AOSC Seminar
October 20, 2016

The zonally resolved atmospheric energy budget: a theory for regional precipitation change

William Boos
Department of Geology and Geophysics, Yale University

A long sought-after but still unattained goal in climate research is a theory that will predict the regional changes in precipitation, over both land and ocean, produced by an arbitrary forcing.  Recently, numerous studies have used the atmospheric energy budget to constrain shifts in the tropical precipitation maximum. This has led to quantitative theories for the precipitation shifts produced by various forcings, but these theories apply only in the zonal mean and have limited relevance to regional climate. Here we present a theory for the regional tropical rainfall response to an arbitrary energy source and apply this to the test case of mid-Holocene climate, which has a large and well-known insolation forcing. First, we demonstrate the qualitative association of zonal and meridional energy fluxes with rainfall changes in the seasonal cycle and the El Nino-Southern Oscillation. Then we present a quantitative theory that, given only Earth's orbital change and the modern climate state, predicts the increase in mid-Holocene rainfall over Africa and South Asia as simulated by global climate models. In this theory, the direct atmospheric response to the orbital forcing does not produce sufficient rainfall to maintain a vegetated Sahara, which instead requires an additional large energy source such as that due to reductions in Saharan surface albedo. This provides theoretical support for the idea that feedbacks with land surface properties are needed to achieve the mid-Holocene African rainfall changes seen in climate proxies.