Climate: Processes that cause extreme heat waves vary by region

The air that causes extreme heat waves (such as the heat wave during late June 2021 in the Pacific Northwest of North America) warms up before the event as it moves through the atmosphere, but the specific warming processes vary regionally, according to a study published in Nature Geoscience.

Extremely hot temperatures on the Earth’s surface represent a severe and growing threat to human and natural systems. High temperatures are generally caused by three processes: warm air can be transported from other regions (advection); air can be warmed up directly by a hot land or ocean surface (diabatic heating); or air masses descending towards the surface warm as they get compressed (adiabatic heating). The relative importance of each mechanism in causing hot extremes is poorly understood, hampering understanding of when and where severe heat waves will occur.

Matthias Röthlisberger and Lukas Papritz analyzed climate data from 1979–2020, determining the mix of the three heating processes that led to extremely hot days within grid points spanning the globe. They tracked the air that was present on the hottest day in each year backwards in time and found that the anomalous warmth typically formed over approximately 1,000 km and 60 hours. The authors found that the dominant heating processes depend on geography: advection over mid-latitude oceans, adiabatic warming near mountain ranges and over sub-tropical oceans, and diabatic heating above tropical and sub-tropical continents. Applying their approach to the heat wave that persisted over the northwestern United States and Canada throughout June 2021, they found that these heat extremes were caused by anomalously warm air from the Pacific Ocean in combination with exceptionally large diabatic heating throughout the region.

The authors conclude that understanding the specific mechanisms that heat the air through time is key to improving and constraining projections of heat waves in a warming climate.