Mechanical forcing of the North American monsoon by orography.

William R. Boos,Salvatore Pascale

doi:10.1038/s41586-021-03978-2

Abstract

A band of intense rainfall extends more than 1,000 km along Mexico's west coast during Northern Hemisphere summer, constituting the core of the North American monsoon1,2. As in other tropical monsoons, this rainfall maximum is commonly thought to be thermally forced by emission of heat from land and elevated terrain into the overlying atmosphere3-5, but a clear understanding of the fundamental mechanism governing this monsoon is lacking. Here we show that the core North American monsoon is generated when Mexico's Sierra Madre mountains deflect the extratropical jet stream towards the Equator, mechanically forcing eastward, upslope flow that lifts warm and moist air to produce convective rainfall. These findings are based on analyses of dynamic and thermodynamic structures in observations, global climate model integrations and adiabatic stationary wave solutions. 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. Our results indicate that the core North American 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.

Highlights

Tropical monsoons occur when a surface of low heat capacity transfers the energy of intense summer solar radiation to the overlying atmosphere, creating thermally direct, precipitating flow
The core North American monsoon (NAM) consists of a narrow tongue of high precipitation stretching over 1,000 km north-south along the western side of the Sierra Madre Occidental (SMO) mountains (Fig. 1a)
One early study stated that it was difficult to determine whether mechanical or thermal effects of orography dominated in forcing the NAM19, most previous work has described the NAM as either (i) similar to though smaller in scale than the South Asian monsoon[15 ], with a central role played by elevated plateau heating[3 ], or (ii) caused by land-ocean thermal contrast[4, 8, 33]

Summary

Introduction

Tropical monsoons occur when a surface of low heat capacity transfers the energy of intense summer solar radiation to the overlying atmosphere, creating thermally direct, precipitating flow. Such circulations supply water to billions of people and set the climate of large swaths of Earth’s surface. The North American monsoon (NAM) is commonly viewed in this paradigm, being a low-latitude summer circulation crucial for the hydrology of western Mexico and the southwestern US1, 2, 6, 7. The core NAM consists of a narrow tongue of high precipitation stretching over 1,000 km north-south along the western side of the Sierra Madre Occidental (SMO) mountains (Fig. 1a). Drier conditions lie east of this precipitation maximum, in central

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