Abstract
The most destructive wildfires recently in Northern California have been linked to the occurrences of Diablo Winds (DWs). This study investigates the climatology of DWs during September–December 1979–2018, and their relationships with various climate modes using observations and two high-resolution reanalysis datasets. Our finding shows that DWs do not have a long-term trend in terms of the annual total number, total duration, and associated maximum wind speeds of DWs over the past 4 decades. However, their associated minimum relative humidity (RH) has decreased significantly, especially in October, which suggests that the dryness during DWs has become more severe with time, possibly leading to an increased chance of fires, and their destructive potential. We also find that the annual total number and duration of DWs have exhibited an quasi-periodic variation, with intervals ranging from 2 to 4 years. The periodic variability of DWs might be attributed to the teleconnections between DWs and climate oscillations, specifically the El Niño–Southern Oscillation (ENSO) and the quasi-biennial oscillation (QBO), through their modulation of pressure systems near California and the location of the Pacific jet stream. It is suggested that when La Niña and the QBO westerly phases co-occur in the spring, DWs in the following fall and winter tend to occur more frequently, and are associated with more intense high winds and dryness as compared to other springtime QBO and ENSO phases. This relationship may be used to predict the seasonal outlook of DWs.
Highlights
IntroductionAs the population grows and wildland and urban interface areas continue to increase (Williams et al 2019), wildfires are becoming an ongoing threat to many of these communities as they adversely affect regional air quality and lead to loss of life and property
California with its Mediterranean climate is a fire-prone state
The climatological values for the event number, duration, associated maximum surface wind speed, and minimum surface relative humidity (RH) based on the past 40 years data are two events per year, 52 h per year, 11 m s−1 and 9%, respectively, from North American Regional Reanalysis (NARR) and two events per year, 35 h per year, 6 m s−1, and 11%, respectively, from ERA5. Both NARR and ERA5 reanalysis datasets consistently show no discernible long-term trends in the past 4 decades for the annual total number and duration of Diablo Winds (DWs) because except for Pearson’s linear correlations from NARR, P values associated with the other coefficients are larger than 0.05 (Fig. 4a and b, and Table S4)
Summary
As the population grows and wildland and urban interface areas continue to increase (Williams et al 2019), wildfires are becoming an ongoing threat to many of these communities as they adversely affect regional air quality and lead to loss of life and property. Top 20 most destructive wildfires in California history, six of them—the Camp Fire (2018), the Tubbs Fire (2017), the Tunnel Fire (1991), the Nuns Fire (2017), the Atlas Fire (2017), and the Redwood Valley Fire (2017)—have been linked to the occurrences of Diablo Winds (DWs). DWs play two important roles in wildfire activities: (1) causing ignition by pulling down power lines or poles, and (2) rapidly spreading fires and increasing the fire size (Mass and Ovens 2019; Monteverdi 1973; Pagni 1993; Smith et al 2018). Several DW events in 2019 prompted the Pacific Gas and Electric Company to shut-off power in several areas of the San Francisco Bay area (SFBA), affecting the daily lives of many residents
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