Simulation of Sahel drought in the 20th and 21st centuries

Abstract

The Sahel, the transition zone between the Saharan desert and the rainforests of Central Africa and the Guinean Coast, experienced a severe drying trend from the 1950s to the 1980s, from which there has been partial recovery. Continuation of either the drying trend or the more recent ameliorating trend would have far-ranging implications for the economy and ecology of the region. Coupled atmosphere/ocean climate models being used to simulate the future climate have had difficulty simulating Sahel rainfall variations comparable to those observed, thus calling into question their ability to predict future climate change in this region. We describe simulations using a new global climate model that capture several aspects of the 20th century rainfall record in the Sahel. An ensemble mean over eight realizations shows a drying trend in the second half of the century of nearly half of the observed amplitude. Individual realizations can be found that display striking similarity to the observed time series and drying pattern, consistent with the hypothesis that the observations are a superposition of an externally forced trend and internal variability. The drying trend in the ensemble mean of the model simulations is attributable to anthropogenic forcing, partly to an increase in aerosol loading and partly to an increase in greenhouse gases. The model projects a drier Sahel in the future, due primarily to increasing greenhouse gases.

Fig. 1.

Observed (CRU) 5-year running mean (July–August–September) Sahel rainfall, normalized by its mean value over 1901–2000 (black line), historical CM2 ensemble mean normalized so that its mean value is unity over the same time interval (thick light blue line), and the historical realization that most resembles the observations in the period 1950–2000 (thick red line). The gray area represents ±1 standard deviation within the ensemble. The future scenarios are B1 (green), A1B (blue), and A2 (red). There are two lines for each scenario, one from CM2.0 and another from CM2.1.

Fig. 2.

Observed and modeled rainfall trends. (Left) The linear trend from 1950 to 2000 in the observed (CRU) July–August–September rainfall over land, in mm/month per 50 years. Blue areas correspond to a trend toward wetter conditions, and brown areas toward a drier climate. (Center) The linear trend for the eight-member ensemble mean of CM2 but plotted over both land and ocean. (Right) Linear trend for an ensemble mean of 10 simulations with the atmospheric/land component of CM2.0 running over observed sea surface boundary conditions.

Fig. 3.

Observed (CRU) Sahel July–August–September rainfall for each year (black), with no additional time smoothing, and ensemble mean of 10 realizations of the atmosphere/land component of CM2.0 (red) forced with observed ocean surface temperatures. Both are normalized to unit mean over 1950–2000. (This fixed SST model is 10% drier than the observations.) The gray area is ±1 standard deviation within the model ensemble.

Fig. 4.

As in Fig. 2 but for the annual mean precipitation. The CM2 ensemble mean trend in Center has been multiplied by 2.

Fig. 5.

The annual mean precipitation response of three atmospheric models to a uniform warming of ocean temperatures. (Left) The atmospheric component of CM2.0. (Center) A model developed at National Aeronautics and Space Administration's Global Modeling and Assimilation Office (J. Bacmeister, personal communication). (Right) The CAM3 model developed at the National Center for Atmospheric Research (J. Kiehl, personal communication).