EVOLUTION OF THE RELATIONSHIP BETWEEN NEAR GLOBAL AND ATLANTIC SST MODES AND THE RAINY-SEASON IN WEST-AFRICA - STATISTICAL-ANALYSES AND SENSITIVITY EXPERIMENTS

Monthly sea surface temperature anomalies (SSTA) at near-global scale (60 degrees N-40 degrees S) and May to October rainfall amounts in Wes t Africa (16 degrees N-5 degrees N; 16 degrees W-16 degrees E) are fir st used to investigate the seasonal and interannual evolutions of thei r relationship. It is shown that West African rainfall variability is associated with two types of oceanic changes: (1) a large-scale evolut ion involving the two largest SSTA leading eigenmodes (16% of the tota l variance with stronger loadings in the equatorial and southern ocean s) related to the long-term (multiannual) component of rainfall variab ility mainly expressed in the Sudan-Sahel region; and (2) a regional a nd seasonally coupled evolution of the meridional thermal gradient in the tropical Atlantic due to the linear combination of the two largest SSTA modes in the Atlantic (11% with strong inverse loadings over the northern and southern tropics) which is associated with the interannu al and quasi-decadal components of regional rainfall in West Africa. L inear regression and discriminant analyses provide evidence that the m ain July-September rainfall anomalies in Sudan-Sahel can be detected w ith rather good skills using the leading (April-June) or synchronous ( July-September) values of the four main oceanic modes. In particular, the driest conditions over Sahel, more marked since the beginning of t he 1970s, are specifically linked to the warm phases of the two global modes and to cold/warm anomalies in the northern/southern tropical At lantic. Idealized but realistic SSTA patterns, obtained from some basi c linear combinations of the four main oceanic modes appear sufficient to generate quickly (from mid-July to the end of August) significant West African rainfall anomalies in model experiments, consistent with the statistical results. The recent negative impact on West African ra infall exerted by the global oceanic forcing is primarily due to the g eneration of subsidence anomalies in the mid-troposphere over West Afr ica. When an idealized north to south SSTA gradient is added in the tr opical Atlantic, strong north to south height gradients in the middle levels appear. These limit the northward excursion of the rainbelt in West Africa: the Sahelian area experiences drier conditions due to the additive effect (subsidence anomalies + latitudinal blocking) while o ver the Guinea regions wet conditions do not significantly increase, s ince the subsidence anomalies and the blocking effect act here in oppo site ways.