An anomalous warm-season trans-Pacific atmospheric river linked to the 2021 western North America heatwave

Abstract

Atmospheric rivers are long and narrow bands of enhanced water vapour transport concentrated in the lower troposphere. Many studies have documented the important role of cold-season atmospheric rivers in producing heavy precipitation and extreme flooding events. Relatively little research has been conducted on the warm-season atmospheric rivers and their impacts on heatwave events. Here we show an anomalous warm-season atmospheric river moving across the North Pacific and its interaction with the extreme western North American heatwave in late June 2021. This system transported moisture and heat energy from Southeast Asia to the northeast Pacific. Its landfall over Southeast Alaska resulted in substantial spillover of moisture and sensible heat beyond the Pacific Coast Ranges. We provide evidence indicating that the sensible heat flux convergence and the short-lived greenhouse effect of the trapped moisture could form a positive feedback mechanism leading to the northward expansion of the heatwave event across western Canada.