West African monsoon dynamics and eastern equatorial Atlantic and Pacific SST anomalies (1970-88)

The Laboratoire de Meteorologie Dynamique atmospheric GCM is used to investigate relationships between West African monsoon dynamics and SST anomalies in the eastern equatorial Atlantic and Pacific for the period 1970-88. Positive SST anomalies in the eastern equatorial Pacific, mainly associated with a larger east-west divergent circulation over the tropical Atlantic, are found to coincide with negative rainfall anomalies over West Africa. This is the case for the composite ENSO warm episodes of 1972, 1976, 1982, and 1983. By contrast, positive SST anomalies in the eastern equatorial Atlantic are accompanied by a southward shift of the intertropical convergence zone along with negative rainfall anomalies in the Sahel and positive rainfall anomalies in the Guinean region. This was the case in 1987. The ENSO warm event during this year had apparently no significant impact on West African monsoon dynamics. A zonal atmospheric coupling associated with differences of SST anomalies between the eastern equatorial Pacific and the Atlantic is evident in the period 1970-88. Positive (negative) phases of this coupling could enhance the impact of ENSO warm (cold) events on West African monsoon dynamics.