Causes of Interannual Variability of Summer Precipitation Intraseasonal Oscillation Intensity over Southwest China

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- to 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- to 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.