The 2012-2016 California (CA) drought culminated in a 500-year snowpack low in 2015. Such severe droughts pose persistent threats to the economy and environment, including water shortages for urban and agriculture consumers, low hydroelectric power generation, and enhanced risk of wildfire and tree mortality. Extreme droughts in CA occur when precipitation deficits coincide with high temperature extremes and such concurrences have become more frequent with rising temperatures in recent decades. In this context, the role of high pressure blocking ridges in reducing moisture delivery from the Pacific Ocean has been extensively evaluated, but the climatological role of the north polar jet (NPJ) stream in fostering dry and fire conditions has received less attention. Here we present independent tree-ring based reconstructions of NPJ behavior, precipitation, and fire activity in CA over the past 500 years. Past dry and fire extremes are strongly associated with a weakening, reduction of southward extent, and enhanced meridional flow of the NPJ in the northeastern Pacific. Similarly, wet and no-fire conditions are strongly associated with a strengthening, greater southward extension, and enhanced zonal flow of the NPJ in this region, allowing enhanced northern Pacific storm track activity and tropical moist airflow into CA. These strong associations allow evaluation of potential changes in the likelihood of extreme drought and fire years through the rest of the 21st century as anthropogenic climate change progresses, based on NPJ conditions extracted from climate model scenarios. I will further discuss past socio-ecological influences on CA fire regimes and will put our results in the context of current CA hydroclimate conditions.
By: Valerie Trouet, Associate Professor, University of Arizona
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