http://197.159.135.214/jspui/handle/123456789/12 2026-04-23T15:09:25Z 2026-04-23T15:09:25Z Coumba, Niang http://197.159.135.214/jspui/handle/123456789/254 2021-08-05T13:10:41Z 2015-01-01T00:00:00Z

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

2015-01-01T00:00:00Z Toure, N’Datchoh Evelyne http://197.159.135.214/jspui/handle/123456789/163 2021-08-05T13:10:41Z 2015-03-01T00:00:00Z

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

2015-03-01T00:00:00Z Sanogo, Souleymane http://197.159.135.214/jspui/handle/123456789/162 2021-08-05T13:10:41Z 2015-01-01T00:00:00Z

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

2015-01-01T00:00:00Z Annor, Thompson http://197.159.135.214/jspui/handle/123456789/160 2021-08-05T13:10:41Z 2015-02-01T00:00:00Z

Potential impacts of Climate Variability and Change on Hydrology and Water resources over the Volta Basin Annor, Thompson The Volta Basin is one of the vital water resources in West African region. Climate variability and change pose serious consequences on water availability in the basin, as a result of its climate-sensitive nature. Therefore, there is the need for information on the expected near future states of the water resources in the basin to support water management planning. Regional climate simulations were performed using a climate version of the Weather, Research and Forecast (WRF) model to assess the potential impact of future climate variability and change on hydrology and water resources over the basin. Two time periods (1976–2005 and 2026–2055) of the ECHAM6 scenario RCP4.5 were dynamically downscaled with WRF in a double nested configuration. The outer domain at 50 km resolution covering the whole of West Africa and the inner domain covers the entire Volta Basin at 10 km. The performance of WRF simulations for the period 1980-2005 was assessed using the ERA-Interim reanalysis data as driving data. Standardized precipitation index (SPI) was computed from the WRF simulation results to project future potential impacts of climate variability and change on the Volta Basin hydrology and water resources. Also, the Hydrologic Model System (HMS) was setup over the Volta Basin and the preliminary results at Lawra and Bui gauge stations for the period 2005-2007 were presented. The present-day results show that generally, GCM biases were transferred into the RCM downscaling simulations, however, there were some additional bias contributed solely by the RCM. The WRF model generally underestimated annual mean temperature over both West Africa and the Volta Basin. For the annual total precipitation, a slight underestimation was simulated over West Africa, while overestimation was produced by the model over the Volta Basin. The general performance of the ECHAM6 model was good, particularly for temperature over both Sahel and Sahara regions, however, the added value to the simulated fields by the WRF model was evident, especially for precipitation over the outer domain on the annual time scale, and over the whole Volta Basin and the Soudano-Sahel for the month of April and spring rainfall respectively where nearly zero bias were simulated. For the future climate projections, whereas simulated temperature changes in all cases showed a clear signal of increase in the future simulations and leaves no doubt about a projected climate change signal, the same cannot be said for precipitation. The WRF model projected a reasonable increase in annual temperature between 0.7 K and 1.6 K over West Africa and between 1.0 K and 1.5 K over the Volta Basin. Over West Africa, precipitation deficit was projected more over the Sahel and the Sahara regions, whereas on the annual scale, precipitation change signal ranging from -6% and 12% was projected over the Volta Basin by the WRF model. Projected changes for precipitation on the seasonal scale over the Volta Basin were quite small. However, the model projected a decrease in April precipitation for the future time period. The SPI results over the basin indicate a high variability in projected soil moisture conditions, streamflow and reservoir levels for the period 2026-2041, but from 2041 to 2055, the projected variability in the various hydrological conditions was rather low. For the 2029-2033 period, a potential hydrological drought over the entire basin is projected due to a projected precipitation deficit for that period. Finally, although the HMS preliminary results of streamflow at the two stations over the Volta Basin showed some bias with those observed, the model performed better at the Bui station than the Lawra station. A change in the future climate of the Volta Basin is therefore, likely to occur, should anthropogenic emissions of greenhouse gases follow the RCP4.5 scenario and furthermore, the projected variability and change in the future climate is likely to impact water availability within the Volta Basin 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

2015-02-01T00:00:00Z