 7–20-day Rossby wave trains along the subtropical jet are crucial for the 7–20-day precipitation over northern SWC. Further analyses suggest that the sea surface temperature (SST) anomalies over the Maritime Continent (MC) and the North Atlantic (NA) are associated with the SPOI dipole pattern. The MC SST warming causes convection anomalies over the tropical Indo-Pacific, consequently triggering a Matsuno-Gill-type atmospheric response conducive to the north-positive-south-negative pattern of SPOI. The NA SST tripole triggers a Rossby wave train across Eurasia that strengthens the East Asian westerly jet and enhances 7–20-day atmospheric variability, consequently favoring the variability of 7–20-day precipitation over northern SWC. Diagnoses of moisture and vorticity budgets further indicate the importance of the interaction between intraseasonal fluctuations and atmospheric background in the formation of the north-south difference in 7–20-day precipitation variability over SWC.