Wildfires and Weather Variability in South-Central Chile

The wildfire regime in South-Central Chile (SCC, 30º to 44ºS) has changed in recent decades due to changes in land use, climate conditions, and characteristics of weather extreme events. While during 1976-2016, the mean annual burned area was ~54,000 ha, during the last decade a sequence of seasons multiplied that value, in particular 2016-2017 with 570,000 ha and 2022-2023 with 450,000 ha. To the north of this region, the fire regime is fuel-limited (e.g. amount and connectivity of biomass), while to the south, it is primarily climate-limited (i.e. plenty of wet fuels). In contrast to all other Mediterranean regions worldwide, SCC has a very low rate of natural ignitions (<1% of wildfires), whereas 99% of fires are caused by humans. In SCC, large-scale plantations of flammable exotic species and invasive trees and shrubs have modified the fuel structure particularly since the mid-1970s, leading to an increase in fire risk. Within this context and beyond climate variability, in this work we unveil crucial aspects on the relationship between wildfires and weather variability in SCC. 

As a first task, we identify weather patterns associated with relatively large wildfires (>520 ha, N~800) within 7 SCC sub-regions, previously delimited according to climate, topography, and land use. Using historical wildfire records (including start date, duration, and burned area) from the National Forestry Corporation (CONAF) spanning 1984-2025, we describe the mean local 15-days evolution of weather conditions centered on the start dates of wildfires. For this, we use daily ERA5 data, including maximum temperature, minimum specific humidity, mean sea-level pressure, and maximum surface wind intensity. Furthermore, within each subregion, a cluster analysis reveals distinct mean weather sequences and typical thresholds for these variables related to wildfires. While subtle weather variability is detected in the northern part of SCC, for the southern part of SCC our analysis reveals the relevance of mid-latitud synoptic variability–in particular blocking patterns induced by migratory anticyclones–, as well as associated mesoscale phenomena, especially coastal lows and foehn wind systems. Moreover, prominent differences in wildfire characteristics are found between distinct extreme weather events, such as heat waves and single hot days.

As a second task, we explore the intra-seasonal evolution leading to selected weather patterns associated with wildfires in SCC. We find groups of events that reveal different sequences of significant mid-latitude and tropical circulation anomalies up to 14 days before the wildfire start dates. For each group, we show that the corresponding weather-fire relationship is in fact mediated by a distinct trajectory of the Fire Weather Index (FWI). Finally, we suggest a scheme based on the Madden-Julian Oscillation (MJO) index and the Standardized Extra-Tropical Index (sETI) to monitor intra-seasonal atmospheric teleconnections favoring weather fire in SCC.