Jet stream change driving California’s floods and wildfires – “Recent California fires during wet North Pacific Jet extremes may be early evidence of this change”
4 March 2019 (NCEI) – Deadly severe wildfires in California have scientists scrutinizing the underlying factors that could influence future extreme events. Using climate simulations and paleoclimate data dating back to the 16th century, a recent study looks closely at long-term upper-level wind and related moisture patterns to find clues.
The new research published by the Proceedings of the National Academy of Sciences USA examines jet stream and moisture patterns in California over a centuries-long time period—1571 to 2013—which is nearly four times longer than the instrumental period of record that begins in the latter part of the 19th century. The length of the study enhances the understanding of dynamics that may contribute to extreme impacts from wildfires, as well as precipitation extremes. The work provides a stronger foundation and a longer-term perspective for evaluating regional natural hazards within California and the economic risks to one of the world’s largest economies.
Between 2012 and 2018, several deadly and costly extreme wildfire events impacted California, including some of the state’s largest and most destructive wildfires on record. In 2018, California experienced several of its costliest, deadliest, and largest wildfires to date, according to records that date back to 1933. Such extreme events, which are tracked by NCEI in its Billion-Dollar Weather and Climate Disasters reports, prompt concern for the future.
Each scientist on the research team brought different perspectives and necessary knowledge to the study. These included expertise in paleoclimatology and paleoecology as well as wildfire research. The international, multi-disciplinary approach needed to execute the research underscored the many factors that can contribute to extreme weather and climate events.
The Jet Stream and Moisture
Moisture in California is largely regulated by the strength and position of the North Pacific Jet (NPJ) stream, high-altitude winds that sweep into the state from the west during the cooler wet season. The study evaluated the NPJ between December and February. The strength and position of the winds influence regional conditions that carry over into the warmer dry season, when wildfires are more prone to occur. The wet-season NPJ thus becomes an important precursor of summer fire conditions.
To build a better understanding of the influence of the NPJ over time, scientists focused on winter NPJ variability in a period of over 400 years. Using paleoclimatological and historical data, such as tree rings and historical fire records, past conditions were reconstructed to show connections between the NPJ and moisture and forest fire extremes.
The team wanted to gain a greater sense of conditions before and after fire suppression methods became more standard in 1904. The researchers constructed a list of low- and high-fire years in the Sierra Nevada for 1600–1903 from the paleo records. Extreme instances from both pre- and post-suppression period were then evaluated.
Very recently, 2017 bucked a pattern seen in the longer record. The severe Tubbs and Thomas fires of 2017, a high-precipitation year, overrode the NPJ’s historical relationship with low-fire extremes after cool seasons of very high moisture. Extreme precipitation had compromised the Oroville Spillway earlier that year in addition to bringing about dangerous floods and landslides. Prior to modern fire suppression, the paleoclimatic reconstruction showed no cases of a high-precipitation year coupled with a high-fire year. If warming continues, as is the scientific consensus, then significant wet season rain and snow may not ensure a quiet fire season afterward.
“Recent California fires during wet NPJ extremes may be early evidence of this change,” the paper states.
Besides fire risk and its associated health and economic impacts, such a change could alter species distribution, forest composition, and ecosystems.
Along with NCEI, contributors to the study came from the Helmholtz-Zentrum Geesthacht Centre for Materials and Coastal Research in Germany and the Integrated Climate System Analysis and Prediction (CLiSAP) Cluster of Excellence at the University of Hamburg, The University of Arizona’s Laboratory of Tree-Ring Research, and Penn State University’s Department of Geography and Earth and Environmental Systems Institute.
Reference: Wahl, Eugene R., E. Zorita, V. Trouet, and A. H. Taylor. Jet stream dynamics, hydroclimate, and fire in California from 1600 CE to present. Proceedings of the National Academy of Sciences Mar 2019, 201815292; DOI:10.1073/pnas.1815292116
A Long View of California’s Climate
ABSTRACT: Moisture delivery in California is largely regulated by the strength and position of the North Pacific jet stream (NPJ), winter high-altitude winds that influence regional hydroclimate and forest fire during the following warm season. We use climate model simulations and paleoclimate data to reconstruct winter NPJ characteristics back to 1571 CE to identify the influence of NPJ behavior on moisture and forest fire extremes in California before and during the more recent period of fire suppression. Maximum zonal NPJ velocity is lower and northward shifted and has a larger latitudinal spread during presuppression dry and high-fire extremes. Conversely, maximum zonal NPJ is higher and southward shifted, with narrower latitudinal spread during wet and low-fire extremes. These NPJ, precipitation, and fire associations hold across pre–20th-century socioecological fire regimes, including Native American burning, postcontact disruption and native population decline, and intensification of forest use during the later 19th century. Precipitation extremes and NPJ behavior remain linked in the 20th and 21st centuries, but fire extremes become uncoupled due to fire suppression after 1900. Simulated future conditions in California include more wet-season moisture as rain (and less as snow), a longer fire season, and higher temperatures, leading to drier fire-season conditions independent of 21st-century precipitation changes. Assuming continuation of current fire management practices, thermodynamic warming is expected to override the dynamical influence of the NPJ on climate–fire relationships controlling fire extremes in California. Recent widespread fires in California in association with wet extremes may be early evidence of this change.
SIGNIFICANCE: North Pacific jet stream (NPJ) behavior strongly affects cool-season moisture delivery in California and is an important predictor of summer fire conditions. Reconstructions of the NPJ before modern fire suppression began in the early 20th century identify the relationships between NPJ characteristics and precipitation and fire extremes. After fire suppression, the relationship between the NPJ and precipitation extremes is unchanged, but the NPJ–fire extremes relationship breaks down. Simulations with high CO2 forcing show higher temperatures, reduced snowpack, and drier summers by 2070 to 2100 whether overall precipitation is enhanced or reduced, thereby overriding historical dynamic NPJ precursor conditions and increasing fire potential due to thermodynamic warming. Recent California fires during wet extremes may be early evidence of this change.
Jet stream dynamics, hydroclimate, and fire in California from 1600 CE to present
