Atmospheric rivers (ARs) are long filaments of intense water vapor transport in the lower atmosphere. These long, narrow corridors carry moisture from the humid tropics to the drier midlatitudes and polar regions, and often cause extreme precipitation events when they make landfall. Our recent study finds that, in addition to transporting moisture, ARs also transport large quantities of heat worldwide. This causes warm temperature anomalies at the surface, as well as being linked to extreme temperature anomalies and humid heat waves.

The term ‘atmospheric river’ was first coined by Zhu & Newell in 1994 [1], but similar atmospheric phenomena have been described in the literature for almost a century. As early as 1937, Carl-Gustav Rossby observed a ‘moist and warm tongue’ in the central US, mapping a distinct narrow structure in isentropic maps which coincided with high relative humidity measurements [2]. Similar features have also been variously called ‘tropical plumes’ or ‘pineapple express’ storms in the Pacific, where they carry warm moist air to the west coast of North America. Most studies of ARs focus on their dramatic hydrologic impacts, such as extreme rainfall, flooding, and damage to local infrastructure. But as we conducted our analyses, it became apparent that the temperature impacts of ARs could be just as dramatic.

ARs have a strong warming effect on local surface air temperatures, on the order of +5°C in the midlatitudes, and +10°C near the poles (Figure 1). Temperature anomalies in the winter season are even higher, with the polar regions experiencing average temperature anomalies of up to +15°C during wintertime AR events. When examining the surface energy fluxes during ARs, we found that while ARs reduce shortwave radiation at the surface (due to cloudiness), this is counteracted by the increase in downward longwave radiation due to the enhanced water vapor. The increase in longwave is coupled with an increase in downward sensible heat flux, on the order of +20-50 W m^-2 over land in the midlatitudes. This enhanced downward sensible heat flux ultimately comes from the anomalous poleward transport and local convergence of sensible heat in the lower levels of the atmosphere that happens during ARs (Figure 2).

Due to these effects, more frequent atmospheric river events are associated with warmer-than-average winters throughout much of the midlatitudes. In addition, we find that ARs are associated with the majority of extreme hourly temperature excursions in many regions: this means that if you experience anomalously warm winter air temperatures in eastern North American or western Europe, there is a >70% chance you are within an AR. Risk ratios show that extreme temperature anomalies in these regions are 7 times more likely to occur within ARs than would be expected if the events were independent, and these values are even higher in the polar regions (Figure 3).

In addition to hourly extremes, we also find that ARs are related to long-duration humid heatwaves, as defined by their high wet-bulb temperatures. These heatwaves are particularly dangerous for human health, as high humidity impedes the body’s ability to cool itself by sweating. In eastern North America, for example, 25-50% of moist heatwave days coincide with AR days, approximately 3 times more than would be expected if the events were independent.

Our results show that ARs significantly affect surface air temperatures on a wide array of timescales, and are impactful for several types of temperature extremes. While the dominant role of ARs in the transport of moisture is well understood [3,4,5], our findings suggest that ARs may also play a much larger role in global energy transport than previously recognized.

References

[1] Zhu, Y., & Newell, R. E. (1994). Atmospheric rivers and bombs. Geophysical Research Letters, 21(18), 1999–2002. https://doi.org/10.1029/94GL01710

[2] Mo, R. (2022). Prequel to the Stories of Warm Conveyor Belts and Atmospheric Rivers: The Moist Tongues Identified by Rossby and His Collaborators in the 1930s. Bulletin of the American Meteorological Society, 103(4), E1019–E1040. https://doi.org/10.1175/BAMS-D-20-0276.1

[3] Zhu, Y., & Newell, R. E. (1998). A Proposed Algorithm for Moisture Fluxes from Atmospheric Rivers. Monthly Weather Review, 126(3), 725–735. https://doi.org/10.1175/1520-0493(1998)126<0725:APAFMF>2.0.CO;2

[4] Nash, D., Waliser, D., Guan, B., Ye, H., & Ralph, F. M. (2018). The Role of Atmospheric Rivers in Extratropical and Polar Hydroclimate. Journal of Geophysical Research: Atmospheres, 123(13), 6804–6821. https://doi.org/10.1029/2017JD028130

[5] Newman, M., Kiladis, G. N., Weickmann, K. M., Ralph, F. M., & Sardeshmukh, P. D. (2012). Relative Contributions of Synoptic and Low-Frequency Eddies to Time-Mean Atmospheric Moisture Transport, Including the Role of Atmospheric Rivers. Journal of Climate, 25(21), 7341–7361. https://doi.org/10.1175/JCLI-D-11-00665.1