Investigating the geographic controls of the North American monsoon in the Community Atmosphere Model

This work examines the influence of topography on the dynamics of the North American monsoon (NAM), including the timing and large-scale characteristics of the monsoon. The monsoon season occurs from July to September in the southwestern United States and northwestern Mexico. Characterized by an increase in rainfall, the NAM accounts for 40 to 80% of the total annual rainfall with the greatest influence over the western foothills of the Sierra Madre Occidental. The western United States and northern Mexico Plateau create a monsoonal circulation pattern over the United States and Mexico. Moisture for the monsoon is advected from the Gulf of California, eastern Pacific and Gulf of Mexico. These geographic and topographic features are unique to the NAM and essential for the development and maintenance of the monsoon. A better understanding of the monsoon and the impact topography has on the circulation will allow for a more accurate representation of the monsoon in projections of future climate. Using a simple “monsoon index” we find that simulations with the Community Atmosphere Model capture the essential nature of the NAM. Comparing standard low-resolution (1o latitude x 1o longitude) simulations where the topography over North America is either retained or removed we evaluate the models’ representations of the NAM through the analysis of the moist static energy budget. Preliminary results from simulations where the topography is retained indicate that the simulated NAM is driven by locally-generated convection, with advection processes being secondary; this is consistent with the hypothesis that the NAM is a result of the thermal contrast between the hot, summertime continent and cool ocean. It is expected that in the simulation without topography the NAM will be relatively weak and driven primarily by locally-generated convection.