IMPACTS OF CLIMATE CHANGE AND REFORESTATION ON RAINFALL ONSET AND CESSATION OVER WEST AFRICA USING REGIONAL CLIMATE MODELS

Abstract

This study used two observation datasets (the Global Precipitation Climatology Project (GPCP) and the Tropical Rainfall Measuring Mission (TRMM)), to evaluate the ability of nine regional climate models (RCMs) that participated in the Coordinated Regional Climate Downscaling Experiment of Africa (CORDEX-Africa), in simulating the rainfall onset and cessation dates (RODs/RCDs) in West Africa. The study also used two of the RCMs (RegCM and WRF) to examine the potential impacts of climate change and reforestations on RODs and RCDs. All the CORDEX RCMs were driven by ERA-Interim reanalysis (ERAIN), while for impacts studies, RegCM and WRF were driven by general circulation models (GCMs) HadGEM and ECHAM respectively. Four definitions of ROD and one definition of RCD based on rainfall data only are used to compute ROD/RCD. The models evaluation focussed on how well the models simulate the observed mean, standard deviation, and inter-annual variability of ROD and RCD over West Africa when compared to observations (GPCP and TRMM), ERAIN and GCMs. The models were also assessed on how well they link ROD/RCD with the northward movement of the monsoon system over the region. This is because reliable forecasts of the RODs and RCDs are crucial for agricultural planning and food security in West Africa. Results from the observations show that the mean ROD and RCD in West Africa have a zonal distribution with onset dates (ROD) increasing (RCD decreasing) from the Guinea coast northward. ERAIN fails to reproduce the spatial distribution of ROD/RCD as observed in GPCP and TRMM. It was found that the performance of some RCMs in simulating the RODs depends on the ROD definition used, for while ARPEGE, RACMO, PRECIS, and CCLM produce better ROD distributions than ERAIN when the first three ROD definitions were used, they give worse ROD distribution than ERAIN using the fourth definition. However, regardless of the definition used, CCRM5, RCA35, REMO, RegCM3 and WRF show a remarkable improvement over ERAIN. Furthermore, the RCMs (RCA35, PRECIS, CRCM5, REMO, RACMO, RegCM3, and WRF) simulate well the RCDs as observed and perform better than ERAIN; ARPEGE and CCLM fail to simulate the RCD well. This study also shows that the CORDEX RCMs ensemble mean best iii simulates the ROD/RCD over West Africa than individual RCMs. The ability of each RCM strongly depends on how well the model reproduces the northward movement of the West African monsoon system and the associated features. For investigating the potential impacts of climate change and reforestation on the future ROD and RCD over West Africa, RegCM and WRF were used to simulate the present-day climate (1970- 2004) and projected future climate (2030-2064) under the representative concentration pathway 4.5 (rcp4.5) conditions. Results show that, for the impacts of climate change due to elevated greenhouse gas (GHGs), the temperature would likely increase over West Africa in both RCMs and also in the GCMs (HadGEM and ECHAM) more consistently over the Sahel. On the other hand, elevated GHGs would lead to a decrease in rainfall as indicated by the RCMs and GCMs although the decrease is much more consistent again over the Sahel. As for the future ROD, increase in GHGs indicates that regardless of the definitions used, northern Nigeria would have delayed RODs. The vertical structure of the monsoon dynamics in the areas where the highest impacts of climate change (i.e. latest ROD) are observed for each definition shows that the elevated GHGs in the future under rcp45 condition would induce shallower monsoon flow essentially over the Sahel. However, there was no agreement between the RCMs (RegCM and WRF) on the potential impact of climate change on the RCDs due to elevated GHGs, for while RegCM indicates delayed RCDs over the Sahel and early RCDs over the Savanna, WRF produces early RCDs in all the areas. From both models, the projected impact of reforestation under the rcp45 condition indicates that West African climate would be cooler in most areas with more rainfall during the rainy season especially over the reforested zone. The cooling effect is more consistent and higher over Savanna but reforestation would induce a warming over some surrounding areas. Regardless of the definitions used, reforestation over Savanna area would induce early ROD over most areas in West Africa as obtained from both models, except over north of Nigeria. The potential impact of reforestation on the RCDs (under rcp45 conditions) again produces divergent results by the RCMs. WRF produces delayed RCDs over all the climatic zones, while RegCM indicates delayed RCDs over the reforested area and early RCDs over Sahel.