Further Case Studies of Tropical Atlantic Surface Atmospheric and Oceanic Patterns Associated with Sub-Saharan Drought

Abstract

Sub-Saharan West Africa (10°?20°N) receives rainfall from westward-propagating disturbance lines that have their base within and receive most of their moisture from the low-level, wedge-shaped, southwest monsoonal flow off the tropical Atlantic. This paper builds on earlier research to further identify the tropical Atlantic surface atmospheric and oceanic patterns that accompany drought in sub-Saharan West Africa. Patterns for the four driest years since 1940 (1972, 1977, 1983, 1984) are compared with counterparts for the wettest of the last 20 years (1975) and 60-year (1911?70) average fields. The key results for the rainy season (July-September) of three of the four severe sub-Saharan drought years (1972, 1977, 1984) duplicate those obtained earlier. They include (i) a distinctive basinwide sea surface temperature (SST) anomaly pattern (positive departure to the south of ?10°N; negative departures between 10°?25°N); (ii) a concomitant southward displacement (relative to the 1911?70 mean) of the zone of maximum SST by 250?500 km; (iii) the North (South) Atlantic subtropical high extending farther (less) equaterward than in the 60-year mean; and (iv) associated southward displacements (by 200?350 km) of the near-equatorial pressure trough, wind direction discontinuity between Northern and Southern hemisphere tmdm and zones of maximum rainfall frequency and total cloud amount. These results offer further evidence that very deficient sub-Saharan rainy seasons tend to coincide with the southwesterly surface monmonal flow not extending as far north along the West African coast as in the 60-year mean and, by extension, a reduced northward penetration of the monsoon wedge into West Africa. Also consistent with earlier findings is that only the SST patterns of the aforementioned results show evidence of evolving during preceding seasons. This further underlines the potential for tropical Atlantic SST to provide the basis for the prediction of sub-Saharan rainy season quality several months in advance. These results were not characteristic of the other extremely deficient sub-Saharan rainy season investigated (1983) or the nondrought rainy season studied for comparative purposes (1975), During July-September 1983, the SST departures were positive over much of the tropical Atlantic, and most of the aforementioned near-equatorial atmospheric-oceanic features were in close to their 1911?70 average positions. The latter was also true of July-September 1975, when the SST anomaly pattern was rather fragmented.