Dust generation and transportation from North Africa are thought to modulate the West African Monsoon (WAM) features. In this study, we investigated the relationship between the Saharan Air Layer located above Atlantic Ocean (OSAL) and WAM features, including Monsoon flow, African Easterly Jet (AEJ) and Tropical Easterly Jet (TEJ) over West Africa using the RegCM4 regional model at 30 km grid resolution. Two sets of experiments with and without dust load were performed between 2007 and 2013 over the simulation domain, encompassing the whole of West Africa and a large part of the adjacent Atlantic Ocean. An intercomparison of the two simulations shows that dust load into the atmosphere greatly influences both the wind and temperature structure at different levels, resulting in the observed changes in the main features of the WAM system during summer. These changes lead to a westward shift with a slight strengthening of AEJ core over tropical Atlantic and weakening of both TEJ and monsoon flux penetration over land. In addition, despite running the RegCM4 with prescribed sea surface temperature, a correlation has been found between Aerosol Optical Depths in OSAL and WAM dynamics suggesting a mechanistic link between dust and WAM well reproduced by RegCM4.

Dust induced changes on the West African summer monsoon features

The central-eastern Sahel (CES) subregion of West Africa has experienced a significant increasing trend in summer monsoon rainfall during 1980–2012, but the reasons remain unclear. In this study, we employ a moisture budget analysis to assess the controlling mechanisms and to quantify the contributions of different factors to this increasing trend. Results reveal that the majority of the increasing rainfall over the CES is balanced by increased vertical moisture advection. We further decompose the vertical moisture advection term into dynamic, thermodynamic, and nonlinear components. The dynamic component exhibits the largest contribution to the trend, followed by the thermodynamic component. These components are modulated by strong convergence in the middle and lower troposphere and increased near-surface specific humidity from a remote source. Diagnosing changes in vertical moisture advection against the background of global warming deserves more attention when projecting changes in climate at subregional scales, especially in arid and semiarid regions.

Dynamic and thermodynamic factors controlling increasing summer monsoon rainfall over the West African Sahel

The geographical and climatic conditions of West Africa make the region an essential area for the description of atmospheric aerosol optical properties across the globe. This study provides an intercomparison of aerosol optical depth (AOD440nm) retrievals from satellite-based CALIPSO, MISR, MODIS and OMI sensors against ground-based sun photometer (AERONET) measurements between 2004 and 2014 from six West African sites, which are Agoufou, Banizoumbou, Ouagadougou, Dakar, Djougou and Ilorin during the period 2004–2014. The analysis revealed that MODISSTD performed better with a high degree of correlation for the six study sites, the CALIPSO and MISR–AERONET comparisons showed strong correlations, and the MODISDB also indicated better correlations, as did the OMI–AERONET comparisons. The root mean square error, mean absolute error and root mean bias error were also computed. The CALIPSO instrument has the lowest RMSE and MAE values over Dakar, while the highest RMSE and MAE values were indicated by the CALIPSO and MODISDB sensors, respectively, over Ilorin. The MISR instrument showed good agreement over Dakar than the other instruments, while CALIPSO AOD retrievals were better than those from the other sensors in Banizoumbou and Ouagadogou. The expected error bounds computed for both MODIS retrievals showed that MODISSTD consistently outperformed MODISDB in all the study sites. High AOD values were averagely observed by the satellite sensors during the local dry months (December–February), due to high concentrations of dust aerosols. High AOD values were observed during March–May, due to the condensation of water vapor on aerosol leading to increase in size and optical depth.

Intercomparison and Validation of Satellite and Ground-Based Aerosol Optical Depth (AOD) Retrievals over Six AERONET Sites in West Africa

Implications of MODIS impression of aerosol loading over urban and rural settlements in Nigeria: Possible links to energy consumption patterns in the country

It has been speculated that the vegetation change and human land use have modulated the dust sources in North Africa and contributed to the observed increase of desert dust since 1960s. However, the roles of surface disturbances on dust generation are not well constrained because of limitations in the available data and models. This study addresses this issue by simulating the Total Ozone Mapping Spectrometer (TOMS) Absorbing Aerosol Indices (AAIs) for model-predicted dust and comparing them with the observations. Model simulations are conducted for natural topographic depression sources with and without adding sources due to vegetation change and cultivation over North Africa. The simulated AAIs capture the previously reported properties of TOMS AAI as well as observed magnitude and spatial distribution reasonably well, although there are some important disagreements with observations. Statistical analyses of spatial and temporal patterns of simulated AAI suggest that simulations using only the natural topographic source capture the observed patterns better than those using 50% of surface disturbance sources. The AAI gradients between Sahara (north) and Sahel (south) suggest that the best mixture of surface disturbance sources is 20-25%, while spatial and temporal correlations suggest that the optimum mixture is 0-15% with the upper bound of 25-40%. However, sensitivity studies show that uncertainties associated with meteorology and source parameterization are large and may undermine the findings derived from the simulations. Additional uncertainties will arise because of model errors in sources, transport, and deposition. Such uncertainties in the model simulations need to be reduced in order to constrain the roles of different types of dust sources better using AAI simulation.

Simulation of absorbing aerosol indices for African dust : Quantifying the radiative and biogeochemical impacts of mineral dust

The Sahelian West Africa (Long 20W:20E, Lat 0:30N) by its climatological and geographical conditions is a key region for the characterization of global atmospheric aerosol optical properties This study evaluates the spatial and temporal variation of the Aerosol Optical Depth (AOD440nm), aerosol particle size characterization (Angstrom exponent (I±440-675nm) at four locations (Agoufou, Banizoumbou, Cape Verde and Ilorin) over a period of January 2005 to December 2009 Results of the day-to-day AOD440nm variations as well as the seasonal and annual variations are presented in order to establish the aerosol climatology in the region We compared satellite derived data of Total Ozone Mapping Spectrometer - Aerosol Index (TOMSAI), MODIS (Terra and Aqua) with those of ground-based Sunphotometer AERONET measurements In general, there exits good relationship between MODIS (Terra and Aqua) and the ground-based AERONET measurements with correlation coefficients, R2 â€o 08 reported in all stations However low coefficients (as low as 040) were obtained in all the stations for regressions between TOMS AI and ground-based Sunphotometer AERONET data

Bernard A

Papers

Multiyear Analysis of Ground-Based Sunphotometer (AERONET) Aerosol Optical Properties and Its Comparison with Satellite Observations over West Africa