Land–Atmosphere Interactions Amplify Wildfires in Southeastern Australia

The temperate climate region of southeastern Australia (SEA) suffers from large periodic wildfires. We investigated how land–atmosphere (L–A) interactions could affect wildfires in the SEA region by applying the forest fire danger index (FFDI) using climate variables from the European Centre for Medium-Range Weather Forecasts reanalysis 5. We calculated FFDI95, which is the number of days exceeding the 95th percentile of daily FFDI during the study period of 1980–2021. The FFDI95 was verified by correlation with the burned areas obtained from satellite data and fire records (r = 0.66, p value < 0.05). We found that FFDI95 in the temperate climate region of SEA increased significantly during the fire season (austral summer) throughout the study period. Using the FFDI95 area averaged over the study region, correlation and composite difference analyses were conducted along with land and atmospheric variables after removing the long-term trend and El Niño–Southern Oscillation variability. During the prefire season (austral spring), significantly reduced soil moisture enhances heat transfer through sensible heat flux, thus raising the temperature from the surface to the lower troposphere. Thermal expansion results in increased atmospheric thickness and strengthened anticyclonic circulation in the midtroposphere. During the fire season, a thicker troposphere, including the intensification of anticyclonic circulation and subsidence, may promote hot and dry conditions, thus intensifying wildfires. The physical processes related to the variations in land conditions and the corresponding responses of the atmosphere indicate that L–A interactions can amplify wildfires in SEA. SIGNIFICANCE STATEMENT: Rising temperatures caused by climate change have increased the risk of wildfires. We focused on the physical processes that amplify wildfire risk in southeastern Australia (SEA) regarding the interactions between land and the atmosphere. We propose a plausible process as follows: With a soil moisture deficit, surface to lower-troposphere temperatures may increase by more sensible heat transfer from drier land. The warming troposphere expands thermally; therefore, atmospheric pressure increases in the midtroposphere. Consequently, high temperatures and low humidity at the surface and in the lower troposphere could be induced by adiabatic warming, and hot and dry conditions promote the danger of wildfires in SEA.