Trends and interannual variability of extreme rainfall indices over Ghana, West Africa

Rainfall plays an important role in the socio-economic development of any nation. Below or above normal rainfall amounts have serious consequences on key socio-economic sectors. This is particularly true for countries such as Ghana where rainfall plays a key role in the agriculture and energy sectors. In this study, the Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS)-homogenized daily rainfall series was used to determine the variability of ten extreme rainfall indices recommended by the World Meteorological Organization-Commission for Climatology (WMO-CCL) and the research project on Climate Variability and Predictability (CLIVAR). The trends of these indices and their links to SST anomalies at oceanic basins were also explored in this study. To understand the variability at interannual to decadal time scales and links with SST forcings, the sum of the first three principal components of the indices were correlated with SST anomalies at oceanic basins. The 35 years mean of frequency indices, CDD, CWD, R10mm and R20mm were found to be in a range of 10-125, 6-14, 30-60 and 8-24 days per year, respectively. On the other hand, the mean over the same period of intensity indices PRCPTOT, R95p, R99p, SDII, RX1day and RX5day were in the range of 900-1700 mm, 200-400 mm, 40-130 mm, 9-13 mm, 30-100 mm and 90-180 mm per year, respectively. The maximum of temporally averaged intensity indices covers southwestern Ghana, while the minimum of the indices lies over northwestern and eastern coasts. A significant decreasing trend in wet indices were observed over the Volta Lake and central portions of the country (7.5 degrees N to 9.5 degrees N), whereas low positive trends were observed over the Northern parts of the country. Wet indices over the country showed significant positive correlations with the Atlantic Ocean SST and negative correlations with the Pacific and Indian basins SSTs. Specifically, the NINO3.4 revealed significant negative correlations with the wet indices over the west-central portions of the country. IOD was observed to have a dipole effect on rainfall indices with the central and southern parts generally covered by negative correlations, while Northern and coastal regions showed positive correlations. The impacts of Atlantic SST on wet rainfall indices are significant over most parts of the country. These results have implications on the improvement of seasonal forecast of the Ghanaian rainfall and its extremes and also provide prior knowledge for better understandings of multidecadal modulations of global interannual teleconnections.