Hot and cold flavors of southern California’s Santa Ana winds: their causes, trends, and links with wildfire

Abstract

Santa Ana winds (SAWs) are associated with anomalous temperatures in coastal Southern California (SoCal). As dry air flows over SoCal’s coastal ranges on its way from the elevated Great Basin down to sea level, all SAWs warm adiabatically. Many but not all SAWs produce coastal heat events. The strongest regionally averaged SAWs tend to be cold. In fact, some of the hottest and coldest observed temperatures in coastal SoCal are linked to SAWs. We show that hot and cold SAWs are produced by distinct synoptic dynamics. High-amplitude anticyclonic flow around a blocking high pressure aloft anchored at the California coast produces hot SAWs. Cold SAWs result from anticyclonic Rossby wave breaking over the northwestern U.S. Hot SAWs are preceded by warming in the Great Basin and dry conditions across the Southwestern U.S. Precipitation over the Southwest, including SoCal, and snow accumulation in the Great Basin usually precede cold SAWs. Both SAW flavors, but especially the hot SAWs, yield low relative humidity at the coast. Although cold SAWs tend to be associated with the strongest winds, hot SAWs tend to last longer and preferentially favor wildfire growth. Historically, out of large (> 100 acres) SAW-spread wildfires, 90% were associated with hot SAWs, accounting for 95% of burned area. As health impacts of SAW-driven coastal fall, winter and spring heat waves and impacts of smoke from wildfires have been recently identified, our results have implications for designing early warning systems. The long-term warming trend in coastal temperatures associated with SAWs is focused on January–March, when hot and cold SAW frequency and temperature intensity have been increasing and decreasing, respectively, over our 71-year record.

Winter wet–dry weather patterns driving atmospheric rivers and Santa Ana winds provide evidence for increasing wildfire hazard in California

“…Downslope Santa Ana winds (SAWs) are characteristically hot, dry, offshore winds that bring fire weather to coastal and inland Southern California (Abatzoglou et al 2021). SAWs are linked with Southern California's most destructive wildfires (Westerling et al 2004;Moritz et al 2010;Kolden and Abatzoglou 2018;Gershunov et al 2021). Typically, the SAW season begins in fall and peaks in December (Guzman Morales et al 2016).…”

“…These winds warm by adiabatic compression as air flows from above the elevated Great Basin, ascends over the 3000 m high Transverse and Peninsular Ranges or is funneled through mountain passes and ultimately descends down to sea level. This warming leads to coastal heat waves in the fall, winter and spring (Gershunov et al 2021). The associated hot, dry, and windy conditions during periods of low fuel moisture in fire-prone ecosystems creates substantial fire hazard in wildland and adjacent communities.…”

Decline in Seasonal Snow During a Projected 20-Year Dry Spell

“…As the contribution of snowmelt to runoff declines [70], warm season recreation and tourism suffers from low river flows, air quality reductions from local or long-range transport, as well as forest, road, and trail closures from increasingly large and frequent wildfires moving upslope into the seasonal snow zone (Figure 4; [6,23,71]). Snowpack loss over the Great Basin and changing synoptic circulations suggests increasing chances of hot and fire-favoring fall and winter season Santa Ana winds [24,72], which bring forth numerous negative economic and health impacts on large populations [73,74]. Additional reductions in streamflow from major rivers once flowing into the Sacramento-San Joaquin River Delta, particularly during the dry and extended warm season (June-September; [75]) will exacerbate the existing suite of widespread problems arising due to changes in ecosystem health, water quality, water availability and electricity generation [11,76,77].…”

Wildfires

“…The southern part of the state is subject to the powerful Santa Ana winds that cause fires to spread three times faster than they normally would. These fires tend to occur closer to urban areas and are related to approximately 80% of the economic damage caused each year 12,13…”

“…These fires tend to occur closer to urban areas and are related to approximately 80% of the economic damage caused each year. 12,13 Ironically, the act of fire suppression also increases the region' s vulnerability. Putting out fires often leaves behind the dry vegetation that would have been consumed, thus increasing the future risk of a large fire.…”