The term “atmospheric river” (AR) is used to describe a long and narrow corridor of strong horizontal water vapour transport concentrated in the lower atmosphere¹. The amount of water vapour flowing in a typical AR can be comparable to the amount of water flowing in the Amazon River^2,3,4. ARs are often driven by an extratropical cyclone-anticyclone couplet.  They can produce heavy precipitation and lead to severe flooding situations in regions where moist air is forced to ascend rapidly^1,5. In North America (NA), some intense ARs over the northeast Pacific are known colloquially as “Pineapple Express” storms for their apparent origin in the subtropical area near Hawaii (where pineapples are grown). These storms usually arrive during autumn and winter seasons. The combined effect of the warm, moist air and orographic uplift along the coastal mountain ranges causes some of the most torrential rains in western NA. The picture below shows a landfalling Pineapple Express on 14 November 2021 with catastrophic consequences: It stalled over southern British Columbia (BC) for about two days, producing extremely heavy precipitation and triggering one of the most destructive weather disasters in Canadian history.

In the above figure, the anticyclonic circulation marked by a blue “H” is a semi-permanent high-pressure system known as the North Pacific High. During the northern hemisphere summer, this feature shifts further to the north and reaches its strongest, effectively blocking most of the Pacific ARs from reaching on the west coast of NA. Sometimes the high-pressure system can act as a heat dome to trap hot air near the ground⁶. This was the case from late June through mid-July 2021, when an extreme heatwave affected a large area in western NA.

In a recent article “An anomalous warm-season trans-Pacific atmospheric river linked to the 2021 western North America heatwave”⁷, we investigate a summer AR that moved across the North Pacific in late June 2021 to interact with a heat dome in western NA. The interaction involves AR-induced heat transport and the short-lived greenhouse effect of trapped moisture. As shown in the below figure and animation, the origin of this super-long AR can be traced to the Southeast Asian summer monsoon. To emphasize this unique connection, we call this system an “Oriental Express”, analogous to the naming of Pineapple Express.

This Oriental Express was driven by a cyclone-anticyclone couplet in the northeast Pacific to make landfall over the Alaska Panhandle around 0000 UTC 24 June 2021. With a strong and stable anticyclone to its southeast, this AR remained quasi-stationary in the northeast Pacific for more than 48 hours, and began to dissipate and disconnect with the Asian monsoon system around 0000 UTC 27 June. During this period, there were a few flooding events in Southeast Alaska that could be attributed to this AR, not only because of heavy rainfall but also from warm advection associated with the system leading to a large build-up of ice and snow melt.

In the paper, we emphasize the impacts of this landfalling AR on the following development of heatwave in western NA. The picture below shows that the hot weather started in northeastern Washington. The surrounding area was under the heat dome associated with the North Pacific High. The hot weather spread out to cover a much larger area during the last five days of June 2021. The small circle indicates the location of the Village of Lytton, BC, where the Canada’s highest ever temperature of 49.6°C was recorded on 29 June, and one day later the village was consumed by flames that destroyed 90 percent of it. Overall, this extreme heatwave caused more than 1,400 deaths, and led to numerous extensive wildfires and widespread smoke pollution across western NA⁸.

We argue in the paper that the Oriental Express transported large amounts of 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. The picture below shows the AR-induced sensible and latent heat fluxes and their convergences, vertically integrated from the earth’s surface to 500-hPa level and averaged over 24-27 June 2021. It becomes obvious that the strong convergence of sensible heat in northern BC and southern Yukon is an important reason for the heatwave expansion to the area from the south.

Given that water vapour is the dominant short-lived greenhouse gas in the atmosphere, we further argue that there could be another mechanism for this AR to influence the heatwave through radiative heating. The landfall of this AR injected a large amount of water vapour into western Canada. We showed evidence of the added moisture acting as greenhouse gas to trap solar radiation and make the lower atmosphere even warmer.

This paper is a follow-up study to a preceding work⁸, in which we investigated the subseasonal predictability of this extreme heatwave and the possible mechanisms responsible for the circulation anomalies. The greenhouse effect of water vapour was considered as one of the possible factors.

¹ American Meteorological Society. Atmospheric River. Glossary of Meteorology, http://glossary.ametsoc.org/wiki/Atmospheric_river (2022).

² Newell, R. E., Newell, N. E., Zhu, Y. & Scott, C. Tropospheric rivers?—A pilot study. Geophys. Res. Lett. 19, 2401–2404 (1992).

³ Zhu, Y. & Newell, R. E. Atmospheric rivers and bombs. Geophys. Res. Lett. 21, 1999–2002 (1994).

⁴ Zhu, Y. & Newell, R. E. A proposed algorithm for moisture fluxes from atmospheric rivers. Mon. Wea. Rev. 126, 725–735 (1998).

⁵ Ralph, F. M., Neiman, P. J. & Wick, G. A. Satellite and CALJET aircraft observations of atmospheric rivers over the eastern North Pacific Ocean during the winter of 1997/98. Mon. Wea. Rev. 132, 1721–1745 (2004).

⁶ National Oceanic and Atmospheric Administration. What is a heat dome? In National Ocean Service website, https://oceanservice.noaa.gov/facts/heatdome.html (2022).

⁷ Mo, R, Lin, H. & Vitart, F. An anomalous warm-season trans-Pacific atmospheric river linked to the 2021 western North America heatwave. Commun. Earth Environ. 3, 127 (2022).

⁸ Lin, H., Mo, R. & Vitart, F. The 2021 western North American heatwave and its subseasonal predictions. Geophys. Res. Lett. 49, e2021GL097036 (2022).