Journal of Vector Borne Diseases (Dec 2010)
Climate indices, rainfall onset and retreat, and malaria in Nigeri
Abstract
Background & objectives: Rainfall in western sub-Saharan Africa is related to seasonal shifts ofthe Inter-tropical Convergence Zone, which moves northward early in the year, retreating in thesecond half of the year. The objective of the present study was to determine significant relationshipsbetween onset and retreat timing and climate indices. The relationship between timing and malariacase reporting was then evaluated.Methods: Relationships between published rainfall onset and retreat dates for Nigeria from 1971–2000 were evaluated in relation to pairs of climate indices using response surface analysis. Graphicalrepresentation of the response surface in relation to the underlying data was used to identify instancesof overfitting. Association of onset and retreat timing with published case reporting records wasevaluated using graphical and correlation analysis.Results: Onset timing and rate of advance were related to ENSO (El Niño-Southern Oscillation),in combination with the Northern Annular Mode (NAM), while retreat timing was related to NAO(North Atlantic Oscillation), in combination with the East Pacific (EP) or West Pacific (WP)index, depending on location. Later onset was associated with faster northward progression ofonset. Retreat date at Kano, the most northerly of the study locations, increased over the period1990–2000, with higher case reporting for Nigeria as a whole being associated with the last threeyears of that period.Interpretation & conclusion: Rainfall retreat occurs much faster than onset, with onset and retreattiming and rate of onset advance being related to combinations of climate indices rather than to asingle index. Threshold for determining a “rainy” day would influence results. The increase innational case reporting with delayed retreat at Kano may be related to the extension of the shorttransmission period in the north.
