Large-scale Tropical Atlantic surface circulation patterns associated with Subsaharan weather anomalies

Identification is made of the Tropical Atlantic (30° N–30° S) surface atmospheric and oceanic patterns of a data set composited for 5 years which were very dry in Subsaharan West Africa (11–20°N). Patterns for a counterpart wet composite data set are also presented. Sixty-year (1911–70) average fields constitute a background reference.

For the rainy season (July–September) of the dry composite, the Tropical Atlantic near-equatorial pressure trough, kinematic axis separating Northern and Southern Hemisphere Trades, and zone of maximum sea surface temperature (SST) are located 200–300 km south of their 60-year average positions. Concurrently, the North Atlantic subtropical high (NAH) extends further equatorward than average, although its centre lies up to 150 km north of its mean latitude. Both Northeast and Southeast Trades are stronger than average, and negative and positive SST anomalies occur north and south of approximately 10° N, respectively. Some of these anomalies evolved during the preceding January-June, which offers encouragement that Subsaharan droughts may be predictable 3 to 6 months in advance.

Patterns for July–September of the wet composite contain fewer and less pronounced anomalies. The NAH is centred 100–150 km further north and extends less equatorward than average, the kinematic axis lies 100–150 km north of its mean latitude, and the Trades are weak in many areas. Except for the northward displacement of the NAH centre, these anomalies are opposite to those for the dry composite July–September. The wet composite July–September anomalies did not evolve during preceding seasons.