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 two 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-day 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 prefer 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.