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Jet stream interactions with tropical circulations: the case of the North American monsoon

Presentation Date
Tuesday, December 14, 2021 at 10:45am
Convention Center - Room 294-296



Tropical monsoons are widely understood to be thermally direct circulations driven by heat sources. In this talk we show, for the specific case of North America, how what is commonly called monsoon rainfall instead requires the interaction of the extratropical jet stream with mountain ranges to organize convective rainfall. The core of the North American monsoon consists of a band of intense rainfall along the west coast of Mexico, and is commonly thought to be caused by thermal forcing from both land and the elevated terrain of that region. We use observations, a global climate model, and stationary wave solutions to show that this rainfall maximum is instead generated when Mexico’s Sierra Madre mountains mechanically force an adiabatic stationary wave by diverting extratropical eastward winds toward the equator; eastward, upslope flow in that wave lifts warm and moist air to produce convective rainfall. Land surface heat fluxes do precondition the atmosphere for convection, particularly in summer afternoons, but these heat fluxes alone are insufficient for producing 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 North American monsoon to past and future global climate change, making trends in jet stream interactions with orography of central importance.

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