Rain over tropical continents: linear theories, abrupt changes, and monsoons

Imparte: William Boos, University of California, Berkeley, USA

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onsoons deliver water to billions of people in Asia, Africa, and the Americas, so we want to know how forcings such as changes in greenhouse gas concentrations, atmospheric aerosols, and land use might affect regional monsoon rainfall.  Furthermore, we want to know if a slow, gradual change in one of these forcings might cause a large, rapid, and thus catastrophic change in monsoon strength.  Here I present a two-dimensional theory for regional tropical rainfall shifts that is based on the atmospheric energy budget, then demonstrate its relevance to understanding tropical rainfall shifts observed during the El Nino-Southern Oscillation and the mid-Holocene, the latter being a period when changes in Earth’s orbit produced enough rainfall over northern Africa to support vegetation in what is now the Sahara Desert.  Using related energy-budget arguments, I also demonstrate the existence of errors in previous studies arguing for an abrupt shutdown of monsoons in response to small changes in anthropogenic forcings; simple theory together with an ensemble of integrations of a global climate model demonstrate a near-linear dependence of monsoon rainfall on a wide range of greenhouse gas, aerosol, and surface albedo forcings.