Section 6.5: JAS Sah el Rainfall Links
109
This may result via SST fit modifying West African rainfall, which creates
circulation and diabatic heating anomalies that infiuence atmospheric circulation in the Mediterranean.
6.5 JAS Sahel Rainfall Links with SeaSurface Temperature and Marine Atmosphere
6.5.1 Introduction
There is now strong evidence for sea-surface temperature (SST) anomaly
patterns forcing a substantial fr action of the seasonal to multidecadal atmospheric variability over at least some continental regions in the tropics. The
body of evidence has grown iteratively through the symbiosis of observational studies and numerical model experiments since the 1970s (Rowntree,
1972; Hastenrath, 1984; Lamb, 1978a,b; Folland et al. 1986a; Lough, 1986;
Palmer 1986; Ropelewski and Halpert, 1987, 1989; Palmer et al., 1992). This
Section summarises the observed SST and near-surface marine atmospheric
relationships with seasonal rainfall in the Sahel region of Africa. The empirical relationships presented should be interpreted alongside the numerical
modelling evidence for the infiuence of SST on Sahel rainfall. For ten past
years, Rowell et al. (1992) show that a GCM forced with the observed worldwide SST pattern can simulate the observed JAS rainfall total in the Sahel
with great accuracy. This suggests that many of the empirical relationships
described in this Section are likely to represent stages in the chain of physical
forcing of the Sahelian atmosphere by the remote SST.
For this study, monthly station rainfall data for Africa have been formed
into 2.5 0 lat x 3.75 0 long grid-box averages (Hulme, 1992 and personal communication).
6.5.2 Rainfall in the Sahel of Africa
The Sahel region of Africa (Figure 6.5a) lies just south of the Sahara desert
and receives 80-90% of its annual rainfall in the JAS season, when the Intertropical Convergence Zone (ITCZ) is at its most northerly location. During these months, the coastal region to the south, which will be called the
Guinea Coast (Figure 6.5a), experiences a relatively dry season, though the
JAS rainfall total varies substantially from year to year. There is so me tendency for the rainfall anomalies on the Guinea Co ast and the Sahel to vary
out of phase (Nicholson, 1980; Janowiak, 1988; Janicot, 1992a; Nicholson
and Palao, 1993; Ward, 1994), and this has guided the definition of the Sahel and Guinea Co ast regions marked on Figure 6.5a. For the two regions, a
mean standardised anomaly rainfall time-series has been calculated (for basic
109
This may result via SST fit modifying West African rainfall, which creates
circulation and diabatic heating anomalies that infiuence atmospheric circulation in the Mediterranean.
6.5 JAS Sahel Rainfall Links with SeaSurface Temperature and Marine Atmosphere
6.5.1 Introduction
There is now strong evidence for sea-surface temperature (SST) anomaly
patterns forcing a substantial fr action of the seasonal to multidecadal atmospheric variability over at least some continental regions in the tropics. The
body of evidence has grown iteratively through the symbiosis of observational studies and numerical model experiments since the 1970s (Rowntree,
1972; Hastenrath, 1984; Lamb, 1978a,b; Folland et al. 1986a; Lough, 1986;
Palmer 1986; Ropelewski and Halpert, 1987, 1989; Palmer et al., 1992). This
Section summarises the observed SST and near-surface marine atmospheric
relationships with seasonal rainfall in the Sahel region of Africa. The empirical relationships presented should be interpreted alongside the numerical
modelling evidence for the infiuence of SST on Sahel rainfall. For ten past
years, Rowell et al. (1992) show that a GCM forced with the observed worldwide SST pattern can simulate the observed JAS rainfall total in the Sahel
with great accuracy. This suggests that many of the empirical relationships
described in this Section are likely to represent stages in the chain of physical
forcing of the Sahelian atmosphere by the remote SST.
For this study, monthly station rainfall data for Africa have been formed
into 2.5 0 lat x 3.75 0 long grid-box averages (Hulme, 1992 and personal communication).
6.5.2 Rainfall in the Sahel of Africa
The Sahel region of Africa (Figure 6.5a) lies just south of the Sahara desert
and receives 80-90% of its annual rainfall in the JAS season, when the Intertropical Convergence Zone (ITCZ) is at its most northerly location. During these months, the coastal region to the south, which will be called the
Guinea Coast (Figure 6.5a), experiences a relatively dry season, though the
JAS rainfall total varies substantially from year to year. There is so me tendency for the rainfall anomalies on the Guinea Co ast and the Sahel to vary
out of phase (Nicholson, 1980; Janowiak, 1988; Janicot, 1992a; Nicholson
and Palao, 1993; Ward, 1994), and this has guided the definition of the Sahel and Guinea Co ast regions marked on Figure 6.5a. For the two regions, a
mean standardised anomaly rainfall time-series has been calculated (for basic