Increasing intensity and frequency of cold fronts contributed to Australia's 2019-2020 Black Summer fire disaster

Human-caused climate changes are increasing the risk of dangerous wildfires in many regions of the world. There are multiple, compounding aspects of climate change that are increasing fire risk, including large-scale climate changes driving hotter and drier conditions that are generally well observed and predicted. However, changes in synoptic-scale processes that can exacerbate dangerous fire weather and promote extreme pyroconvective events are often not well known in historical observations and are poorly represented in climate models, making it difficult to fully quantify and anticipate changing fire risk. In this study, we statistically test the association between synoptic-scale cold front passage and large fires in southeast Australia during Australia's 2019-2020 'Black Summer' fire disaster, and analyse daily gridded temperature data to detect long-term changes in the intensity and frequency of strong cold fronts over southeast Australia. We demonstrate that the passage of cold fronts over southeast Australia significantly increased the likelihood of large fire days during the entire Black Summer fire season. Additionally, the intensity and frequency of strong cold front events were anomalously high during the Black Summer, and this is part of a long-term significant increase in the intensity and frequency of strong cold fronts since the 1950s. These changes in fire-promoting cold front activity are expected to imminently emerge above the range of historical experience across large areas of southeast Australia if current trends continue. Our results provide new insights into a previously poorly constrained contributor to fire risk in southeast Australia, highlighting the potential of synoptic-scale weather changes to compound previously documented broad-scale climate changes in intensifying future forest fire risk.