Diversity of High-Frequency (10–25-Day) Boreal Summer Intraseasonal Oscillation over the Western North Pacific

Abstract The high-frequency (HF) (10–25 days) boreal summer intraseasonal oscillation (BSISO) connects tightly to day-to-day weather and is inimitable in regulating extreme weather events. The western North Pacific (WNP) exhibits the most robust HF-BSISO variability worldwide, overshadowing the low-frequency (25–90 days) BSISO variability. Revealing the diversity of the HF-BSISO over the WNP area is crucial for understanding the BSISO physics and the week two-to-four subseasonal forecast. This study objectively identified four types of HF-BSISO by cluster analysis of the (two-dimensional) propagation of outgoing longwave radiation (OLR) anomalies, including two northwestward-propagating patterns, with trajectories predominantly being located to the east and west of the Philippines, a westward-propagating Rossby wave train from the Philippine Sea to the Bay of Bengal, and a quasi-standing oscillation pattern centered around the Luzon Island. We have explored the propagation mechanisms of each archetype through a column-integrated moist static energy (MSE) budget diagnosis. Potential factors influencing the diverse propagation patterns are identified and compared. Basically, advections by the background flow predominantly drive the three propagating patterns, while the quasi-standing oscillation pattern is more sensitive to the surface turbulent heat fluxes. It is suggested that the diverse propagation patterns of the HF-BSISO over WNP could be impacted by the background states. However, the origins of these diversified propagation forms are complex and deserve further exploration.