Regional modeling of decadal rainfall variability over the Sahel |
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| Authors: | Deborah Herceg Adam H Sobel Liqiang Sun |
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| Institution: | (1) Institute of Marine and Coastal Sciences (IMCS), Rutgers University, 71 Dudley Road, New Brunswick, NJ 08901, USA;(2) Department of Applied Physics and Applied Mathematics, Department of Earth and Environmental Sciences, Columbia University, New York, NY, USA;(3) International Research Institute for Climate and Society (IRI), Columbia University, Palisades, NY, USA |
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| Abstract: | A regional climate model is used to investigate the mechanism of interdecadal rainfall variability, specifically the drought
of the 1970s and 1980s, in the Sahel region of Africa. The model is the National Center for Environmental Prediction’s (NCEPs)
Regional Spectral Model (RSM97), with a horizontal resolution of approximately equivalent to a grid spacing of 50 km, nested
within the ECHAM4.5 atmospheric general circulation model (AGCM), which in turn was forced by observed sea surface temperature
(SST). Simulations for the July–September season of the individual years 1955 and 1986 produced wet conditions in 1955 and
dry conditions in 1986 in the Sahel, as observed. Additional July–September simulations were run forced by SSTs averaged for
each month over the periods 1950–1959 and the 1978–1987. These simulations yielded wet conditions in the 1950–1959 case and
dry conditions in the 1978–1987 case, confirming the role of SST forcing in decadal variability in particular. To test the
hypothesis that the SST influences Sahel rainfall via stabilization of the tropospheric sounding, simulations were performed
in which the temperature field from the AGCM was artificially modified before it was used to force the regional model. We
modified the original 1955 ECHAM4.5 temperature profiles by adding a horizontally uniform, vertically varying temperature
increase, taken from the 1986–1955 tropical mean warming in either the AGCM or the NCEP/National Center for Atmospheric Research
Reanalysis. When compared to the 1955 simulations without the added tropospheric warming, these simulations show a drying
in the Sahel similar to that in the 1986–1955 difference and to the decadal difference between the 1980s and 1950s. This suggests
that the tropospheric warming may have been, at least in part, the agent by which the SST increases led to the Sahel drought
of the 1970s and 1980s. |
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