Case Studies of Tropical Atlantic Surface Circulation Patterns During Recent Sub-Saharan Weather Anomalies: 1967 and 1968

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  • 1 Department of Meteorology, The University of Wisconsin, Madison, 53706
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Abstract

Sub-Saharan West Africa (10–20°N) receives moisture from the tropical Atlantic via low-level south-westerly flow across the southwestern coast of West Africa. This paper utilizes a 1arge data set to identify the tropical Atlantic (30°N–30°S) surface atmospheric and oceanic patterns for two years when sub-Saharan West Africa experienced anomalous weather. Comparison is made with 60-year (1911–70) average fields.

The following tropical Atlantic surface features were located/centered 300–500 km further south in the deficient sub-Saharan rainy season (July-September) of 1968 than the more abundant 1967 rainy season— the kinematic axis between the Northern and Southern Hemisphere trades, the near-equational convergence tune, the near-equatorial pressure trough, the zone of maximum sea surface temperature (SST), the mid-Atlantic maxima of precipitation frequency and total cloudiness, and the center of the North Atlantic subtropical high. Sixty-year mean positions of these features were generally intermediate between the 1967 and 1968 locations. Rainfall was more frequent immediately south of the Gulf of Guinea coast and more abundant along this coast, during the 1968 sub-Saharan drought than in 1967. During the dry July-September 1968, positive SST departures occurred south of 10°N and east of 35°W, with a southwest-northwest oriented negative SST anomaly immediately to the northwest. The opposite SST departure pattern characterized July-September 1967.

The July-September 1968 departures from 60-year average patterns were largely characteristic of April-June 1968. In contrast, the July-September 1967 anomalies showed little evidence of evolving during preceding

Abstract

Sub-Saharan West Africa (10–20°N) receives moisture from the tropical Atlantic via low-level south-westerly flow across the southwestern coast of West Africa. This paper utilizes a 1arge data set to identify the tropical Atlantic (30°N–30°S) surface atmospheric and oceanic patterns for two years when sub-Saharan West Africa experienced anomalous weather. Comparison is made with 60-year (1911–70) average fields.

The following tropical Atlantic surface features were located/centered 300–500 km further south in the deficient sub-Saharan rainy season (July-September) of 1968 than the more abundant 1967 rainy season— the kinematic axis between the Northern and Southern Hemisphere trades, the near-equational convergence tune, the near-equatorial pressure trough, the zone of maximum sea surface temperature (SST), the mid-Atlantic maxima of precipitation frequency and total cloudiness, and the center of the North Atlantic subtropical high. Sixty-year mean positions of these features were generally intermediate between the 1967 and 1968 locations. Rainfall was more frequent immediately south of the Gulf of Guinea coast and more abundant along this coast, during the 1968 sub-Saharan drought than in 1967. During the dry July-September 1968, positive SST departures occurred south of 10°N and east of 35°W, with a southwest-northwest oriented negative SST anomaly immediately to the northwest. The opposite SST departure pattern characterized July-September 1967.

The July-September 1968 departures from 60-year average patterns were largely characteristic of April-June 1968. In contrast, the July-September 1967 anomalies showed little evidence of evolving during preceding

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