Abstract
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°–25°N, 120°–170°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.
Significance Statement
The subseasonal variation in monsoon rainfall typically manifests as fluctuations between active events with abundant rainfall and break events with deficit rainfall, which can induce converse extremes such as floods and droughts, significantly affecting water management, agriculture planning, and economic activities. This study aims to reveal the characteristics, formation, and occurrence regularity of active and break events during the western North Pacific summer monsoon (WNPSM). We note that the occurrence of these active and break events is determined not only by intraseasonal oscillations but also by seasonal mean backgrounds, which result in certain asymmetrical features between the two types of events. These results will enhance our understanding of the monsoon’s intraseasonal variability and the physical association between multiple time scales over the WNP domain (0°–25°N, 120°–170°E).
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