Future Trends in Extreme Temperatures over the Western U.S.: The Influence of Terrain and Coastlines

A number of studies have suggested that heat waves will increase in frequency, intensity, and duration under anthropogenic global warming. However, these trends are less understood in regions of complex terrain or in coastal regions where temperature gradients are relatively larger and mesoscale features are important. Thus it is important to understand the regional impacts of terrain and land/water contrasts on heat extremes in a warming world, and identify changes in the conditions that drive such extremes. The northwest U.S. is a region with such surface complexity, where regional heat extremes arise from synoptic/mesoscale interactions between the large-scale flow and local topography.

This talk will analyze changes to the synoptic and mesoscale conditions associated with heat extremes over the northwest U.S. by utilizing CMIP5 global climate model simulations, and dynamically downscaled regional climate models. An analysis of changes in mid- to low-level tropospheric temperature, offshore/onshore flow, soil moisture, precipitation, and cloud fraction are described, as well as how these changes interact with regional terrain and land/water contrasts to alter the temperature distributions over the region.

CMIP5 models suggest that low-level zonal wind distributions over the northwest U.S. become narrower, leading to fewer days with strong offshore flow. Soil moisture is projected to decline over the region due to reductions in clouds and precipitation, as well as general warming. Historical and future regional temperature distributions will be described, and it will be shown that even though climate models predict increases in heat extremes for western Oregon and Washington, these increases are far fewer compared to inland areas. The variance over most inland locations increases, with the exception of the northwest U.S., where variance doesn't change. The importance of regional terrain and land/water contrasts in explaining these changes will be described.