http://197.159.135.214/jspui/handle/123456789/12 Thu, 06 Aug 2026 04:27:57 GMT 2026-08-06T04:27:57Z http://197.159.135.214/jspui/handle/123456789/1307 Impacts of Climate Change and Reforestation on Rainfall onset and Cessation over West Africa using Regional Climate Models Mounkaila Saley, Moussa 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 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. A Thesis submitted to the West African Science Service Centre on Climate Change and Adapted Land Use and the Federal University of Technology, Akure, Nigeria, in partial fulfillment of the requirements for the degree of Doctor of Philosophy Degree in West African Climate Systems Sun, 01 Feb 2015 00:00:00 GMT http://197.159.135.214/jspui/handle/123456789/1307 2015-02-01T00:00:00Z http://197.159.135.214/jspui/handle/123456789/254 influence of madden-julian oscillation (mjo) on rainfall variability over west africa at intraseasonal timescale Coumba, Niang intraseasonal variability of rainfall over West Africa plays a significant role in the economy of the region that are highly linked to agriculture and water resources. This study therefore has two aims. The study first evaluates the ability of the Atmospheric Model Intercomparison Project (AMIP) simulations performed by Atmosphere General Circulation Models (GCMs) forced with prescribed Sea Surface Temperature (SST) in producing the mean state of West African Monsoon (WAM). This is achieved by analysing the performance of models in reproducing the summer rainfall and temperature climatology, the moving rainbelt and the main dynamical features of WAM such as the strength and position of the African Easterly Jet (AEJ) and Tropical Easterly Jet (TEJ). Secondly, this research study investigated the relationship between the Madden Julian Oscillation (MJO) and rainfall over West Africa during the boreal summer as well as the dynamical processes involved using the AMIP type simulations. The results reveal that most of the models are capable of simulating the main features of the West African monsoon and also produce a realistic summer low-level circulation overWest Africa with more intense westerly anomalies over the maximum rainbelt zone. However, some models simulate an equatorward and earlier maximum of rainfall over Guinean coast from March to the end of May. As for the MJO, the simulations show in general good skill in capturing its main characteristics as well as its influence on rainfall over West Africa. On the global scale, most models simulated an eastward propagation of enhanced and suppressed convection similar to the observed one. Over West Africa the MJO signal is too weak in some models although there is good coherence in the eastward propagation. In addition, the ensemble average of models v gives better performance in reproducing these features. The influence on rainfall is well captured in both Sahel and Guinea regions thereby adequately producing the transition between positive and negative rainfall anomalies through the different phases as in the observation. Futhermore, the results show that a strong active convection phase is clearly associated with the AEJ but the weak convective phase is associated with a much weaker AEJ particularly over coastal Ghana. In assessing the mechanisms which are involved in the above impacts the convectively equatorial coupled waves (CCEW) are analysed separately. The analysis of the longitudinal propagation of zonal wind at 850hPa and outgoing longwave radiation (OLR) shows that the CCEW are very weak and their extention are very limited beyond West African region. It was found that the westward coupled equatorial Rossby waves are needed to bring out the MJO-convection link over the region and this relationship is well reproduced by all the models. However, Kelvin waves do not account for the overall impact of MJO signal on convection over West Africa. Results also confirmed that it may be possible to predict the anomalous convection over West Africa with a lead time of 15-20 day with regard to anomalous convection events over the Indian Ocean and AMIP simulations performed well in this regard. A Thesis submitted to the West African Science Service Centre on Climate Change and Adapted Land Use and the Federal University of Technology, Minna, Nigeria, in partial fulfillment of the requirements for the degree of Master of Science Degree in Climate Change and Adapted Land Use Thu, 01 Jan 2015 00:00:00 GMT http://197.159.135.214/jspui/handle/123456789/254 2015-01-01T00:00:00Z http://197.159.135.214/jspui/handle/123456789/163 West African Aerosols and their Impacts on Regional Climate Toure, N’Datchoh Evelyne West Africa is one of the most important source of aerosols in the World due to the large extent of Sahara and Sahel regions of Africa which have been identified as the first source of mineral dust. In addition, fires occur in several vegetated ecosystems across the World, especially in tropical and subtropical savannah where fire is widely used by the population, during dry season for mainly social and economic purposes. Therefore West Africa, due to its location (between Sahara and Atlantic Ocean) is subject to complex interaction between dust, combustion (biomass burning and fossil fuel) particles and maritime aerosols which may impact the regional climate. Using a regional climate model (RegCM4) coupled to an interactive dust module, treating dust emission, transport, and deposition processes, investigations of effects of dust and carbonaceous aerosols particles from biomass burning were conducted on the West African climate. The study further investigates the relationship between the Saharan Air Layer located above Atlantic Ocean (OSAL) and West African Monsoon (WAM) features, including Monsoon flow, African Easterly Jet (AEJ), and Tropical Easterly Jet (TEJ) over West Africa. To achieve these set purposes, two sets of experiments from 2000-2010 were performed, one including dust and one without dust effect over the West African domain, encompassing the whole West Africa and a large part of the adjacent Atlantic Ocean. Results from simulations performed in this study show that dust load into the atmosphere has an effect on both the wind and temperature structure at different levels, inducing observed changes in WAM system during June-July-August-September (JJAS) seasons. These changes lead to a westward shift and slight strength of AEJ core over tropical Atlantic which is associated to a weak TEJ. Moreover despite the prescribed Sea Surface Temperature (SST), good correlation was noted to exist between Aerosol Optical Depths in OSAL and regional wind, suggesting that the mechanism between dust and WAM features is well reproduced by RegCM4. Moreover, assessing dust-induced radiative forcing over the study domain revealed that dust induced cooling both at TOA and surface throughout the year. The radiative forcing at the Top of Atmosphere (TOA) is minimum during June-July-August (JJA) both over the Ocean (- 30 to -40 W.m-2) and land (-10 to -20 W.m-2), and maximum during December-January- February (DJF) with transitional value during MAM and SON. Also, the daily satellite products (L3JRC) of burned areas from the SPOT– VEGETATION sensor at a moderate spatial resolution of 1 km × 1 km between 2000 and 2007 were analyzed in this work. Results from seasonal analysis revealed a large increase in burned areas from November to February with consistent peaks in December at regional scale and 30% of the L3JRC pixels were burned in approximately 2 years intervals over the West African Savannah. Dividing West Africa into sub-regions broadly according to climate and vegetation, revealed existence of several fire regimes across the region following climate and vegetation gradient. Fires regime is regular in Guinean and Sudanian savannahs with less impact of climate variability on the fires. A Thesis submitted to the School of Postgraduate Studies, in Partial Fulfillment of the Requirement for the award of the Degree of Doctor of Philosophy in Meteorology and Climate Science of the Federal University of Technology, Akure, Ondo State in Nigeria Sun, 01 Mar 2015 00:00:00 GMT http://197.159.135.214/jspui/handle/123456789/163 2015-03-01T00:00:00Z http://197.159.135.214/jspui/handle/123456789/162 Empirical Analysis of the Recent Rainfall Recovery in West Africa Sanogo, Souleymane In this study, daily and monthly rainfall data from 167 and 254 stations, respectively, across West Africa with at least 80% data availability for the 31-year period 1980-2010 and the gridded African Rainfall Climatology- Version 2 (ARC2) data for the period 1983-2013, are used to investigate the monthly, annual and inter-annual rainfall variability over West Africa. Precipitation-related indices of the Expert Team on Climate Change Detection Indices (ETCCDI) are used to investigate the implication of the recovery in terms of the occurrence of precipitation extremes, intensity and frequency. Also, trends in the rainy season onset and retreat dates are analysed to assess the implication of the recovery on monsoon season length. The Standardised Precipitation Index (SPI) for various running time scales is used to assess the consistency of the rainfall recovery with the change from the drought states toward wet or normal conditions. Using the joint global observational dataset from the National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR), the study further examined the physical mechanisms and teleconnections that led to the observed recovery in the West African rainfall. This analysis involved the influence of Sea Surface Temperature (SST) Anomaly on West African rainfall variability and the feedbacks from land surface condition changes. A projection for future scenarios of the recovery in West African rainfall is proposed based on the Empirical-Statistical Downscaling (ESD) applied to the output of the Hadley Global Environment Model 2 (HadGEM2) projections. The study reveals that the majority of stations in the Sahel between the West Coast and 15°E show statistically significant positive (increasing) rainfall trend for annual totals. The August-October period shows the largest rainfall recovery in the Sahel and the date of the retreat of the rainy season significantly moved later into the year by 2 days per decade over that region. The Sahel rainfall recovery is reflected in more rainy days associated with longer wet spell duration and more extreme rainfall events. In contrast, stations along the Guinea Coast show constant or weak trends generally statistically non-significant. However, a tendency toward a more intense 2nd rainy season indicates a later retreat of rains from the Guinea Coast. Results also establish that the Atlantic Multidecadal Oscillation (AMO) changes its sign at the same time that the drought in West Africa started to recover. Evidence is found for a significant feedback between land surface variables (i.e., soil moisture, vegetation index and surface albedo) and rainfall variability at monthly and annual time scales. The feedback of soil moisture increases the rainfall variation by as much as 30% of the variability in annual precipitation in several areas in the Sahel. Based on the climate projection scenarios of the Coupled Model Intercomparison Project Phase 5 (CMIP5) used in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC-AR5), the Empirical-Statistical Downscaling (ASD) applied to data from HadGEM2 underscores a projection of future wetter rainy season over West Africa with a small delay to both rainy season onset and retreat in the ongoing 21st Century for the Representative Concentration Pathways 4.5 (RCP4.5). A Thesis submitted to the School of Postgraduate Studies, in Partial Fulfillment of the Requirement for the award of the Degree of Doctor of Philosophy in Meteorology and Climate Science of the Federal University of Technology, Akure, Ondo State in Nigeria Thu, 01 Jan 2015 00:00:00 GMT http://197.159.135.214/jspui/handle/123456789/162 2015-01-01T00:00:00Z