Mechanical forcing of the North American monsoon by orography

The core of the North American monsoon (NAM) is characterized by a band of intense rainfall along the western coast of Mexico. This rainfall band is commonly understood to be caused by thermal forcing from both the elevated terrain of that region (Sierra Madre) and land. This fits into the general paradigm of monsoons as thermally direct circulations driven by heat sources. In this talk, instead, we show that the NAM rainfall is generated by the interaction of the extratropical jet stream with mountain ranges. We reach this conclusion using observations, a high-resolution global climate model, and stationary wave solutions to show that the NAM core rainfall band arises when Mexico’s Sierra Madre mountains mechanically force an adiabatic stationary wave through the equatorward diversion of extratropical westerly winds; westerly, upslope flow associated with that stationary wave then lifts warm and moist air to cause convective rainfall. Heat fluxes at the surface precondition the atmosphere for convection, especially in summer afternoons, but they alone are insufficient to produce the observed rainfall maximum.

 

These results, together with dynamical structures in observations and models, indicate that the core monsoon should be understood as convectively enhanced orographic rainfall in a mechanically forced stationary wave, not as a classic, thermally forced tropical monsoon. This has implications for the response of the NAM to past and future global climate change, making trends in jet stream interactions with orography of central importance.