The Evolution of “Hot” Droughts in Southern California, USA from the 20th to the 21st Century

Abstract:
<p>Drought intensity and duration in southern California, USA during the 21^st century has been exceptional, and the changing climate dynamics of drought in this arid and semiarid region of North America have been linked to anthropogenic warming. We examine the frequency, intensity, and persistence of drought conditions in southern California’s two climatic divisions during 1900–2022 and use the monthly instrumental record of the Palmer Drought Severity Index (PDSI) to track how drought events have changed. We introduce an empirical definition of “hot drought” by combining mean summer PDSI values with standardized scores of mean summer minimum temperature. By all measures we examined, droughts in southern California, USA have become increasingly more severe, frequent, and long-lived in the 21^st century. Our decadal analysis showed that moderate drought conditions (PDSI &lt; -2) were recorded during more than half of the months in the decade ending in 2020, and one drought event in this decade persisted for 4.6 years. Similarly, hot droughts, droughts associated with both water deficits and significantly above-normal temperatures, have become increasingly frequent in recent decades. The first hot drought in southern California was not recorded until 1959, but by the 21^st century, hot droughts were occurring in approximately half of the summers. Our findings support the idea that anthropogenic warming results in a changing drought climatology for arid and semiarid regions of southern California and that hot droughts will likely become the dominant drought type if trends toward warmer and drier conditions continue for western North America.</p>

Keywords: Drought, Hot droughts, Southern California USA, Palmer Drought Severity Index, 500 hPa geopotential height anomalies

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