The relationship between large-scale atmospheric circulations, extreme meteorological conditions, and the severe wildfire outbreak in California, USA during July 2024

doi:10.11755/j.issn.1006-7639-2025-05-0678

Abstract

Abstract:

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.

Key words: California wildfire, extreme meteorological conditions, high temperature and drought, subtropical high, El Niño

摘要：

2024年7月加州北部爆发的特大山火在极端高温与强风的共同作用下迅速蔓延，造成严重损失。本文利用美国干旱监测的干旱程度和面积指数（Drought Severity and Coverage Index，DSCI）、欧洲中期天气预报中心（European Centre for Medium-Range Weather Forecasts， ECMWF）发布的ERA5（ECMWF Reanalysis v5）再分析资料和美国国家海洋和大气管理局（National Oceanic and Atmospheric Administration，NOAA）提供的海温等多源数据，分析山火爆发前后的气象条件与环流背景，可为极端气候背景下山火预警与防控提供科学依据。结果表明：2024年7月加州山火爆发是极端气象条件导致的结果，持续数周的高温（日最高气温达32.1 ℃）、极低湿度（平均相对湿度<50%）和持续干旱（DSCI>26.0）使植被高度易燃。环流系统极端异常加剧了高温干旱，北美西海岸持续受高压环流控制，局地强风加速了山火的空间传播。2024年厄尔尼诺事件（Niño 3.4区正异常）通过遥相关作用增强高压系统，抑制冬季降水并延长干旱期，热带东太平洋暖海温使得哈德莱环流增强，加强了加利福尼亚州的下沉气流，最终形成“高温-低湿-干旱”的耦合模式，显著增加山火面积和持续时间。

关键词: 加利福尼亚山火, 极端气象条件, 高温和干旱, 副热带高压, 厄尔尼诺

CLC Number:

P467

ZHAO Cailing, YANG Jinhu, YUE Ping, YAN Pengcheng, LI Yiping, LI Hong, LI Danhua. The relationship between large-scale atmospheric circulations, extreme meteorological conditions, and the severe wildfire outbreak in California, USA during July 2024[J]. Journal of Arid Meteorology, 2025, 43(5): 678-688.

赵采玲, 杨金虎, 岳平, 颜鹏程, 李忆平, 李红, 李丹华. 2024年7月美国加利福尼亚州山火与干旱和高温等极端气象条件的关系及大气环流成因[J]. 干旱气象, 2025, 43(5): 678-688.

Figures/Tables 9

Fig.1 State boundary （black solid line） and topography （the color shaded， Unit： m） of California， USA （The red dot marks the wildfire ignition point）

Fig.2 The spatial distribution of differences between average geopotential height field （black contour lines， Unit： dagpm） and temperature field （red contour lines， Unit： ℃） at 500 hPa from June 29 to July 3 （a）， July 4 to July 8 （b）， July 9 to July 13 （c）， July 14 to July 18 （d）， July 19 to July 23 （e）， 2024 and the climatological state from 1991 to 2020 （The solid lines denote positive anomalies， dashed lines denote negative anomalies）

Fig. 3 The spatial distribution of differences between the 700 hPa average specific humidity field （Unit： kg·kg-1） from June 29 to July 3 （a）， July 4 to July 8 （b）， July 9 to July 13 （c）， July 14 to July 18 （d）， July 19 to July 23 （e）， 2024 and the climatological state from 1991 to 2020

Fig.4 The latitude-time sections of the average vertical wind fields （arrows， Unit： m·s-1）， total horizontal wind speed （contours， Unit： m·s-1）， and divergence （the color shaded， Unit： 10-6 s-1） from June 29 to July 3 （a）， July 4 to July 8 （b）， July 9 to July 13 （c）， July 14 to July 18 （d）， July 19 to July 23 （e）， 2024

Fig.5 Monthly sea surface temperature （SST） anomalies in the Ni?o 3.4 region from January to August 2024

Fig.6 Weekly variations of DSCI in California in 2024 （The black dashed line denotes the wildfire outbreak date， the same as below）

Fig.7 Daily variations of mean temperature， daily maximum temperature， relative humidity （a） and precipitation （b） in California， USA from April to October 2024

Fig.8 Daily variations of 10 m wind speed in California， USA from July to August 2024.

Fig.9 Changes in aerosol optical depth （AOD） before and after the outbreak of wildfire in California on 23 （a） and 31 （b） July， 2024

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