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.

How Unexpected Was the 2021 Pacific Northwest Heatwave?

“…By June 25th, an omega‐block had developed over the PNW, which progressed eastward and intensified over the course of the heatwave (Neal et al., 2022; Philip et al., 2021). A cross‐Pacific atmospheric river also transported latent heat into the region (Mo et al., 2022). The block caused an extended period of clear skies, increased solar radiation at the surface, and subsidence, all of which increased temperatures.…”

How unexpected was the 2021 Pacific Northwest heatwave?

“…By June 25th, an omega-block had developed over the PNW, which progressed eastward and intensified over the course of the heatwave (Neal et al, 2022;Philip et al, 2021). A cross-Pacific atmospheric river also transported latent heat into the region (Mo et al, 2022). The block caused an extended period of clear skies, increased solar radiation at the surface, and subsidence, all of which increased temperatures.…”

Reliable heatwave attribution based on successful operational weather forecasts

“…The date at which we initialise our perturbed forecasts is a key choice that allows us to condition our attribution analysis on different synoptic drivers of the heatwave, which become predictable at different leads 36,37 . The climate change response of drivers already present in the initial conditions is clearly not incorporated into our attribution results for each lead time due to this conditioning.…”

“…Based on current understanding, the heatwave arose from an optimal combination of proximal drivers 35,36,37,2 . Development of an omega block between the 23rd-27th coincided with the landfall of an atmospheric river (AR) on the 25th.…”

“…The seasonal forecast from ECMWF captured one important aspect of the event: it predicted a thicker troposphere than average (measured by 500 hPa geopotential height) over the Pacific Northwest during the summer. A key change in the predictability of the exceptional temperatures occurred around June 21st, being the earliest point at which the penetration of the AR over land was well represented 37 . The success of these forecast models provides an opportunity to use them to examine the influence of anthropogenic climate change on the event as it actually occurred.…”