Climate Change Impacts on Mountains in Coastal Santa Barbara Based on 30 Years of High Resolution Downscaling

Abstract:

Coastal Santa Barbara County (SB) is among the most exposed communities to wildfire hazards in southern California. The Santa Ynez Mountains (SYM) rise abruptly from coastal SB separating the Pacific Ocean on its south face from the Santa Ynez Valley (SYV) on its north face. The SYM is influenced by the semi-stationary North Pacific anticyclone, and receives most of the rainfall from frontal systems and atmospheric rivers during the winter months. Downslope windstorms (“sundowner winds”), which intensify after sunset, are frequently observed on the southern-facing slopes of the SYM. Sundowners are responsible for the fast spread of wildfires toward populated areas in SB. The number of wildfires has increased 6 times in the last 20 years compared to the last 50 years of the 20^th century. Although ignitions are mostly of anthropogenic origin, changes in circulation, temperature and humidity have played a significant role in increasing the probability of major wildfires. We use 30yr of downscaling with the Weather Research & Forecast (WRF) model at 1km grid resolution and 1 hour temporal resolution to identify trends in atmospheric variables near surface and in the lower troposphere. We show significant trends in fire weather conditions during the fire season (July-November), and these trends are seasonally dependent. During summer, significant warming trends increase with topographic elevation. During fall, these trends are significant on the coast and foothills of the mountains. We found that trends in 2m temperature are largely linked to the decrease in mesoscale circulation (sea-breeze) that result in less thermal contrast between mountain slopes and the free atmosphere, reduced upslope winds and enhanced temperature at mountain tops. In the fall, conversely, we observe positive trends in anabatic winds, which offset the warming at the top. Enhanced temperature in the foothills of the mountains is explained by enhanced katabatic winds. Moreover, we investigate how these trends are linked to the characteristics of mountain waves. We show that trends in circulation result from large-scale (rather than regional scale) climatic forcing. Patterns in temperature trends, on the other hand, are explained by local feedback processes.