Trend, decadal and interannual variability in annual rainfall of subequatorial and tropical North Africa (1900–1994)

Abstract

Annual rainfall anomalies over subequatorial and northern tropical Africa are analysed for interannual and interdecadal variability over the time interval 1900–1994. Then, the main modes of variation in the annual rainfall field are related to the same frequencies of variation in several SST indexes chosen in key-areas. First, empirical orthogonal function (EOF) analysis is performed on the annual rainfall anomalies field for extracting the main dominant spatio-temporal modes, and leading principal components are analysed through multi-singular spectrum analysis (M-SSA). Regional rainfall anomaly indices, constructed from EOF analysis and describing, respectively, the temporal variability of Sahel, Guinea Gulf area, equatorial East Africa and Gabon–Congo area, are analysed individually through Monte Carlo SSA for assessing the level of significance of spectral peaks. Low-frequency variability, strongest north of 5°N, is a combination of a non-linear trend mixed with an irregular oscillation near 25–40 years. Together, these modes contribute about 35–40 per cent of the Sahelian regional variance and about 15–20 per cent of the Guinean one. The low-frequency variability shows weak negative values till 1915–1920, then positive ones till 1967–1970, with a relative minimum near 1940–1945, and lastly, strong negative values. This behaviour must be placed in a larger context of at least Atlantic-basin scale, or at a global scale in which the strongest signal seems to be the reversing of the interhemispheric thermal gradient around 1970. A quasi-decadal pulse (near 12–13 years) is also observed over Sahel and explains about 12–14 per cent of the regional rainfall index variance. Even if associated SSA components are not significant when a red noise null-hypothesis is considered, some physical consistency is found between this mode and a similar oscillation concerning the thermal gradient between tropical northern and equatorial Atlantic. At the interannual scale, significant oscillations of 5·1–5·8 years (quasi-quinquennial oscillation—QQO) (contributing less than 5 per cent of the Sahelian variance to almost 20 per cent of the East African one), and of 3·2–3·6 years (quasi-triennial oscillation—QTO) (explaining from 5 to 10 per cent of the Sahelian variance to 20–25 per cent of the East African one) are the first ones before quasi-biennial oscillations (2·0–2·8 years) which explain in general less than 5 per cent of the variance and at maximum, near 10 per cent of the variance in Equatorial Africa. The QQO and QTO seem to be modulated frequencies of the same phenomenon connected to central and eastern tropical Pacific (CETP): (i) QQO, which is characterized by an out-of-phase pattern between East Africa (more/less rainfall than normal when CETP and western Indian Ocean is anomalously warm/cold) and central Africa seems to be associated mainly with in-phase behaviour between the CETP and western Indian Ocean. The relation is stable during the twentieth century; (ii) QTO, which is characterized by an in-phase pattern from East Africa to the Sahel (less/more rainfall when CETP is anomalously warm/cold) seems to be related only to CETP and this relation is strongest at the end of the period (after 1960–1970). Relationships between QQO and QTO of rainfall with the same oscillations of the Atlantic SST index are weak. © 1997 by the Royal Meteorological Society. Int. J. Climatol., 17: 785–805 (1977) (No. of Figures: 13. No. of Tables: 3. No. of