Summer Midlatitude Stationary Wave Patterns Synchronize Northern Hemisphere Wildfire Occurrence

Abstract

Midlatitude stationary waves are relatively persistent large-scale longitudinal variations in atmospheric circulation. Although recent case studies have suggested a close connection between stationary waves and extreme weather events, little is known about the global-scale linkage between stationary waves and wildfire activity, as well as the potential changes in this relationship in a warmer climate. Here, by analyzing the Community Earth System Model version 2 large ensemble, we show that a zonal wavenumber 5-6 stationary wave pattern tends to synchronize wildfire occurrences across the Northern Hemisphere midlatitudes. The alternation of upper-troposphere ridges and troughs creates a hemispheric-scale spatial pattern of alternating hot/dry and cold/wet conditions, which increases or decreases wildfire occurrence, respectively. More persistent high-pressure conditions drastically increase wildfire probabilities. Even though the dynamics of these waves change little in response to anthropogenic global warming, the corresponding midlatitude wildfire variability is projected to intensify due to changes in climate background conditions. Plain Language Summary Wildfires are natural disasters that often cause severe public emergencies. Increasing our ability to predict wildfire occurrence has the potential to reduce the loss of life and property associated with them. In this study, we analyze new large-ensemble simulations conducted with the Community Earth System Model, version 2 in 1 degree resolution to elucidate the linkage between atmospheric stationary waves and clustered Northern Hemisphere wildfire occurrences, as well as the potential changes in this relationship in a warmer climate. Our analysis shows that wildfire occurrences tend to be clustered regionally by a zonal wavenumber 5-6 stationary wave pattern. More persistent regional high-pressure conditions further increase wildfire probabilities. This relationship can be largely explained by the quasi-stationary moisture and relative humidity conditions which are associated with the summer midlatitude wave trains. More persistent regional high-pressure conditions further increase likelihood of wildfire occurrences.