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University of Cape Town (2016)

Future changes in extreme rainfall events and African easterly waves over West Africa

Egbebiyi, Temitope Samuel

Titre : Future changes in extreme rainfall events and African easterly waves over West Africa

Auteur : Egbebiyi, Temitope Samuel

Université de soutenance : University of Cape Town.

Grade : Master of Science (MS) 2016

Résumé
This study examines the relationship between African Easterly Waves (AEWs) and extreme rainfall events over West Africa, and investigates how climate change could alter this relationship in the future. Satellite observations, reanalysis data, and regional climate model (RCA4) simulations (forced with eight global climate simulations) were analysed for the study. The study used the 95th percentile of daily rainfall as a threshold to identify extreme rainfall events, and applied spectral analysis to extract 3-5 days and 6-9 days AEWs from 700hPa meridional wind component over West Africa. The capability of RCA4 to reproduce the rainfall climatology, extreme rainfall events, the characteristics of AEWs and the contribution of AEWs to extreme rainfall events over the region during the past climate (1971-2005) was examined and quantified using statistical analysis. The future changes (2031-2065) in these parameters were projected for the RCP4.5 and RCP8.5 climate-change scenarios. The results of the study show that RCA4 gives a realistic simulation of the West African climate, including the annual rainfall pattern, the structure of AEWs, and the characteristics of the African Easterly Jet that feeds AEWs. The bias in the simulated threshold of extreme rainfall is within the uncertainty of the observed values. The model also captures the link between the structure of AEWs and the rainfall pattern over West Africa, and shows that the percentage contribution of AEWs to extreme rainfall events over the region ranges from 20 to 60%, as depicted by reanalysis data. For the RCP4.5 and RCP8.5 scenarios, the RCA4 ensemble mean projects a future increase in annual rainfall and in the frequency and intensity of extreme rainfall events over the sub-continent, but the increase is generally higher for the RCP8.5 scenario. It also projects a decrease in the frequency of rain days, no changes in the structure of the AEWs, and an increase in the variance of the waves. However, the simulations from the ensemble mean shows no substantial changes in the contribution of AEWs to the extreme rainfall events, suggesting that the increase in the frequency and intensity of the extreme rainfall events may not be attributable to the changes in AEWs. The study’s application is in understanding and mitigating the future impact of climate extremes over West Africa.

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