West African water vapor variations between recent contrasting Subsaharan rainy seasons
Lamb, Peter J.
- Publisher: Co-Action Publishing
(issn: 1600-0870, eissn: 0280-6495)
Investigation is made of the interannual and intraseasonal variability of the southwest monsoon layer's thickness and moisture content over West Africa, and its water vapor supply to this region from the tropical Atlantic, for the Subsaharan (10°–20° N) rainy season of July–September. Results for the very deficient 1968. 1971. and 1972 Subsaharan rainy seasons. whose departures from the 1941–74 mean ranged between −0.78σ and − 1.39σ. are compared with those for the near-average rainy seasons of 1967, 1969, and 1975 (–0.27σ to + 0.06σ). The observational base consists of monthly mean rawinsonde data from West African stations and seasonal estimates of the tropical Atlantic near-surface specific humidity field. Subsaharan drought does not appear to be associated with the northward supply of unusually dry surface air to West Africa from the tropical Atlantic. Over West Africa, the magnitude of the vertically integrated monsoon layer advective water vapor flux tends to be proportional to the layer thickness. The extremely poor 1972 Subsaharan rainy season (–1.39σ) coincided with particularly shallow southwesterly flow across the Gulf of Guinea coast (–5°N); thicker monsoon layers are found characteristic of the less severe 1968 (–0.78°) and 1971 (–0.88°) droughts. However, the monsoon layer depth here during the nondrought study years does not always exceed that for drought months. The direction of the low-level water vapor flux above Dakar, a sensitive Subsaharan location (−15°N) on the West African coast, shows a stronger tendency to be from north of west during unproductive Subsaharan rainy seasons (− 0.78σ to − 1.39σ) than in near-average ones (–0.27σ to + 0.06σ). This suggests the southwest monsoon does not extend as far north along the West African coast during Subsaharan droughts as in more abundant rainy seasons. In contrast to the foregoing Gulf of Guinea results, the monsoon layer over the interior of the Subsaharan zone at ˜ 13°N exhibits minimal variability for the drought study years. The better developed nondrought southwesterly flows here tended to be thicker than those characteristic of most drought months.DOI: 10.1111/j.1600-0870.1983.tb00197.x