The Impact of Atmospheric Rivers on Warm Winters and Extreme Heat Events

Academic Background

Atmospheric Rivers (ARs) are narrow regions of intense water vapor transport in the Earth’s atmosphere, typically carrying large amounts of moisture from the subtropics to mid-latitudes and polar regions. These transient phenomena dominate global moisture transport and significantly influence precipitation and water resources in many regions. In addition to transporting moisture, ARs also transport heat, but their impact on global near-surface air temperatures has not been fully characterized. With the increasing frequency and intensity of extreme weather events due to global climate change, understanding the impact of ARs on temperature is crucial for predicting and responding to extreme weather events.

This paper, authored by Serena R. Scholz and Juan M. Lora from the Department of Earth and Planetary Sciences at Yale University, was published in Nature on December 19-26, 2024. The study aims to reveal the impact of ARs on global near-surface air temperatures, particularly their contribution to warm winters and extreme heat events.

Research Process

Data Sources and AR Identification

The study used MERRA-2 reanalysis data from 1980 to 2022, including variables such as 2-meter air temperature, surface shortwave radiation, net surface longwave radiation, surface sensible heat flux, and latent heat flux. To identify ARs, the study employed seven global algorithms from the Atmospheric River Tracking Method Intercomparison Project (ARTMIP). All calculations were performed independently, and the final results were averaged across the algorithms.

Relationship Between Temperature Anomalies and ARs

The study found a significant correlation between AR frequency and average air temperatures in many mid-latitude regions, especially during winter. In the Northern Hemisphere winter, the correlation coefficient between AR frequency and temperature exceeded 0.6 in Northern Europe, with significant positive correlations also observed along the eastern coast of North America and the North Pacific. In the Southern Hemisphere winter, most of the Southern Ocean also showed significant positive correlations.

Temperature Anomalies During ARs

During AR events, the average temperature anomaly in North America and Eurasia was approximately +5°C, while in Greenland and Antarctica, it exceeded +10°C. Temperature anomalies in polar regions were particularly pronounced in winter, averaging up to +15°C. Temperature anomalies were positive during both day and night, but nighttime warm anomalies were more significant.

Changes in Surface Heat Fluxes

Surface heat fluxes changed significantly during ARs. Increased cloudiness reduced surface shortwave radiation, but this was offset by increased longwave radiation and sensible heat flux. In desert regions, AR events brought an average of +50 W m⁻² of anomalous downwelling longwave radiation. Additionally, during ARs, sensible heat flux was transported from the atmosphere to the surface in mid-latitude and polar regions, leading to increased surface temperatures.

Horizontal Transport of Heat and Moisture

ARs were associated with anomalous horizontal transport and convergence of sensible heat and moisture in the lower atmosphere. The study found that poleward transport of sensible heat increased significantly during ARs, particularly in North America and Antarctica. Moisture convergence also increased significantly, leading to condensation and latent heat release, further exacerbating surface temperature increases.

Relationship Between ARs and Extreme Heat Events

The study also found that ARs were closely related to extreme heat events. In the North Pacific and North Atlantic storm tracks, 70-80% of extreme hourly temperature anomalies occurred during AR events. In polar regions, the probability of extreme temperature anomalies occurring during AR events was more than ten times higher than independent events. Additionally, ARs were closely associated with moist and compound heatwaves, particularly in eastern North America and northeastern Asia.

Main Results

1. Correlation Between AR Frequency and Temperature: There was a significant positive correlation between AR frequency and average air temperatures in mid-latitude regions, especially during winter.
2. Temperature Anomalies During ARs: During AR events, the average temperature anomaly in North America and Eurasia was approximately +5°C, while in polar regions, it exceeded +10°C.
3. Changes in Surface Heat Fluxes: During ARs, surface longwave radiation and sensible heat flux increased significantly, leading to higher surface temperatures.
4. Horizontal Transport of Heat and Moisture: During ARs, poleward transport and convergence of sensible heat and moisture increased significantly, further exacerbating surface temperature increases.
5. Relationship Between ARs and Extreme Heat Events: AR events were closely related to extreme hourly temperature anomalies and moist and compound heatwaves.

Conclusion

The study demonstrates that ARs significantly impact near-surface air temperatures across multiple timescales, from seasonal averages to hourly extreme temperature events. Through anomalous transport and convergence of heat and moisture, ARs alter surface heat fluxes, leading to increased temperatures in mid-latitude and polar regions. Additionally, the close relationship between ARs and extreme heat events suggests that ARs play a more significant role in global energy transport than previously recognized.

Research Highlights

1. Key Findings: ARs not only influence moisture transport but also significantly impact global near-surface air temperatures, particularly in mid-latitude and polar regions.
2. Methodological Innovation: The study employed multiple AR identification algorithms, ensuring the robustness of the results.
3. Application Value: The findings help improve the predictive capability for extreme heat events, especially in the context of global climate change.

Other Valuable Information

The study also noted that ARs may carry dust and other particulate matter, further enhancing the trapping of longwave radiation, particularly in summer, potentially creating dangerous high-temperature conditions. As global warming continues, ARs may become more frequent and intense, further impacting global temperatures and extreme weather events.

Through this research, we have deepened our understanding of ARs and provided important scientific insights for future climate prediction and extreme weather response.