Drying climate in Ghana over the period 1960-2005: evidence from the resampling-based Mann-Kendall test at local and regional levels

Trends in rainfall series were investigated at 16 stations in Ghana over the period 1960-2005. Time series were first de-correlated using an effective pre-whitening methodology and then submitted to the resampling-based Mann-Kendall test. Field significances were assessed using the regional average Kendall statistic. Although no significant changes were observed in annual rainfall, the analysis reveals: (a) a reduction in the number of wet season days totalling less than 20 mm of rainfall, between latitudes 6 and 9.5N; (b) a delay (about 0.5 d year?1) in the wet season onset at several locations throughout the country; and (c) a lengthening (about 0.1 d year?1) of rainless periods during the wet season in the south and centre of Ghana. All these changes, which remained insignificant at more than half of the individual stations, were found to be regionally significant at the 95% confidence level. The results highlight the importance of evaluating regional significance when investigating climate trends.