Compounding Risk Factors Affecting Heatwave Severity in Southern California Urban Regions

Research on heatwaves has gained significant impetus over the past decade due to a warming planet and rapid 21^st century urbanization. This study investigates compounding risk factors that influence heatwaves across Southern California (SoCal) urban areas. We show that heatwave severity characteristics (frequency, duration, intensity) are strongly linked to nighttime minimum temperatures, urbanization, humidity, and severe drought. Over the past century the mean and variance of urban heatwaves in SoCal urban areas have increased at a rapid rate to present day average values of ~5±2 heatwaves per year and duration of 6±4 days in urban areas. Heatwave apparent nighttime temperatures in SoCal urban areas have increased at ~1°C/decade since the 1980’s driven by an anomalous moisture source off the Baja California coast linked to sea surface warming trends in the region. Coastal urban areas experienced a rapid three-fold increase in heatwave trends over the past two decades, with more heatwaves occurring later in the year during the fire season and exposing densely populated communities along the coast to greater risk. Droughts and heatwaves were found to have strong linkages, particularly for inland urban areas that have high statistical probability of heatwaves doubling in number and duration during severe drought conditions. Heatwave nighttime intensification and increasing humidity are linked to signals of anthropogenic climate change and mean warming that is anticipated to continue and accelerate through the 21st century. Better understanding of heatwave climate drivers and underlying physical processes will improve future predictions and help advise effective adaptation and mitigation strategies in vulnerable urban regions. To this end we further show collaborative work with the Los Angeles (LA) County Sustainability office in developing a future climate sustainability plan for LA. We will show examples of heat vulnerability index maps of LA throughout the diurnal cycle derived from high resolution ECOSTRESS thermal data, and the ability to pinpoint persistent hotspots in the LA region to help guide implementation of cooling technologies and planting more trees.