Wahl Eugene R, Zorita Eduardo, Trouet Valerie, Taylor Alan H
Center for Weather and Climate, National Centers for Environmental Information, National Oceanic Atmospheric Administration, Boulder, CO 80301;
Institute of Coastal Research, Helmholz-Zentrum Geesthacht, 21502 Geesthacht, Germany.
Proc Natl Acad Sci U S A. 2019 Mar 19;116(12):5393-5398. doi: 10.1073/pnas.1815292116. Epub 2019 Mar 4.
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.
加利福尼亚州的水分输送在很大程度上受北太平洋急流（NPJ）的强度和位置调节，冬季高空风会影响随后暖季的区域水文气候和森林火灾。我们利用气候模型模拟和古气候数据重建了公元1571年以来冬季NPJ的特征，以确定NPJ行为在近期火灾抑制之前和期间对加利福尼亚州水分和极端森林火灾的影响。在火灾抑制前的干旱和高火极端时期，NPJ纬向最大风速较低且向北偏移，纬向跨度较大。相反，在湿润和低火极端时期，NPJ纬向最大值较高且向南偏移，纬向跨度较窄。这些NPJ、降水和火灾之间的关联在20世纪前的社会生态火灾状况中都存在，包括美洲原住民的用火、接触后的破坏和原住民人口减少，以及19世纪后期森林利用的强化。在20世纪和21世纪，极端降水和NPJ行为仍然相关，但由于1900年后的火灾抑制，极端火灾变得不再相关。模拟的加利福尼亚州未来状况包括更多作为降雨（而非降雪）的湿季水分、更长的火灾季节和更高的温度，导致与21世纪降水变化无关的更干燥的火灾季节条件。假设目前的火灾管理做法持续下去，热力变暖预计将超越NPJ对控制加利福尼亚州极端火灾的气候-火灾关系的动力影响。加利福尼亚州近期与极端湿润相关的广泛火灾可能是这种变化的早期证据。
