Monday, 20 June 2005
Climate variability over Senegal (West Africa) and its relationships with global climate is examined for the period 1979-1998. The analysis is based on based on monthly rainfall amounts, number of rainy days and temperature data from obtained from the Senegalese National Office of Meteorology, monthly SST data which consist in NOOA Extended Reconstructed SST (Smith and Reynolds), and rainfall indices (Lamb). Monthly observed rainfall for 20 stations over Senegal and monthly CMAP precipitation were averaged for the months of June, July, August and September, to generate seasonal rainfall totals for the wet season and climate indices averaged over the study period. The spatial distribution patterns are mapped and analyzed using ArcGIS Spatial Analyst. To investigate the climate variability over Senegal an EOF analysis are performed for the 1979-7998 period, using rain-gauge and CMAP rainfall data over Senegal, and CMAP data only over West Africa. The first West African mode agrees strongly with Lamb's rainfall index. This agreement confirms that EOF1 for West Africa is the representative mode of rainfall variability over the region. One of our major findings is that EOF2 for West Africa is well correlated with EOF1 for Senegal rainfall. This relationship is supported by the projection of NCEP winds on EOF2 mode, and the grid-point correlation between the time series of EOF2 over West Africa and the Atlantic SST. The typical circulation associated with positive anomalies over Senegal is a moisture convergence which takes place over the Guinea Gulf, in conjunction with warm waters in this area. Time series for rainfall over Senegal show positive correlations with the South Atlantic SST : the highest coefficients are observed during the wet season (up to 0.70 in August). However, significant coefficients are also observed during the pre-wet season (up to 0.57 in May). These values indicate that there is a positive correlation between south Atlantic SSTs and rainfall anomalies over Senegal. In the Pacific Ocean, the greatest coefficients (up to -0.72) are observed during the April-July period, which can provide modest hints for the rainy season over Senegal. This study (i) shows that the CMAP data is robust and suitable for analyses over West Africa. Based on this liability, it has been possible to use CMAP data as a validation for analyses based on rain-gauge precipitation; (ii) highlights the teleconnections linking Senegal and West Africa with the Pacific and Atlantic Oceans, as suggested by the global wind patterns associated with EOF2 for West Africa; (iii) confirms the difference between the coastal West Africa and rest of the region. Given this specificity, traditional indices used by policy makers and end users for the whole Sahel-Sudan region will not work for Senegal.
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