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.
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 [
Soon after the drought in Sahel became known as a major climatic phenomenon, Angell and Korshover [
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 [
Considering the challenges that GCMs face in capturing the variability of Sahel precipitation, in this study we elucidate the governing mechanisms with a different approach. The rationale for this is as follows. In terms of basic meteorological processes, precipitation occurs over the Sahel when the lower atmosphere is unstable and clouds are present over the region. A satisfactory mechanism for explaining variations in Sahel precipitation must include variations in atmospheric stability and cloud cover over the Sahel. A direct indication of changes in stability and cloud cover is given by changes in the distribution of pressure in the lower atmosphere. In an early paper Angell and Korshover [
Average SLP for July, August, and September 1975–2005. Although the center of the SAL is situated over the region from the Arabian Peninsula eastward to northern India, its influence extends westward over the Sahara and the Sahel region.
For this study we calculate objective indices of the monthly averaged pressures and locations of these centers as described in Section
Rossby et al. [
In order to quantify the changes in the centers of action, objective indices for the pressure, latitude, and longitude locations for the centers can be calculated using gridded SLP data as described by Hameed et al. [
Monthly indices of the pressure, latitude, and longitude for these centers were calculated according to (
For Sahel precipitation data we use the monthly mean rain gauge data set by Hulme et al. [
Interannual variations of precipitation over the Sahel during the rainy season July-August-September (JAS) are considered in this paper. As discussed above, the two atmospheric centers of action in the proximity of the Sahel region, and likely to influence the circulation over it, are the Azores High and the South Asia Low. Each center of action is characterized by fluctuations in its pressure, latitude position, and longitude position. We have therefore calculated correlations between the JAS averaged indices characterizing pressure, latitude, and longitude of each center of action with Sahel precipitation. The results are shown in Table
COA variable, corresponding correlation coefficient with Sahel precipitation, and effective sample size. Bold: value statistically significant at the 5% level. The Azores High and South Asia data are for 1948–1998, and the SOI values are for 1951–1998.
COA variable | Correlation with Sahel precipitation | Effective sample size |
---|---|---|
SAL pressure |
|
9 |
SAL latitude | 0.44 | 10 |
SAL longitude | 0.16 | 17 |
AZH pressure |
|
29 |
AZH latitude |
|
51 |
AZH longitude |
|
43 |
SOI (1951–1998) |
|
48 |
From Table
Columns 2 and 3 give the mean values and standard deviations of COA indices related to Sahel precipitation, and columns 4, 5, and 6 are their mutual correlations. The Azores High and South Asia data are for 1948–1998, and the SOI values are for 1951–1998.
COA | Mean | Std Dev | AZH longitude | SAL pressure | SOI |
---|---|---|---|---|---|
AZH longitude | −36.2 | 1.3 | 1.0 | 0.20 | 0.025 |
SAL pressure | 1005.1 mb | 0.92 mb | 0.20 | 1.0 |
|
SOI | −0.22 | 1.49 | 0.025 |
|
1.0 |
The correlation between SAL pressure and SOI is statistically significant at the 5% level, and therefore they cannot be used in the same regression equation. The two possible regressions are as follows:
(I) SAL Pressure and AZH Longitude as independent variables (Sahel precipitation in mm yr−1, SAL pressure in hPa, and AZH longitude in degree):
For this regression
(II) SOI and AZH Longitude as independent variables:
the regression equation is
For this regression
Equation (
Several studies have documented a relationship between ENSO and Sahel Precipitation [
Partial correlation analysis for the independent variables identified in Table
Correlating variables | Adjusting variable | Partial correlation coefficient |
---|---|---|
Sahel precipitation-SAL Pressure | SOI | − |
Sahel precipitation-SAL Pressure | AZH longitude | − |
Sahel precipitation-SOI | SAL pressure | 0.09 |
Sahel precipitation-SOI | AZH longitude |
|
Sahel precipitation-AZH Longitude | SAL pressure | − |
Sahel precipitation-AZH Longitude | SOI | − |
Precipitation values obtained from (
Sahel precipitation in mm yr−1: data and regression on the basis of SAL pressure and AZH longitude.
The Sahel precipitation in Figure
(a) 5-year moving averages: Sahel precipitation (red), AZH longitude (blue), and SAL pressure (green, its axis has been inverted in the figure to facilitate comparison with Sahel precipitation). (b) Sahel precipitation anomalies (blue) and SAL pressure anomalies (red).
To investigate the interannual variability, we define the anomaly
From the analysis above we infer that westward migration of the Azores High and decrease in the pressure of the South Asia Low correspond to increases in Sahel precipitation, and vice versa. To shed light on possible underlying physical mechanisms of the existing correlations, we constructed composite maps of meteorological variables using the NCEP reanalysis data. We averaged over the 10 years when the AZH longitude was most westward and most eastward and examined the difference of these averages, which corresponds to enhanced precipitation over the Sahel. We also performed an analogous analysis for SAL pressure. The years when the JAS average of AZH was most westward are 1999, 1956, 1950, 1960, 1994, 1964, 1967, 1952, 1951, and 2001. The JAS average of AZH was most eastward in 1995, 1987, 1948, 1971, 1990, 1979, 1969, 1997, 1978, and 1981. The years of lowest JAS SAL pressure are 1961, 1958, 1959, 1962, 1954, 1960, 1964, 1963, 1955, and 1956. The years of highest JAS SAL pressure are 2004, 1980, 1982, 1993, 1992, 1987, 1979, 2002, 1986, and 1997.
Figure
(a) Difference of SLP composites in hPa. Average over 10 years for AZH most westward minus average over 10 years for AZH most eastward. (b) Differences of AZH longitude composites for
The Azores High is located to the west of North Africa and this configuration is similar to that of the Indian Ocean High with respect to western Australia. It has been shown that east-west shifts in the position of the Indian Ocean High significantly influence rainfall in southwestern Australia where a multidecadal drought is in progress [
Figure
(a) Differences of SLP composites in hPa. Average over 10 years with SAL lowest pressure minus average over 10 years with SAL highest pressure. (b) Differences of SAL pressure composites at 850 hPa for
Using a 3-dimensional hydrostatic primitive equation model, Rodwell and Hoskins [
Note that this does not imply that the strength of South Asia monsoon is related to the Azores High pressure system, which encompasses large sections over the Atlantic besides the Mediterranean and North Africa. As mentioned before, the South Asia Low pressure and the Azores High longitude position, as calculated by (
(a) Vertical cross-section differences for
This suggests the following scenario: a stronger South Asia Low related to a stronger monsoon induces descending air over the Sahara and the Mediterranean, resulting in an anomalous southward flow over North Africa and convergence over the Sahel.
This paper has presented evidence that July-August-September precipitation in the Sahel is related to the east-west displacements of the Azores High and to the pressure fluctuations of the South Asia Low. A regression equation was developed that could explain 50 percent of the variance of the Sahel precipitation over 1948–1998. This result supports the basic meteorological picture that precipitation occurs when clouds are present and the local atmosphere is unstable. The results presented suggest these atmospheric conditions are strongly influenced over the Sahel by changes in the Azores High position and the South Asia Low pressure. The SOI is also related to the interannual variability of Sahel precipitation but its contribution is less significant than that of the SAL pressure.
The results suggest that a GCM can give a physically correct simulation of Sahel rainfall trends and interannual variability if it can adequately represent the dynamics of the AZH and the SAL pressure centers in terms of both their pressures and locations. Since the AZH is located over the ocean it is likely that interaction between the atmosphere and the ocean plays an important role in its variation. The SAL is primarily a land-based phenomenon, and interaction with the ocean is likely to be less important to its simulation than for the AZH.