Rising trends in urban heatwave metrics and their climate drivers in Southern California, from 1950–2020

This study examines climate drivers influencing urban heatwave metrics and interannual variability in Southern California from 1950–2020. The frequency, duration, and intensity of urban heatwaves are rapidly increasing with a greater tendency towards more humid and intense nighttime events at a rate of ~1°C/decade since the 1980s, elevating heat stress and mortality risk. Increased humidity is associated with an anomalous moisture source off the coast of Baja California, which intensified over the past decade, and linked to marine heatwaves and changes in the California current system. The interannual variability in heatwave events shows significant correlations between Pacific Decadal Oscillation modulations and heatwave frequency and duration for urban areas, irrespective of El Niño Southern Oscillation phases. The coupling of heatwave duration-intensity strengthens from the 1990s, with a longer duration occurring above a ~3°C intensity rise. Droughts and urban heatwaves are strongly linked, with a high statistical probability of increase in heatwaves frequency (42%), duration (26%), and daily mean temperature (2.2%) during severe drought conditions. Heatwaves now persist later in the year during peak fire season, potentially intensifying wildfires by enhancing aridity and drying out fuels. This can be disastrous when they coincide with strong Santa Ana winds as experienced in January 2025 across Altadena and Pacific Palisades in Los Angeles county. Better understanding of heatwave climate drivers and underlying physical processes could help with prediction skill and provide effective data‐driven recommendations for climate mitigation strategies in Southern California cities.