The Relative Role of Different Mechanisms of Northward-Propagating Intraseasonal Oscillation Over the Indian Ocean, the South China Sea, and the Western North Pacific

Previous studies have proposed several well-recognised mechanisms for the northward-propagating boreal summer intraseasonal oscillations (BSISOs), while their relative roles tend to be qualitatively analysed. In the present study, based on the planetary boundary layer (PBL) moisture mode theory and the associated moisture budget equation, we quantitatively calculated the contribution of each mechanism, including PBL moisture advection, vertical easterly wind shear, vorticity advection, and external sea surface temperature (SST) forcing, to the northward propagation of BSISO by using the Japanese 55-year reanalysis (JRA-55). The results show that the dominant mechanisms are the vertical easterly wind shear effect (51.75%–56.1% of the positive contribution) and the SST forcing (18.89%–23.6%) over the Indian Ocean, the vertical easterly wind shear effect (40.72%–51.87%) and the vorticity advection effect (24.65%–31.4%) over the South China Sea, and the vorticity advection effect (56.37%–65.67%) and the air-sea interaction (19.92%–22.2%) over the western North Pacific, which favour the PBL moisture asymmetry, and then the northward-propagating BSISOs. Moisture advection is a supplementary mechanism, while the contributions of vertical moisture advection and nonlinear effects can be ignored in all three regions. These results help us to further understand the northward propagation mechanism of ISO in the Asia-Pacific monsoon region, provide a clear reference for diagnosing persistent extreme weather associated with ISO, and provide quantitative criteria for evaluating tropical ISO propagation in numerical models.