Summer wildfires in the Northern Hemisphere occur more and more frequently under global warming, posing a serious threat to the ecological environment. This study utilized ERA5 daily atmospheric reanalysis data spanning from 1980 to 2024 and daily fire weather index data from the European Forest Fire Information System, and analyzed the causes of wildfire events in California in summer 2024 from the perspective of anomalous circulation in mid-high latitudes both on transient and intraseasonal timescales through the physical decomposition principle of transient asymmetric anomaly and diagnosis of wave activity. The results are as follows: 1) In early July 2024, the energy of quasi-steady Rossby waves over the North Pacific propagated eastward, which strengthened and maintained the anomalous anticyclone off the west coast of the North American continent. The temperature increase caused by adiabatic downward motion favored the occurrence and spread of wildfires. 2) The transient asymmetric anomalous flows indicate that the anomalous anticyclone over California has strengthened the subtropical high and increased the risk of wildfires. 3) The decomposition results of transient asymmetric anomaly show that the transient asymmetric anomalous flows can indicate the meteorological conditions and risk of wildfires more clearly. Therefore, the prediction of transient asymmetric flows should play a key role in the forecast and early warning for extreme weather events.
Under a warming climate, the frequency and intensity of wildfires present a remarkable increasing trend globally, and human activities can significantly amplify wildfire risk by altering the wildfire-associated climate conditions. In January 2025, record-breaking wildfires swept through Los Angeles (referred to as the “25·1” Los Angeles wildfire event for short), which occurred in a favorable previous meteorological condition of scarce rainfall and atmospheric aridity. Yet, the quantitative contributions of the natural variability and human influences on such high-impact drought extremes remains unclear. Based on the Detection and Attribution System of Institute of Atmospheric Physics, Chinese Academy of Sciences (CAS-DASys), we conduct large ensemble simulations and a comprehensive attribution of the vital drought-related meteorological factors associated with the January 2025 Los Angeles wildfires. Observations show that in the second half of 2024, prior to the “25·1” Los Angeles wildfire event, the western United States was in a state of persistent lack of precipitation and dry air, and its precipitation was approximately 60% lower than the climatology (1981-2010), while the vapor pressure deficit (VPD) was 0.33 kPa higher. The standardized anomalies of these two variables reached -1.83 and 2.13, respectively, ranking the second highest in the recent 44 years (1981-2024). Attribution analysis suggests that anthropogenic forcings increase the probability of 2024-like scarce rainfall and atmospheric aridity extremes mainly by elevating the VPD in western United States, with very slight impacts on the rainfall changes. The 2024-like high VPD extremes would almost never occur under natural forcings, while the likelihood increases to 0.012% (uncertainty range: 0.000 46%-0.110 00%) under all forcings. Thus, the anthropogenic forcings largely increase the occurrence of 2024-like compound drought extremes. Our study reveals quantitative contributions of anthropogenic forcings to the key drought-related meteorological factors for regional wildfires under global warming, and provides a scientific basis for the prediction and forecasting of wildfire-associated meteorological conditions and corresponding risk prevention and decision-making.
A catastrophic wildfire on 24 July 2024 erupted in northern California, USA, rapidly intensifying under the synergistic effects of extreme heat and strong winds, leading to significant ecological and socioeconomic losses. Multi-source data, including the drought severity and coverage index (DSCI), ERA5 reanalysis datasets from the European Centre for Medium-Range Weather Forecasts (ECMWF), and sea surface temperature (SST) data provided by the National Oceanic and Atmospheric Administration (NOAA), were utilized to analyze the meteorological conditions and circulation patterns before and after the wildfire outbreak. The findings can provide a scientific basis for wildfire early warning and prevention under extreme climate conditions. The main conclusions are as follows: The July 2024 California wildfire was a result of extreme meteorological conditions, where weeks of persistent high temperatures (daily maximum temperature reaching 32.1 ℃), extremely low humidity (average relative humidity less than 50%), and sustained drought (DSCI greater than 26.0) rendered vegetation highly flammable. Extremely anomalous circulation systems exacerbated the high temperatures and drought, with the North American west coast persistently controlled by high-pressure circulation, and localized strong winds accelerated the spatial spread of the wildfire. The 2024 El Niño event (positive anomaly in the Niño 3.4 region) enhanced the high-pressure system through teleconnection effects, suppressed winter precipitation, prolonged the drought period, and warm sea surface temperatures in the eastern tropical Pacific intensified the Hadley circulation, strengthening the subsidence airflow over California, ultimately forming a coupled “high temperature-low humidity-drought” pattern that significantly increased the wildfire area and duration.
High temperature and drought are key climatic drivers of wildfires. Based on historical wildfire vector data from 1984 to 2023 and multi-source high-resolution climate datasets in California, USA, this study comparatively analyzes the seasonal trends of maximum temperature, maximum and minimum vapor pressure deficit, precipitation and the standardized precipitation evapotranspiration index, and their relationships with burned area in forested regions. The results show that both wildfire frequency and burned area have increased significantly over the past four decades, with summer and autumn being the main fire seasons. Wildfire activity is mainly concentrated in topographically complex and densely vegetated regions, such as the southern Transverse Ranges, the western Coast Ranges, and the northern mountainous areas. Meteorological variables exert a pronounced seasonal influence on wildfire activity, among which maximum temperature, maximum and minimum vapor pressure deficit, during summer exhibit significant positive correlations with burned area, indicating that the co-occurrence of high temperature and drought is the primary climatic driver of frequent and large-scale wildfires. Results from the random forest model further confirm that during the main wildfire seasons, maximum temperature together with maximum and minimum vapor pressure deficits play dominant roles in driving wildfire activity.
