Dry and wet conditions in the Niger River Basin and its link with atmospheric moisture transport

Rogert Sorí,Anita Drumond,Raquel Nieto,Luis Gimeno

doi:10.3390/ecas2017-04150

Abstract

In West Africa, is located the Niger River Basin (NRB). Dry and wet conditions were investigated in this basin during the rainy (May-October) and dry (November-April) seasons, from 1980 to 2014. To do this was, calculated the Standardized Precipitation-Evapotranspiration Index (SPEI) at the time scale of 6-months for the whole NRB. The Lagrangian model FLEXPART v9.0 has been used to compute over the main semi-annual climatological moisture sources of the NRB, the budget of evaporation minus precipitation (E-P) over 10-day backward trajectories from the NRB itself. Positive (negative) (E-P) values indicate moisture uptake (loss). This permit evaluating the role of continental and oceanic sources of moisture separately for composites of extremely and severely dry and wet conditions in the basin. The results show for the dry season the negative trend of the April-SPEI6 values and the (E-P)>0 values obtained over the tropical east-north Atlantic Ocean (NAtl), the western Sahel and the Mediterranean region. Over these sources, the anomalies of (E-P) for driest and wettest composites indicate their direct response. On the contrary, for the rainy season, the October-SPEI6 values trend is positive, as well it occurs for the moisture uptake over the South Sahel (SSah) and the NRB itself. The anomalies of the (E-P) values for driest and wettest rainy seasons composites suggest a direct relationship with those obtained mainly over SSah, SAtl and the NRB itself.

Highlights

The Niger River Basin (NRB) is located in West Africa (Figure. 1)
Differences in the 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 Ocean and the Gulf of Guinea [4]
They documented the most extreme droughts which occurred during past 50 years in 1983–1984 and 1991–1992. In accordance with these results, from 1980 to 2014 we identified two changepoint in the mean of the SPEI6 series; the first in July/1982 and later in April/1988 (Figure 2)

Summary

Introduction

The Niger River Basin (NRB) is located in West Africa In WA and in the NRB the mean annual cycle of precipitation is characterized by minimum values at the beginning of the year, that increase month by month, reaching a maximum in August, to later decrease until December [1, 2]. Differences in the 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 Ocean and the Gulf of Guinea [4]. According to Belen et al [5], at interannual time scales, a warming of the equatorial Atlantic and Pacific/Indian Oceans results in rainfall reduction over the Sahel. Positive Sea Surface Temperature anomalies over the Mediterranean Sea tend to be associated with increased rainfall

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