Extreme rainfall system transitions of west African countries

This study uses monthly average rainfall data spanning from 1900 to 2020 to investigate the annual exceedance rates of extreme rainfall in five West African countries (Nigeria, Niger, Ghana, Chad and Cameroon) and further constructs transition matrices for the countries having thresholds above 200 mm of rainfall within any 12-month period. Employing the Markov chain technique, the analysis establishes a spectrum of four to seven states representing the number of exceedance: 0, 1, 2, 3, 4, 5 and 6 times. These states transition over successive 12-month intervals or years, reflecting the dynamic nature of extreme rainfall events. Flood risk transition matrices are computed for each country, encapsulating the probabilities of transitioning between the aforementioned states, and the probabilities of a few scenarios are obtained. Additionally, the research provides graphical representations of the transition matrices, offering visual insights into the patterns of transition and the evolving probabilities of different exceedance scenarios. The results indicate that Cameroon is most prone to floods, with the average number of exceedances per annum ranging between 3 and 4, while Nigeria follows closely. The evolution of flood, proxied by the 200 mm threshold, indicates at least one flood event following a 2-year period of a similar event, with probabilities ranging across 21% for Cameroon, 43% for Ghana and 55% for Nigeria. This suggests an extreme flood clustering phenomena in the region. The simulated sequence of 1000 state variations provides similar estimates to the observed results of the steady state probabilities, which assigns the highest probability to an exceedance rate of three times per year in the long-run, for Nigeria and Cameroon. Through this comprehensive analysis, the study contributes to a deeper understanding of the dynamics of extreme rainfall events in the region, offering valuable insights for risk assessment, mitigation strategies, and policy formulation regarding climate change adaptation and disaster management. The implications of the study for insurance product development, flood risk assessment, climate finance and public health are provided.