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快乐十分前三组窍门:Jet stream dynamics, hydroclimate, and fire in California from 1600 CE to present
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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.
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.
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Author contributions: E.R.W., E.Z., and V.T. designed research; E.R.W. and E.Z. performed research; E.R.W., E.Z., V.T., and A.H.T. analyzed data; and E.R.W., E.Z., V.T., and A.H.T. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Data deposition: Data are archived with the National Oceanic and Atmospheric Administration Paleoclimatology/World Data Service for Paleoclimatology, https://www.ncdc.noaa.gov/paleo/study/26030.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1815292116/-/DCSupplemental.
Published under the PNAS license.