Drought is a threat to food security and water availability in West Africa. As part of efforts to improve drought early warning systems, this study investigates the characteristics of West African droughts, evaluates the ability of CORDEX regional climate models to simulate the droughts and examines the impacts of future climate change (2031-2060, under the scenario RCP45) and reforestation on the characteristics of droughts. The Standardized Potential Evapotranspiration Index (SPEI) was used to define drought occurrence.
The study applied the Principal Component Analysis(PCA) on Climate Research Unit (CRU) dataset and model simulations to identify spatial patterns in West African droughts. It studied the relationships between the droughts and atmospheric teleconnections. The capability of the models in reproducing the characteristics of West African droughts was investigated. Furthermore, Regional Climate Model (RCM) experiments, using RegCM4 and WRF, were undertaken to investigate the potential impact of reforestation over Savanna (8◦N-12◦N) on drought over West Africa in near future (2031-2060) under the scenario RCP45. RegCM4 and WRF were forced with HadGEM2-ES and the ECHAM6 respectively. The study evaluated the performance of both regional climate models to simulate the present-day(1971-2000) mean climatology on seasonal basis while the impacts of global warming and reforestation on drought regimes were investigated using Self-Organizing-Map(SOM).
The study showed that about 60% of West African droughts can be grouped into 4 patterns (or modes). The first pattern featured drought over east Sahel, the second over west Sahel, the third over Savanna and the fourth over Guinea coast. Most CORDEX models reproduced at least two droughts patterns, but only one model(REMO) reproduces all the four drought patterns. The correlation between observed and simulated drought patterns reveals that some models perform better than ERAINT reanalysis in simulating the seasonal variation of the patterns.
The model evaluation showed that both models were able to capture the seasonal variability of the rainfall and the Sahel Heat Low; however, they struggled to accurately simulate the onset and retreat of the West African monsoon and overestimated precipitation over the Fouta Djalon and the Gulf of Guinea. Both models project warmer conditions over West Africa with a significant increase in temperatures from 1◦C near the coasts up to 3◦C over the Sahel. This zonal warming induced an increase in water demand and severe droughts frequencies from 2 events/decade to 9 events/decade. As for rainfall, models are not consensual: REGCM projects a significantly decrease by -1 mm/day to -2 mm/day over Nigeria, Cameroon and Chad and an increase by 0.6 to 2.2 mm/day over Guinea plateau, Cote d’Ivoire, Northern Ghana and South-West Nigeria.Meanwhile, rainfall over the entire Savanna is expected to decrease by -0.6 to -2 mm/day with WRF model.
The study found that reforestation over Savanna reduces the increase in temperature by -0.1◦C to -0.8◦C and the frequency of severe drought by 1-2 events/decade over the reforested area. It also increases precipitation by 0.8 to 1.2 mm/day over the reforested area. As for drought regime, the impacts of reforestation could follow two opposite paths: while it could decrease the frequency of drought events over Savanna and the Guinea coasts by 1-2 events/decade, it increases it by 1 event/decade over Sahel
especially in July-August-September.