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University of Reading (2018)

Characterising and understanding trends and variability in African rainfall seasonality.

Dunning, Caroline M.

Titre : Characterising and understanding trends and variability in African rainfall seasonality.

Auteur : Dunning, Caroline M.

Université de soutenance : University of Reading.

Grade : Doctor of Philosophy (PhD) 2018

Seasonal rainfall over Africa is critical for rain-fed agriculture, domestic water use, energy production and health. Thus, variability and change in precipitation patterns can potentially inflict socio-economic damage, exacerbated by the low adaptive capacity in many countries. This thesis aims to characterise and interpret the seasonality of rainfall over Africa and its changes over time to improve capability for robust, societally relevant projections of future climatic change over Africa. An objective methodology is developed to quantify the seasonal regime and timing across continental Africa, applicable to both models and observations. Compatibility with known physical drivers of African rainfall, consistency with indigenous methods, and generally strong agreement between satellite-based rainfall datasets confirm that the method realistically captures the seasonal progression of rainfall. Present day seasonality and the role of the El Ni˜no Southern Oscillation are characterised. Examination of the representation of African rainfall seasonality in the CMIP5 climate models reveals deficiencies in some regions. These include timing biases over the Horn of Africa (long rains 20 days late on average) and an unrealistic seasonal regime over the southern West African coastline, with a failure to capture the agriculturally important Little Dry Season. Further analysis using targeted climate model simulations indicates that the misrepresentation of the biannual regime is related to Atlantic sea surface temperature biases. Over much of the continent it is found that the CMIP5 models accurately capture the seasonal cycle in rainfall, which adds credence for future projected changes in seasonality. Assessment of future changes in wet season characteristics under RCP4.5 and RCP8.5 scenarios indicates significant projected changes in several regions of Africa. A delay in the wet season over West Africa and the Sahel of over 5-10 days on average and later onset of the wet season over Southern Africa is identified. This is linked with increasing strength of the Saharan Heat Low in late boreal summer and a northward shift in the rain belt over August-December. Average rainfall per rainy day is projected to increase, while the number of rainy days in the wet season declines in regions of stable or declining rainfall (West and Southern Africa) while remaining unchanged in Central Africa where rainfall is projected to increase. Identified changes in seasonal rainfall characteristics over Africa have implications for crop yields, water supplies, and urban flooding, requiring adaptation strategies to reduce potentially detrimental consequences.


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