West African rainfall deficits and sea surface temperatures

Fontaine, Bernard; Bigot, Sylvain

doi:10.1002/joc.3370130304

Cited by 95 publications

(48 citation statements)

References 35 publications

(38 reference statements)

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“…Lamb, 1978aLamb, , 1978bHastenrath, 1984;Lough, 1986;Palmer, 1986;Wolter, 1989;Folland et al, 1986Folland et al, , 1991Lamb and Peppler, 1992;Janicot, 1992b;Fontaine and Bigot, 1993;Fontaine and Janicot, 1996;Fontaine et al, 1998). These relationships hold for both annual and interdecadal timescales (Hastenrath, 1990;Ward, 1998).…”

Section: Links With Sst Distributionsmentioning

confidence: 88%

A physical basis for the interannual variability of rainfall in the Sahel

Nicholson¹,

Webster²

2007

Quart J Royal Meteoro Soc

108

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A major factor in rainfall variability over Sahelian West Africa is the latitudinal location of the tropical rainbelt. When it is displaced abnormally far northward, the Sahel experiences a wet year. An anomalous southward displacement results in drought. In this paper we examine the question of what controls the location during the boreal summer, hypothesizing that inertial instability plays a role. An analysis of surface pressure and temperature fields, wind fields, divergence and vertical motion show that the criteria for inertial instability are satisfied in wet Augusts but not in dry ones. The key determinant appears to be the surface pressure gradient between the continent and the equatorial Atlantic. When this is large, inertial instability results in the development of a low-level westerly jet. The presence of this jet enhances the horizontal and vertical shear, and displaces the African Easterly Jet northwestward. Associated with this situation is strong vertical motion over the Sahel and subsidence over the Guinea Coast, producing dry conditions over the latter. The result is a rainfall dipole, one of two major modes of variability over West Africa. Important factors include sea surface temperatures (SSTs) in the equatorial Atlantic and pressure in the South Atlantic. The first of these factors suggests a link with the Atlantic Niño mode of tropical Atlantic variability; while the second suggests a possible link with the Pacific and the extratropical South Atlantic. Overall, our study relates the well-known SST influence on Sahel rainfall to atmospheric dynamics over the continent.

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Section: Links With Sst Distributionsmentioning

confidence: 88%

A physical basis for the interannual variability of rainfall in the Sahel

Nicholson¹,

Webster²

2007

Quart J Royal Meteoro Soc

108

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“…A number of studies have shown that dry years with and without the rainfall dipole differ with respect to both circulation anomalies over west Africa and sea surface temperature (SST)-rainfall associations (Semazzi et al, 1988;Janicot, 1992b;Fontaine and Bigot, 1993;Fontaine et al, 1995;Janicot et al, 1996;Ward, 1998). Nicholson and Grist (2001) hypothesize that the dipole and dry patterns have fundamentally different causes.…”

Section: The Spatial Patterns Of Rainfall Variabilitymentioning

confidence: 99%

The intensity, location and structure of the tropical rainbelt over west Africa as factors in interannual variability

Nicholson

2008

Intl Journal of Climatology

110

111

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This study examines the nature of the tropical rainbelt over west Africa in 4 years corresponding to the four most common patterns of rainfall anomalies in the region. These include 2 years that were anomalously wet in the Sahel, 1955Sahel, and 1950Sahel, , and 2 years, 1983Sahel, and 1984, that were anomalously dry. The year 1955 is ubiquitously wet, while a rainfall dipole prevails in 1950, with abnormally high rainfall in the Sahelian latitudes but drought conditions in the Guinea Coast latitudes to the south. The year 1983 is ubiquitously dry and 1984 is a year with a rainfall dipole but opposite in phase (i.e. Sahel dry, Guinea Coast wet). The main results of the study are that the contrast between wet and dry years is associated with a weakening and contraction of the tropical rainbelt. In addition, a north/south displacement of the rainbelt is associated with the dipole patterns, with a northward/southward shift bringing good rains to the Sahel/Guinea Coast. These shifts are associated with north/south shifts in the latitude of the African Easterly Jet (AEJ) over west Africa. The latitudinal position and extent of the rainbelt correspond to the latitudinal position and extent of an immense 'tongue' of strong ascending motion. The core of the ascending motion is bound by the axes of the AEJ on the north and the Tropical Easterly Jet (TEJ) in the south. This importance of these two jets in determining the character of the summer rainy season implies that the Sahelian region provides the link between the Sahel and global scale phenomena.

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“…Research in dynamic tropical climatology over the last 20 years shows that Sea Surface Temperature (SST) on the regional and global scale strongly influences climate (Fontaine and Bigot 1993;Moron et al 1995;Bigot et al 1997). It is possible to characterise the relations between the monsoon rains and the SST more precisely.…”

Section: Conclusion: the Major Role Of Climatic Conditions And Their mentioning

confidence: 99%

“…The second mode presents an opposed distribution for the African continent and the ocean. A north-south tropical Atlantic 'dipole' has thus been found which oscillates between one mode and the other (Fontaine and Bigot 1993;Wotling et al 1995;Bigot et al 1997). An important research result has been to show that over recent millennia the spatial distribution of paleo-climatic anomalies over tropical Africa (more humid regions and more dry ones; Figure 6) and of SST on the neighbouring Atlantic, is very similar to the spatial distribution observed for the two dominant anomalies over the last four decades (Maley 1997;Maley et al 2000).…”

Section: Conclusion: the Major Role Of Climatic Conditions And Their mentioning

confidence: 99%