The dynamics associated with drought in the Sahel have attracted considerable attention in the recent literature. This paper evaluates Sahel precipitation using the paradigm of the atmospheric centers of action, that is, the extended semipermanent highs and lows that dominate regional circulations and are evident in sea level pressure patterns. We find that Sahel precipitation is significantly influenced by changes in the Azores High and the South Asia Low. Specifically, about 50 percent of the variance of July to September rainfall over the Sahel is explained by changes in the Azores High Longitude position and South Asia Low pressure. In contrast, the contribution of the Southern Oscillation to Sahel precipitation is smaller in comparison. Results presented in this paper suggest that a key test for a climate model in simulating variability of Sahel rainfall is the accuracy with which the model simulates the dynamics of South Asia Low and the Azores High. 1. Introduction Since the 1950s, the semiarid African Sahel zone has undergone a persistent drought, although there is some amelioration observed since the 1980s. Several theories on the cause of this phenomenon have been discussed, with a final conclusion and a quantification of human versus natural causes still to be reached. The Sahel drought does not only have devastating consequences for the Sahel region itself [1], but it also influences regions far away because of the increased output of soil-derived aerosol particles into the atmosphere [2, 3]. Soon after the drought in Sahel became known as a major climatic phenomenon, Angell and Korshover [4] analyzed the trends in the atmospheric centers of action and suggested that “the southward or southeastward movement of the Azores High after 1945 is related to the recent drought just south of the Sahara (Sahelian Zone).” Later other more local processes were put forth as influencing Sahel rainfall such as changes in albedo due to land use change [5], the amount of moisture in the soil [6], and the surface roughness [7]. These can be influenced by overgrazing or deforestation, which led to the hypothesis that human activity contributes to the continuing drought by impacting the land surface processes, and which in turn affect atmospheric circulation. More recently, several global climate models have been used to study Sahel precipitation by using the paradigm that the Sahel rainfall responds to sea surface temperature changes in certain ocean basins [8, 9]. Some models have been used to make projections for Sahel precipitation in the future under
References
[1]
M. B. K. Darkoh, “The nature, causes and consequences of desertification in the drylands of Africa,” Land Degradation and Development, vol. 9, no. 1, pp. 1–20, 1998.
[2]
N. Riemer, O. M. Doherty, and S. Hameed, “On the variability of African dust transport across the Atlantic,” Geophysical Research Letters, vol. 33, Article ID L13814, 4 pages, 2006.
[3]
O. M. Doherty, N. Riemer, and S. Hameed, “Saharan mineral dust transport into the Caribbean: observed atmospheric controls and trends,” Journal of Geophysical Research D, vol. 113, no. 7, Article ID D07211, 2008.
[4]
J. K. Angell and J. Korshover, “Quasi-biennial and long-term fluctuations in the centers of action,” Monthly Weather Review, vol. 102, no. 10, pp. 669–678, 1974.
[5]
J. G. Charney, “Dynamics of deserts and drought in the Sahel,” Quarterly Journal of the Royal Meteorological Society, vol. 101, no. 428, pp. 193–202, 1975.
[6]
W. M. Cunnington and P. R. Rowntree, “Simulations of the Saharan atmosphere-dependence on moisture and albedo,” Quarterly Journal of the Royal Meteorological Society, vol. 112, no. 474, pp. 971–999, 1986.
[7]
Y. C. Sud and W. E. Smith, “The influence of surface roughness of deserts on the July circulation—a numerical study,” Boundary-Layer Meteorology, vol. 33, no. 1, pp. 15–49, 1985.
[8]
K. M. Lau, S. S. P. Shen, K.-M. Kim, and H. Wang, “A multimodel study of the twentieth-century simulations of Sahel drought from the 1970s to 1990s,” Journal of Geophysical Research D, vol. 111, no. 7, Article ID D07111, 2006.
[9]
K. H. Cook and E. K. Vizy, “Coupled model simulations of the West African monsoon system: twentieth- and twenty-first-century simulations,” Journal of Climate, vol. 19, no. 15, pp. 3681–3703, 2006.
[10]
M. K. Tippett, “Filtering of GCM simulated Sahel precipitation,” Geophysical Research Letters, vol. 33, no. 1, Article ID L01804, 2006.
[11]
C. G. Rossby, et al., “Relation between variations in the intensity of the zonal circulation of the atmosphere and the displacement of the semi-permanent centers of actions,” Journal of Marine Research, vol. 2, no. 1, pp. 38–55, 1939.
[12]
S. Hameed, M. J. Iqbal, S. Rehman, and D. Collins, “Impact of the Indian Ocean high pressure system on winter precipitation over western and Southwestern Australia,” Australian Meterological and Oceanic Journal, vol. 61, no. 3, pp. 159–170, 2011.
[13]
S. Hameed, W. Shi, J. Boyle, and B. Santer, “Investigation of the centers of action in the North Atlantic and North Pacific in the ECHAM AMIP simulation,” in Proceedings of the 1st International AMIP Scientific Conference, p. 221, Monterey, Calif, USA, 1995, WCRP 92.
[14]
K. E. Trenberth and D. A. Paolino, “The Northern Hemisphere sea-level pressure data set: trends, errors and discontinuities,” Monthly Weather Review, vol. 108, no. 7, pp. 855–872, 1980.
[15]
E. Kalnay, M. Kanamitsu, R. Kistler, et al., “The NCEP/NCAR reanalysis 40-year project,” Bulletin of the American Meteorological Society, vol. 77, pp. 427–471, 1996.
[16]
M. Hulme, “Validation of large-scale precipitation fields in general circulation models,” in Global Precipitations and Climate Change, M. Desbois and F. Desalmand, Eds., NATO ASI Series, pp. 387–406, Springer, Berlin, Germany, 1994.
[17]
M. Hulme, T. J. Osborn, and T. C. Johns, “Precipitation sensitivity to global warming: comparison of observations with HadCM2 simulations,” Geophysical Research Letters, vol. 25, no. 17, pp. 3379–3382, 1998.
[18]
S. E. Nicholson, “The nature of rainfall fluctuations in subtropical West Africa (Guinea Sahel Soudan),” Monthly Weather Review, vol. 108, no. 4, pp. 473–487, 1980.
[19]
C. K. Folland, J. Owen, M. N. Ward, and A. Colman, “Prediction of seasonal rainfall in the Sahel region using empirical and dynamical methods,” Journal of Forecasting, vol. 10, no. 1-2, pp. 21–56, 1991.
[20]
D. S. Wilks, Statistical Methods in the Atmospheric Sciences, Academic Press, New York, NY, USA, 2011.
[21]
B. Wang, The Asian Monsoon, Springer Praxis Books, 2006.
[22]
M. N. Ward, “Diagnosis and short-lead time prediction of summer rainfall in tropical North Africa at interannual and multidecadal timescales,” Journal of Climate, vol. 11, no. 12, pp. 3167–3191, 1998.
[23]
M. J. Rodwell and B. J. Hoskins, “Monsoons and the dynamics of deserts,” Quarterly Journal of the Royal Meteorological Society, vol. 122, no. 534, pp. 1385–1404, 1996.
