Identification of historic shifts in daily rainfall regime, Wenchi, Ghana
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Identification of historic shifts in daily rainfall regime, Wenchi, Ghana Kwadwo Owusu & Peter R. Waylen
Received: 25 March 2008 / Accepted: 16 July 2012 / Published online: 22 January 2013 # Springer Science+Business Media Dordrecht 2013
Abstract The characteristics of rainfall regime and its variability in the sub-humid region of mid-Ghana is analyzed using daily rainfall data from the Wenchi, 1950–2000. Prior research in the area, suggests that climatic variability occurs at lower frequencies than the typical ENSO signal and may be more closely related to noted shifts in global climatic patterns. Fifty-six possible starting dates encompassing the traditional growing season, extending from late January to early November, at 5 day increments, are used to define temporal units of varying lengths, variable temporal units (VTUs). Rainfall characteristics in each unit are described by two variables: total rainfall and number of rainy days. Given the widely noted global climatic shift in the 1970s and the results of regional analyses, the 50 year record is sub-divided into two 20 year periods, 1950–1969 (period 1) and 1980–2000 (period 2). The means and variances of the two variables in each VTU are compared to identify any times when the changes in rainfall characteristics are most noticeable within the rainfall regime between periods. Both variables yield results consistent with the main rainy season and the long dry season being relatively unchanged, however the short dry spell is becoming wetter and the minor rainy season (September/October) has become significantly drier and shorter. The observations are consistent with the general north–south erosion of the bi-modal regime in West African which is associated with the southward shift of the ITCZ and the monsoonal system. This phenomenon is believed to be ongoing for the last 10,000 years in step with the hypothesized shift of the perihelion into the boreal winter. The use of independent arbitrary starting dates and durations (VTU) advances the understanding of temporal variability of rainfall, at a scale appropriate to agricultural practices in the study area.
K. Owusu (*) Department of Geography and Resource Development, University of Ghana, PO Box LG59, Legon, Ghana e-mail: [email protected] P. R. Waylen Department of Geography, University of Florida, 3141 Turlington Hall, PO Box 117315, Gainesville, FL 32611-7315, USA e-mail: [email protected]
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Climatic Change (2013) 117:133–147
1 Introduction Generalizations about rainfall variability in West Africa based on knowledge of the Sahel pose major problems for planning and modeling purposes. Considerable spatial variability in rainfall has been observed (Ibe and Ojo 1994), and frequently associated with different causal mechanisms, reflected in the uni-modal and bi-modal regimes in the drier Sahel and the humid Guinea Coast region (below 8°N) respectively. Although the consequences of rainfall variability in the sub-humid region are not as dire in the short-run, the phenomenon needs to be studied since d
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