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North Carolina State University (2017)

Rainfall in East Africa : Uncertainties in Data Availability and the Recent Drying Trend.

Angus, Michael Peter

Titre : Rainfall in East Africa : Uncertainties in Data Availability and the Recent Drying Trend.

Auteur : Angus, Michael Peter

Université de soutenance : North Carolina State University

Grade : Doctor of Philosophy (PhD) 2017

Résumé
Reliable observation networks are an essential component in predicting and understanding the mechanisms underlying seasonal rainfall in East Africa. However between 1980 and 2010, the number of station observations in the region declined substantially, reducing the ability of these networks to reliably inform our understanding of the seasonal rainfall. Simultaneously, the networks recorded a significant decline in the total rainfall amount of the Long Rains. Several mechanisms have been proposed for the decline of the Long Rains overall precipitation, but no attempt has been made to assess the impact of station data loss on the observed seasonal rainfall change. This study aims to address this to increase confidence in future projections of rainfall variability for this vulnerable region. The characteristics of the recent drying trend are first defined through a comprehensive review of the previous literature and by directly comparing multiple precipitation data. The Long Rains trend is observed to be independent across the three months of the season and as such conclusions are drawn for each separately. The relationship between precipitation and station decline is clearly observed in May, where a significant drying trend is only observed in gauge networks which retain local station information. No significant trends are observed during March in any available dataset, motivating a move away from the standard "MAM" Long Rains season. A two pronged approach is taken to quantify the uncertainty surrounding the Long Rains decline ; a simulation of station loss in East Africa between 1951 and 1980 is developed to isolate the impact of varying gauge number, and alternative measures of drying are analyzed to isolate the impact of rainfall decline. Regional trends driven by station data loss are of equal magnitude to the observed drying trend, particularly in southern Tanzania where station decrease has been most extensive, casting doubt on the observed rainfall trend during April in this region. Proxy measures of rainfall all agree that Somalia and eastern Ethiopia have observed a recent drying trend during May, necessitating increased station coverage in these countries. Based on these results it is proposed that the search for causes of the recent drying trend shift focus from April to the unambiguous rainfall decline signal during May.

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Page publiée le 10 octobre 2018