Land and atmospheric conditions amplifying the forest fires in southeastern Australia

Abstract

Southeastern Australia (SEA), which contains large cities and forests, often suffers from forest fires. In the SEA, the surrounding ocean contributes significantly to the fire climate. Hence, extensive research has focused on the relationships between the risk of bushfires and the atmospheric–oceanic oscillation of various temporal and spatial scales. However, studies on the forcing from the land to the atmosphere are relatively rare. Therefore, we attempted to investigate the interactions between the land and atmosphere in leading to forest fires in Australia by analyzing the climatic variables from the near-surface to the upper atmosphere from ERA5 and Forest Fire Danger Index (FFDI) from the Copernicus Emergency Management Service. First, we performed an empirical orthogonal function (EOF) analysis of the FFDI during fire season (DJF) and derived the principal component (PC) time series from 1980 to 2021. The first EOF captured dominant spatial variation in the SEA with the highest percent variance explained (64.9%). Using the FFDI PC1 time series, we conducted the detrended correlation and composite difference analyses with the land and atmospheric variables. Using the results of the correlation and composite analyses, we propose a plausible physical process by which the dry land condition during the pre-fire season (SON) can affect the near-surface to upper atmosphere by thermal expansion and, thus, increase the danger of bushfires during DJF in the SEA by enhancing the anticyclonic circulation and descending motion at the mid-atmosphere. The vertical cross-section of temperature and geopotential averaged by latitude covering Australia supported the process.