Exploring Factors Influencing Large-Scale Atmospheric Circulation and Precipitation over North America

Abstract

The observed large-scale atmospheric circulation and precipitation pattern over North America consists of substantial zonal (east-west) asymmetries. Despite such a pattern's relevance to regional climate, a thorough explanation behind how such features arise remains somewhat lacking in existing literature. Here we further investigate the dynamical causes of zonal asymmetries in atmospheric circulation and precipitation over North America by performing and exploring a series of global climate model experiments with altered geography and land surface characteristics over North America and analyzing observations. In Chapter 2, we explore the role of the Gulf of California in the North American monsoon by comparing a realistic climate simulation with a water-filled Gulf of California to one where the Gulf of California is instead replaced with flat land. We find that the Gulf of California enhances North American monsoon precipitation, and the results provide numerous additional insights on the nature of the North American monsoon. In Chapter 3, we examine how North America's topography shapes the atmospheric circulation and precipitation pattern over North America and beyond during the cold season (December-March). Here, we perform and analyze a series of climate model simulations with altered topography over North America and show how the simulated impact of topography is consistent with observations. We find that North America's topography mechanically diverts the large-scale midlatitude westerly flow and subtropical low-level easterly flow meridionally, contributing substantially to the observed high-amplitude atmospheric wave pattern and zonal asymmetries in mean precipitation over North America. In Chapter 4, we return to the warm season and investigate how topography and land surface heating shape the summer atmospheric circulation and precipitation pattern over North America. We use the topography experiments performed in Chapter 3 and perform additional land albedo perturbation experiments. We find that both topography and land surface heating are essential. Whereas the northern Rockies primarily mechanically divert the large-scale midlatitude westerly flow so as to contribute to the observed ridge-trough pattern over northern North America, we explain how the circulation over the southern Rockies is largely driven by strong surface sensible heating over the region's sloping terrain. As such, in contrast to some recent work, we argue that well-known features such as the Great Plains low-level jet and North American monsoon can be largely characterized as thermally-driven.