A Comparative Investigation of Monsoon Active and Break Events over the Western North Pacific

This study identifies 86 active events and 66 break events of the western North Pacific summer monsoon (WNPSM) from 1979 to 2020. These active and break events exhibit sharp contrast in large-scale convection and circulation fields. During the active period, deep convection almost covers the entire WNP domain (0 degrees-25 degrees N,120 degrees-170 degrees E), accompanied by the strengthening of a local monsoon trough and monsoon westerlies extending from the Arabian Sea to the WNP. In contrast, during the break period, along with weakened monsoon westerlies, the subtropical high replaces the monsoon trough to dominate the WNP domain, leading to the disappearance of deep convection. Results about the convection evolution at multiple time scales indicate that active/break events are primarily contributed to by the wet/dry phases of intraseasonal oscillations (ISOs). The phase transitions of ISOs are nearly symmetric between active and break events. However, these two types of events present notable asymmetric features in convection anomalies relative to climatology, which can be attributable to the modulation of the seasonal mean component. Specifically, both active and break events correspond to anomalously strong seasonal mean convection, which amplifies (weakens) the convection enhancement (suppression) during the active (break) period and results in normal (enhanced) convection during adjacent periods. The preferred occurrence of active and break events in strong summer monsoons is because related mean backgrounds, including increased low-level positive vorticity, specific humidity, and easterly vertical shear, facilitate the intensification of ISOs. These mean backgrounds are likely induced by SST cooling in the north Indian Ocean and warming in the equatorial central Pacific during the simultaneous summer.