This article is based on the Symons Memorial Lecture ‘The dynamics of deserts’, delivered by the author to the Royal Meteorological Society on 20 March 1974. In that lecture he proposed a bio-geophysical feedback mechanism to account, at least partially, for the drought in the Sahel. Since then, numerical integrations have been performed which appear to confirm his hypothesis. They have been reported elsewhere and are described in an appendix to this article.
Dynamics of deserts and drought in the Sahel†
Version of Record online: 15 DEC 2006
Copyright © 1975 Royal Meteorological Society
Quarterly Journal of the Royal Meteorological Society
Volume 101, Issue 428, pages 193–202, April 1975
How to Cite
Charney, J. G. (1975), Dynamics of deserts and drought in the Sahel. Q.J.R. Meteorol. Soc., 101: 193–202. doi: 10.1002/qj.49710142802
- Issue online: 15 DEC 2006
- Version of Record online: 15 DEC 2006
- National Science Foundation. Grant Number: GA-28724X
It is suggested that the high albedo of a desert contributes to a net radiative heat loss relative to its surroundings and that the resultant horizontal temperature gradients induce a frictionally controlled circulation which imports heat aloft and maintains thermal equilibrium through sinking motion and adiabatic compression. In the subtropics this sinking motion is superimposed on the descending branch of the mean Hadley circulation but is more intense. Thus the desert feeds back upon itself in an important manner.
If one takes into account the biosphere, this feedback mechanism could conceivably lead to instabilities or metastabilities in desert border regions. It is argued that a reduction of vegetation, with consequent increase in albedo, in the Sahel region at the southern margin of the Sahara would cause sinking motion, additional drying, and would therefore perpetuate the arid conditions. Numerical integrations with the general circulation model of NASA's Goddard Institute for Space Studies appear to substantiate this hypothesis. Increasing the albedo north of the ITCZ from 14% to 35% had the effect of shifting the ITCZ several degrees of latitude south and decreasing the rainfall in the Sahel about 40% during the rainy season.