Abstract
The hydroclimatology of the Niger River basin, located in West Africa, is very complex. It has been widely studied because of its importance to the socioeconomic activities of the countries that share its natural resources. In this study, to better understand the causes and mechanisms that modulate the rainfall over the Niger River basin, we identified the most relevant moisture sources for precipitation within the basin. The Lagrangian model FLEXPART was utilised to track backward trajectories of air parcels initially losing humidity over climatological rainfall zones of the basin. Along 10-day backward trajectories, we computed the budget of the difference between evaporation and precipitation (E − P) from 1000 to 0.1 hPa, permitting the identification of those regions where moisture uptake ((E − P) > 0) prevail. The study was conducted for the period 1980–2017. Monthly maps of ((E − P) > 0 were developed to illustrate the regions from where moisture is transported, contributing to precipitation in the Niger River basin. The spatial variability of the sources matches the precipitation variability over the basin restricted to surrounding areas of the Niger River basin during months with low average precipitation and widely spreading over the continent and the Atlantic Ocean in months with high average precipitation. During climatological dry months (e.g., December, January and February) the continental sources of West and Northeast Africa and the climatological rainfall zones themselves provide most of the moisture for precipitation. However, during the rainy season, the moisture supplies from oceanic sources increase, becoming greater than the contribution from land-based sources during August (the rainiest month). Dry conditions were identified for each climatological rainfall zone using the Standardised Precipitation Index. Similar to many previous studies, we found that the 1980s were highlighted by dry conditions. Local recycling and particularly moisture uptake from the tropical South Atlantic Ocean seem to be highly related to dry and wet conditions in the basin. A reduction on the moisture uptake from surrounding continental sources and the tropical South Atlantic Ocean is almost persistent during extremely dry conditions. Ascending movements are restricted to the lower troposphere during extremely dry conditions and oscillate latitudinally as well as precipitation.
Highlights
Hydro-climatic changes in the Niger River basin (NRB) have had significant impacts on the local population since the 1970s [1]
In this study we identified the most relevant moisture sources for precipitation over the NRB
To do so we delimited the monthly climatological rainfall zones (CRZs) within the NRB, from which we tracked backward in time those air parcels that lost moisture ((E − P) < 0)
Summary
Hydro-climatic changes in the Niger River basin (NRB) have had significant impacts on the local population since the 1970s [1]. The Sahelian precipitation (P) alterations are primarily a consequence of contrasting circulation, together with recycling of local evaporation and moisture. West African Monsoon (WAM) development during the rainy season. The determining factors are the positional shifts in the African Easterly Jet and West African Westerly Jet (AEJ and WAWJ, respectively), the African easterly waves and the Intertropical Convergence Zone (ITCZ) [6,8,9]. For West Africa (WA) and the NRB, dry season approximately occurs from November to April and rainy season from. Observed changes in the onset and demise of seasonal rains over WA and the
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