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

Abstract

Floods caused by atmospheric rivers and wildfires fanned by Santa Ana winds are common occurrences in California with devastating societal impacts. In this work, we show that winter weather variability in California, including the occurrence of extreme and impactful events, is linked to four atmospheric circulation regimes over the North Pacific Ocean previously named and identified as the “NP4 modes”. These modes come in and out of phase with each other during the season, resulting in distinct weather patterns that recur throughout the historical record. Some phase combinations favor atmospheric river landfalls and extreme daily or multi-day precipitation, while other phase combinations favor anomalously hot weather and drying Santa Ana wind conditions over Southern California. This historical perspective of atmospheric circulation and impacts over 70 years reveals that weather patterns are changing in a way that enhances wildfire hazard in California, while the frequency of weather patterns linked to historical floods is not diminishing. These changes highlight the rising hazards of cascading weather extremes in California’s present and future.

Midwinter Dry Spells Amplify Post‐Fire Snowpack Decline

“…Guirguis et al. (2022) found increasing frequencies of three midwinter dry patterns that parallel observed declines in California snowpack (Mote et al., 2018). These same atmospheric circulation patterns are associated with the MWDSs in WY2013 and WY2022.…”

“…(c) As in (b) but for WY2013. Primary dry weather regimes (WR) and their frequency derived from atmospheric reanalysis products (Guirguis et al., 2022) during the MWDS of 30 December–18 February of (d) WY2022, and (e) WY2013.…”

“…While both years demonstrate similar weather patterns, WY2013 had slightly more active weather compared to WY2022 (Figure S3 in Supporting Information S1). Because warmer-than-average conditions are expected during WRs 1, 4, and 5 (Guirguis et al, 2022), we also investigated the role of temperature. Tahoe City is influenced by nocturnal inversions, so we compared MWDS temperatures at the mid-slope Echo Summit SNOTEL.…”

“…Analysis of weather regimes (WR) reveals broad similarities between WY2013 and WY2022 during the MWDS (Figures 3d and 3e) and throughout the accumulation season (Figure S3 in Supporting Information S1). The bulk of snow accumulation in December during both years was associated with WR favoring wet conditions and snowfall (Figures S3 and S4 in Supporting Information S1;Guirguis et al, 2022). Beginning in late December (WY2013) or early January (WY2022) a WR shift occurred bringing atmospheric ridging conditions over/offshore from California (Figures S3 and S4 in Supporting Information S1).…”

Autumn precipitation: the competition with Santa Ana winds in determining fire outcomes in southern California

“…Over seven decades of record from 1948 to 2018, substantial precipitation events (defined as 8.5 mm or more over a 3-day interval) occurred, on average, once every 5 years in September, but these events increased in frequency to more than one per year in November, in response to the activation of North Pacific storminess as autumn grades toward winter (Cayan and Peterson 1989). In contrast, days with SAWs occurred, on average, about once every 2 years in September, and increased to more than seven per year in November, reflecting the increase in weather patterns having high pressure air masses over the Great Basin (Gershunov et al 2021;Guirguis et al 2022). The very same windward slopes that accumulate moisture and support dense vegetation experience SAWs (Moritz et al 2010), becoming leeward during these dry gusty north-easterly wind events.…”