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 western North Pacific. Significance Statement The monsoon break usually refers to significant rainfall reduction during the monsoon 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.