Compound heat wave, drought, and dust events in California

Abstract California is one of the nation’s top agriculture producers and is vulnerable to extreme events such as droughts and heat waves. Concurrent extreme events may further stress water and energy resources, exerting greater adverse socioeconomic, environmental, and health impacts than individual events. Here we examine the features of compound drought, heat wave, and dust events in California during spring and summer. From 2003 to 2020, 16 compound events are found in warm seasons, with a mean duration of about four days. Compound events are characterized by enhanced surface temperature up to 4.5 °C over northern and western California, reduced soil moisture and vegetation density, and an increase in dust optical depth (DOD) by 0.05 to 0.1 over central to southern California. The enhanced DOD is largely associated with severe vegetation dieback that favors dust emissions and southeasterly wind anomalies that support northward transport of dust from source regions in southern California. Surface fine dust and PM2.5 concentrations also increase by more than 0.5 and 5 μg m-3, respectively, during compound events associated with both enhanced dust emissions and a relatively stable atmosphere that traps pollutants. The development of the compound events is related to an anomalous high over the west coast in the lower to middle troposphere, a pattern favoring sinking motion and dry conditions in California. The anomalous high is embedded in a wave train that develops up to seven days before the events. In comparison with heat wave extremes alone, compound events show significantly higher DOD and lower vegetation density associated with droughts.