Substantial cold bias during wintertime cold extremes in the southern
Cascadia region in historical CMIP6 simulations
Abstract
Global climate models often simulate atmospheric conditions incorrectly
due to their coarse grid resolution, flaws in their dynamics, and biases
resulting from parameterization schemes. Here we document the magnitude
and extent of minimum temperature biases in the CMIP6 model ensemble,
relative to ERA5. Bias in the southern Cascadia region (i.e. Pacific
Northwestern United States and southwestern British Columbia, Canada,
spanning from the coast to the Rocky Mountains) stands out relative to
the rest of North America, with some models showing a bias in excess of
-10°C in the 1st percentile of daily winter minimum temperature. During
the coldest minimum temperature days, the CMIP6 models show an anomalous
high in mean sea level pressure in the Northeast Pacific – an
atmospheric blocking pattern that is also present in ERA5. While this
atmospheric blocking pattern is typically concurrent with cold
temperatures across much of North America, terrain barriers such as the
Rockies and Cascades prevent the cold air from reaching the Pacific
Northwest in observation and reanalysis. Our results suggest that the
bias in CMIP6 minimum temperatures is a result of unresolved topography
in the Rockies and Cascade mountain ranges, such that the terrain does
not adequately block cold air advection from the interior of the
continent.