Insights on the North American Monsoon from Climate Simulations with Altered Topography

The North American Monsoon (NAM) is a distinct atmospheric circulation responsible for summer rainfall maxima in northwest Mexico and the southwest US. Here we explore the role of topography in generating the NAM. We use a series of 50 km climate model simulations with altered topography and compare to dynamic and thermodynamic analyses of the observed NAM system using reanalysis products. Simulations include a flattening and raising of North American topography, regional flattening of the United States and Central America, and raising/lowering of the Baja Peninsula. In agreement with previous model studies, as well as the observed local precipitation enhancement near orography, simulated NAM precipitation is strongly enhanced by regional topography. A clear signal of vertical ascent over land and subsidence over ocean in the NAM region related to land-sea thermal contrast is present in flattened orography experiments, but topography shapes this ascent and increases the vertical extent so as to induce concentrated upslope flow of moist air, leading to locally high precipitation. The strong seasonal contrast of circulation changes due to orography in our simulations, observed regional thermodynamic/dynamic structure, and robust seasonal and diurnal cycles suggest a strong thermal component to the NAM circulation and precipitation. This is further evidenced in a simulation with raised topography over the Baja Peninsula, in which there is a strong enhancement of southeasterly surface flow up the Gulf of California, and a large increase in precipitation in far northwestern Mexico and the southwest US during the NAM season, and near-reversal of these changes during the cold season.