The 2021 Pacific Northwest heatwave would have been more severe without the influence of Asian summer monsoon

The Pacific Northwest experienced a record-breaking heatwave during the summer of 2021, resulting in significant adverse effects on both human society and ecosystems. This event was so extreme and shattered previous temperature records by an astounding 5 ℃, highlighting the need for a comprehensive understanding of the underlying physical mechanisms. In this study, we employ a hierarchical approach with increasing complexity to demonstrate that the Asian summer monsoon, when accounting for all relevant convective activities, contributed to suppressing the intensity of this event. Without the variability of the Asian summer monsoon, the heatwave's amplitude is estimated to be approximately 0.4°C (3%-4%) greater than the already extraordinary observed amplitude. Since this extreme event occurred against a context of imperfect synchronization among climate systems, it serves as a warning that even more intense heatwave is likely to occur in the future even if global warming remains at current levels.