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Natural Climate Variability on Decade-to-Century Time Scales
Schematic representation of the mechanisms affecting Sahel rainfall. Both the natural variability and the human-induced changes are contributing to the reduction in Sahel rainfall.
variability of the global climate system, it is reasonable to assume that at some unpredictable future time the natural variability (global SST patterns, for example) is likely to produce excessive rainfall over the Sahel and thus reverse the current drought. However, it is unclear whether the changes in the land-surface characteristics can also be reversed. Therefore, even if the global circulation shifts in such a way as to be favorable for reversing the current drought, the global natural changes will have to be stronger than before, and more favorable, to overcome the opposing effects of local atmosphere-land interactions. In fact, it is entirely possible that the Sahel drought will continue indefinitely, because the natural variability effects may never be large enough to counteract the persistent effects of the local land-surface changes.
In a hypothetical GCM sensitivity experiment, Xue and Shukla (unpublished manuscript) have shown that rainfall over the Sahel can be increased by replacing the broadleaf shrubs over ground cover and the shrubs over bare soil by broadleaf trees over ground cover over a large region that includes the Sahel. This suggests that although human effects were not responsible for the initiation of the Sahel drought, a suitable combination of large-scale natural variability and human intervention might contribute toward reversing the current drought. Modification of surface albedo, evapotranspiration, or surface roughness would all be of assistance, though of course it would be extremely difficult to sustain vegetation on a scale sufficiently large to halt the spread of the desert.
DESER: Do you think local forcing applies to the string of wet years in the 1950s and 1960s too?
NICHOLSON: Absolutely. What I feel is the critical factor involved in the forcing is the fluxes to the atmosphere, including dust. The land surface forcing should be reversible, or symmetric, for the wet and dry periods.
RIND: How successful have people been in quantifying the albedo change between the wet and dry periods, as far as the land surface is concerned?
NICHOLSON: One study made 10 or 15 years ago took the available satellite data and reconstructed the albedo over about 8 years. The small changes found were not consistent with rainfall fluctuations, and I think basically showed that albedo is not one of the major forcing factors. More recently, Vivian Gornitz searched historical archives for material on land-surface changes, and constructed a beautiful map of West Africa showing changes due to human activities. Again, in terms of absolute values of albedo, it's a very small change, and sometimes in the wrong direction for the Charney mechanism. So I think that not albedo but something coupled to soil moisture, vegetation, and dust will be the major factor.
DIAZ: To what degree do you think that time differences in the frequency of the westward-propagating tropical disturbances affect the variability you're seeing?
NICHOLSON: I think they're key to understanding the region. This may be the one place on the globe where the land-surface forcing per se has a major impact on the overall large-scale dynamics. The jet stream at the mid-levels here, which is critical in disturbance development, is due to nothing other than the temperature gradient across the land surface down to the ocean.
Some of the models done years ago, during the GATE days, showed that if you change the relationship between baroclinic and barotropic instability you can change the characteristics of the disturbances. The difference between the wet 1950s and the dry 1970s can be explained by one or two disturbances a month that produce, say, 150 mm of precipitation a day, where the dry periods have nothing over 50. I think the large-scale circulation is clearly coupled to the land surface there. Incidentally, the dust in that region is right at the level of the African easterly jet, about 650700 mb, which is another reason I think it is so important.
CANE: I can see that the land-surface processes can affect what's happening, but I'd like to hear what you see as the feedback