The Influence of North American Topography on Atmospheric Winter Circulation and Precipitation

Abstract It has long been known that North America’s mountains play a key role in shaping Northern Hemisphere atmospheric winter circulation and precipitation. However, much remains to be understood within this subject. Here, we further investigate how North America’s topography alters winter (December–March) circulation and precipitation by performing and exploring a series of general circulation model experiments with altered topography over North America and analyzing observations. We find that the mountains of western North America divert the large-scale midlatitude westerly flow, leading to an anomalous ridge over the northern Rockies and a broad anomalous trough extending from southwest of the United States to eastern North America and the northwest Atlantic, contributing significantly to the observed high-amplitude atmospheric wave pattern over North America. The blocking and diversion of large-scale subtropical easterly flow by Mexico/Central America’s mountains also have a substantial impact on large-scale circulation, contributing to a realistic separation of the North Atlantic and Pacific subtropical anticyclones. North American topography leads to strongly enhanced precipitation along the western slopes of the Rocky Mountains and eastern slopes of Central America where the large-scale low-level flow is forced upslope, with a drying over the interior of North America and a notable wetting over the southeast United States, where precipitation is increased by more than 100% in some areas when realistic topography is incorporated. In this work, we perform an in-depth dynamic analysis of our topography experiments and show how the simulated impact of North America’s topography is consistent with observations. Significance Statement This work seeks to improve our understanding of how North America’s mountains shape regional climate during the Northern Hemisphere winter. Our results suggest North America’s topography contributes substantially to observed east–west asymmetries in atmospheric circulation, temperature, and precipitation near North America. Thus, topography–circulation interactions should be taken into careful consideration in attempts to better understand regional patterns of long-term climate variability and future projections due to anthropogenic forcing. Additionally, climate model deficiencies in simulating topography–circulation interactions, such as due to artificially smoothed topography in coarse model grids, should be further explored as a source of climate model biases.