Changes in Pacific Northwest Heat Waves under Anthropogenic Global Warming

Though western Oregon and Washington summers are typically mild due to the influence of the nearby Pacific Ocean, this region occasionally experiences heat waves with temperatures in excess of 35ºC. These heat waves can have a substantial impact on this highly populated region, particularly since the population is unaccustomed to and generally unprepared for such conditions. A comprehensive evaluation is needed of past and future heat wave trends in frequency, intensity, and duration. Furthermore, it is important to understand the physical mechanisms of Northwest heat waves and how such mechanisms might change under anthropogenic global warming.

Lower-tropospheric heat waves over the west coast of North America are the result of both synoptic and mesoscale factors, the latter requiring high-resolution models (roughly 12-15 km grid spacing) to simulate. Synoptic factors include large-scale warming due to horizontal advection and subsidence, as well as reductions in large-scale cloudiness. An important mesoscale factor is the occurrence of easterly flow, resulting in an adiabatically warmed continental air mass spreading over the western lowlands rather than the more usual cool, marine air influence. To fully understand how heat waves will change under global warming, it is necessary to determine the combined impacts of both synoptic and mesoscale effects in a warming world.

General Circulation Models (GCM) are generally are too coarse to simulate mesoscale effects realistically, and thus may provide unreliable estimates of the frequency and magnitudes of West Coast heat waves. Typical resolutions of GCMs (100-200 km grid spacing) cannot properly define the terrain and land/water contrasts of the western U.S. Therefore, to determine the regional implications of global warming, this work made use of long-term, high-resolution WRF simulations, at 36- and 12-km resolution, produced by dynamically downscaling GCM grids.

This talk will examine the predicted trends in Pacific Northwest heat wave intensity, duration, and frequency during the 21st century (through 2100). The spatial distribution in the trends in heat waves, and the variability of these trends at different resolutions and among different models will also be described. Finally, changes in the synoptic and mesoscale configurations that drive Pacific Northwest heat waves and the modulating effects of local terrain and land/water contrast will be discussed.