AbstractPaleo-climatic proxies indicate that significant summertime rainfall reached the Sahara region during the mid-Holocene, presumably in response to stronger summertime heating in the northern hemisphere. Climate models generally do not replicate the enhanced precipitation. As a step toward understanding the response and possible role of model errors, we conducted a series of idealized experiments with the Community Earth System Model in which local atmospheric heat sources of increasing magnitude are applied in the boundary layer over the Sahel / Sahara. In response to this local heating, the cold and moist southwesterly monsoon inflow encroaches farther northward. A source strength of roughly 1 K/day produces similar responses as a simulation with mid-Holocene orbital forcing imposed globally, while that of 1.5 K/day produces a precipitation response similar to that from paleoproxies.The precipitation increases non-linearly, with a jump at heating of around 1 K/day, even though the low-level monsoon inflow increases linearly. Competition at low-to-mid levels between drying by a shallow return flow just above the boundary layer and moistening by vertical advection within the layer affects convection and determines the northward extension of precipitation. When the heating becomes 1.5 K/day, the boundary-layer flow encroaches sufficiently northward to weaken the shallow return flow, further aiding precipitation. This novel nonlinear mechanism operates without biogeophysical feedbacks, and suggests that poor representation of the local thermodynamic processes may hamper a models ability to simulate dynamical feedbacks and hence the strength and poleward extension of monsoon rains under forcings like those during the mid-Holocene.
Journal of Climate – American Meteorological Society
Published: Oct 13, 2017
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