Impacts of Pacific Ocean SST on the interdecadal variations of tropical Asian summer monsoon onset: new eastward-propagating mechanisms

Understanding the variability and mechanisms of monsoon onset is extremely prominent for water management and rain-fed agriculture. Previous studies have shown a significant interdecadal advance in Asian summer monsoon (ASM) onset after the late-1990s and attributed it to the sea surface temperature anomalies (SSTA) in the tropical Pacific. However, the westward-propagating mechanisms revealed by previous studies (Walker circulation, equatorial Rossby wave response) are gradually decaying westward, which cannot explain the observational facts of stronger low-level winds over the Arabian Sea than the South China Sea. Based on longer datasets and multiple methods, this study reveals the influences of Pacific SST on the interdecadal changes of ASM onset through two eastward-propagating mechanisms: the equatorial Kelvin wave response to the SSTA in the equatorial central Pacific, and the extratropical Rossby wave train associated with SSTA in the subtropical North Pacific. These two eastward-propagating mechanisms mainly modulate the ASM onset via altering the meridional temperature gradient, which is more evident over the Arabian Sea and is more consistent with the observations. Special attention has been paid to the generation and maintenance of the extratropical Rossby wave train, which is less understood compared to the other mechanisms. This Rossby wave train can be excited by the upper-level divergence associated with the warm SSTA in the subtropical North Pacific. In addition, it can effectively gain available potential energy and kinetic energy from the basic flow, and exhibits strong positive interactions with the synoptic-scale eddies. This Rossby wave train is a newly recognized mechanism by which the extratropical Pacific SSTA influences the tropical ASM.

Automated summary

Understanding the variability and mechanisms of monsoon onset is crucial for water management and rain-fed agriculture. This study reveals that the interdecadal changes of Asian summer monsoon onset are influenced by two eastward-propagating mechanisms: the equatorial Kelvin wave response and the extratropical Rossby wave train associated with sea surface temperature anomalies in the Pacific. The Rossby wave train, which is a newly recognized mechanism, plays a significant role in modulating the monsoon onset by interacting with synoptic-scale eddies.