Impact of Topography-Induced Precipitation Biases on Summer Temperature Extremes in Western North America

Soil moisture is a critical parameter for understanding summertime temperature variance and the duration of heat waves in semi-arid to arid climates. Given the key role of precipitation in driving antecedent soil moisture anomalies, precipitation biases in general circulation models (GCMs) may obscure the nature of extreme heat events in present and projected climate simulations. Recent work has shown that most GCMs exhibit wet precipitation biases in the western U.S. due in large part to overly-smooth Rocky Mountains and resulting biases in stationary wave patterns and storm tracks. In this study, we explore the impact of this pervasive topographic bias on the regional distribution of summer temperatures and the frequency and duration of extreme heat events. We compare pre-industrial and projected future climate simulations in two GCMs (GFDL CM2.5-FLOR and NASA GISS E2.1) forced with control topography vs. a modified boundary condition that better captures mountain peak height in North America. The modified topographic boundary conditions resolve much of the wet precipitation biases in the western U.S., allowing assessment of evolving heat risks from a more accurate baseline climate state. Surface energy and moisture budget analyses reveal the dominant physical processes by which precipitation-induced soil moisture anomalies impact seasonal temperature extremes. We also reassess projected shifts in heat wave intensity and frequency in response to elevated greenhouse gas conditions in 2xCO2 simulations forced with elevated topography. These results provide important constraints on the effect of common modeled precipitation biases in the climatology and projected future changes of extreme heat events.