Comparisons between Short- and Long-Lived Break Events during the Western North Pacific Summer Monsoon

The break events of the western North Pacific summer monsoon vary significantly in duration, ranging from a few days to more than 2 weeks. In this study, we classify the monsoon break events into short-lived (#8 days) and long-lived (.8 days) events, which account for 78% and 22% of the total events during 1979-2020, respectively. The results show that convection suppression is stronger and broader for long-lived events than for short-lived events. In addition, the temporal distributions of the two break categories are distinct: short-lived events present a roughly even distribution from late July to late September, while long-lived events are highly concentrated, with a striking frequency peak around early September. The mechanisms responsible for break events are investigated. Results indicate that both break categories are co-contributed by 10-25-and 30-60-day oscillations. Short-lived events result from a phase lock of the two oscillations, which explain 54% and 35% of the convection suppression, respectively. By contrast, long-lived events are initiated by both oscillations but maintained only by 30-60-day oscillations. In addition, 30-60-day oscillations reach the peak intensity after the monsoon onset due to seasonal background changes, which is critical for forming the frequency peak of long-lived events around early September. Furthermore, it is found that long-lived events tend to occur in the developing phase of positive SST anomalies in the tropical central Pacific, when 30-60-day oscillations are abnormally enhanced over the west-ern North Pacific.SIGNIFICANCE STATEMENT: The monsoon break usually refers to significant rainfall reduction during the mon-soon season, which can induce abnormal weather, such as heatwaves and droughts, resulting in huge economic losses. To date, however, there are only a few studies on the break of the western North Pacific summer monsoon (WNPSM), in sharp contrast to the well-known Indian monsoon break. We note that break events of the WNPSM vary widely in duration. This study aims to reveal the remarkable difference between short-and long-lived events, including intensity, spatial extent, occurrence time, and physical mechanisms. These results will help us better understand the diversity of the break phenomenon and may serve as a scientific basis for improving the subseasonal prediction of the WNPSM.