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Kent State University (2020)

Relative Long-Term Changes in West African Rainfall Components

Obarein, Omon Aigbovboise

Titre : Relative Long-Term Changes in West African Rainfall Components

Auteur : Obarein, Omon Aigbovboise.

Université de soutenance  : Kent State University

Grade : Master of Arts (MA) 2020

Rainfall components likely differ in the magnitude and direction of their long-term changes for any given location, and some rainfall components may carry a greater regional signal of rainfall change than rainfall totals. This study, therefore, sought to evaluate the magnitude of change of each rainfall component relative to other components, and the greatest locations or regions of change across all the rainfall components in West Africa. Hourly rainfall data from the ERA5 reanalysis dataset was used to derive twelve rainfall components, which were evaluated for long-term means, interannual variability and long-term changes. The spatio-temporal magnitude of changes among the components was estimated using absolute z-score values of the slopes of each component and the count of significant grid-point trends. Teleconnections with ENSO were also assessed for each rainfall component and each region (within each rainfall component). Rainfall components were mostly similar in the spatial patterns of long-term means and interannual variability, but considerable differences exist in the spatial pattern of long-term trends. For rainfall totals and heavy rainfall frequency, the central Sahel is witnessing increasing trends while the western Sahel is experiencing significant decreasing trends, and this dichotomy has been widely reported in the literature. In general, decreasing trends predominate in the study area, especially in the northwestern Congo Basin, where annual rainfall is decreasing by 120mm per decade. Rainfall frequency accounts for 62% of all significant grid-point trends for the whole domain. In contrast, rainfall totals account for 27% of all combined significant trends across the domain, while rainfall intensity (4.6%), rainfall timing (5.2%), and rainfall seasonality (1.2%) account for the remaining signals of change. Most of the changes among the rainfall components are in the Tropical Wet and Dry regions (59% of all significant trends) ; the Saharan and Equatorial regions account for the least changes (19% of all significant trends). The study finds evidence that rainfall frequency is more sensitive to regional rainfall signals of change compared to rainfall totals and should be explored as rainfall inputs in climate models to potentially improve regional predictions of future rainfall

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