Abstract
The Niger River basin (NRB) is located in the important climatic region of the African Sahel. In this study we use the Lagrangian tridimensional model FLEXPART v9.0 to identify and characterise the moisture sources for the NRB. This method allows the integration of the budget of evaporation minus precipitation over 10-day backward trajectories, thereby identifying the origins of the air masses residing over the NRB. The analysis was performed for the 35-year period from 1980 to 2014, which allowed us to identify the main semi-annual climatological moisture sources of the NRB, for November–April (NDJFMA) (dry season) and May–October (MJJASO) (wet season), and to quantify the respective moisture uptakes. Throughout the year, the NRB main moisture sources are located on the tropical eastern North Atlantic Ocean near Africa, the tropical eastern South Atlantic Ocean in the Gulf of Guinea, in the regions surrounding the Sahel and in the Mediterranean Sea. The extents of these sources vary between dry and wet seasons. In NDJFMA two regions appear in the east of the basin, which then join up, forming a larger source to the northeast of the basin in MJJASO, when three other less important moisture sources can be seen in central-equatorial Africa, the tropical western Indian Ocean and the Persian Gulf. In NDJFMA the majority of the moisture uptake comes from the NRB itself but then, later in MJJASO, when the precipitation increases over the basin the greatest uptake of moisture occurs over the tropical eastern South Atlantic Ocean, northeast Africa and the NRB, which suggests that these are the effective sources of precipitation in the basin in overall terms. The seasonal moisture uptake quantification over the moisture sources of the NRB, reveals that largest fraction of moisture income to the basin from outside its boundaries. Despite providing moisture to the NRB the source located in the tropical eastern North Atlantic Ocean does not contribute that much to precipitation in the basin. A daily (ten-day) backward analysis shows the importance of the moisture uptake within the NRB and from near moisture sources during the first few (backward) days, while the Atlantic Ocean sources and the Mediterranean became more important during the last five (backward) days of the analysis.
Highlights
Several authors have investigated the moisture sources for precipitation in the Sahel and WestAfrica (WA) by using a range of different methods
Results of van der Ent et al [8], using a moisture recycling approach to study the complete process of continental moisture feedback demonstrated that the Sahel region receives its moisture from three large water bodies: the Mediterranean Sea, the South Atlantic Ocean, and the Indian Ocean, and on average about 50% to 60% of the precipitation originates from continental evaporation
For the Niger River basin (NRB), Stohl and James. [13] utilized a Lagrangian approach for a five-year period (1999–2003), finding that about half of the moisture provided to it originates from the Atlantic Ocean and half originates from the land
Summary
Several authors have investigated the moisture sources for precipitation in the Sahel and WestAfrica (WA) by using a range of different methods. Several authors have investigated the moisture sources for precipitation in the Sahel and West. The identification of mechanisms and sources of moisture responsible for the precipitation regimes is crucial for the understanding of the global hydrological cycle and for improving the predictive power of numerical models [2]. Atmosphere 2017, 8, 38 identification of moisture sources as part of the analysis of extreme events has become a major research area (e.g., for flooding and droughts), but it is increasingly important for regional and global climatic assessments, including paleoclimatic reconstructions and future climate change scenarios [3]. Differences in Sahelian precipitation rate are primarily a consequence of the contrasting circulation, together with recycling of local evaporation and moisture advected from the tropical North Atlantic. Evaporation in the tropical Atlantic Ocean, WA, and Central
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