Journal of Climate, in press.

SST forcings and Sahel rainfall variability in simulations of the 20th and 21st centuries

Michela Biasutti
Lamont-Doherty Earth Observatory, Earth Institute at Columbia University, Palisades, NY

Isaac M. Held
Geophysical Fluid Dynamics Laboratory, Princeton, NJ

Adam H. Sobel
Department of Applied Physics and Applied Mathematics and Department of Earth and Environmental Sciences, Columbia University, New York, NY.

Alessandra Giannini
International Research Institute for Climate and Society, Earth Institute at Columbia University, Palisades, NY

Abstract

The outlook for Sahel precipitation in coupled simulations of the 21st century is very uncertain, with different models disagreeing even on the sign of the anomalies. Such disagreement is especially surprising in light of the robust response of the same coupled models to the 20th century forcings.

We present a statistical analysis of the pre-industrial, 20th century and A1B scenario simulations performed by thirteen modeling groups in preparation for the IPCC fourth assessment report. We show that the relationship that links Sahel rainfall anomalies to tropical SST anomalies at interannual time scales in observations is reproduced by a vast majority of models in all simulations, independently of the change in the basic state as the world warms. Moreover, the same SST/Sahel relationship can be used to predict the simulated 20th century changes in Sahel rainfall from each modelŐs simulation of changes in Indo-Pacific SST and Atlantic SST meridional gradient. Conversely, such a relationship does not explain the rainfall trend in the 21st century in a majority of models.

We argue that consideration of African environments in a global climate context is indispensable if we are to exploit advances in our physical understanding of recent variability and trends to shape our outlook on future climate change, to inform the debate between proponents of mitigation v. adaptation strategies, and to plan and implement well-informed policymaking action at national, regional and continental scales.