Western North Pacific Summer Monsoon Circulation Patterns Regulate Tropical Cyclogenesis Productivity

Abstract Prior studies have documented that changes in the large-scale summer monsoon circulation, particularly monsoon intensity and location, significantly impact western North Pacific (WNP) tropical cyclogenesis (TCG). However, how changes in the large-scale monsoon circulation pattern affect TCG has been less studied. This study finds that changes in monsoon patterns play an important role in modulating TCG productivity. Using an index of daily monsoon trough (MT) intensity, all days during boreal summer of June–October from 1979 to 2022 were categorized into three monsoon patterns: the MT pattern (MTP), the monsoon gyre pattern (MGP), and the weak monsoon pattern (WMP). We find that the daily TCG rate is highest in the MGP (13.3%), followed by the MTP (8.4%), and is lowest in the WMP (4.5%). These differences in TCG rate can be largely explained by changes in large-scale factors, especially midlevel vertical motion and barotropic energy conversion. Barotropic energy conversion mainly arises from the low-frequency circulation in the MTP, from both the low-frequency and the intraseasonal circulation in the MGP and from the intraseasonal circulation in the WMP. We also find that phase changes of the boreal summer intraseasonal oscillation can modulate changes in the large-scale monsoon circulation pattern. These findings provide new insights into how monsoon circulation patterns modulate TCG rates and underscore the importance of considering these patterns for improving tropical cyclone prediction in the WNP region.