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University College London (2008)

Decadal rainfall variability over southern Africa

Preece, David John

Titre : Decadal rainfall variability over southern Africa

Auteur : Preece, David John

Université de soutenance : University College London

Grade : Doctor of Philosophy (PhD) 2008

Southern Africa is a region of vulnerability, with high seasonal and interannual rainfall variability combined with rain-dependent agriculture. This variability is underpinned by influences from ENSO and other oceanic forcing, and known to exhibit quasi-decadal variability at 16-20 years (Tyson, 1975). This study uses a combination of proxy, observed and climate model data to explore the nature, mechanisms and anthropogenic forcing of rainfall variability at decadal scales. The spatial nature of rainfall variability, demonstrated through EOF analysis and composite decadal events, is similar to that at interannual scales. Spectral analysis of observed data confirms the presence of variability at ENSO-related timescales, combined with influences in the 10-12 year and 16-20 year band. Subtle differences appear in model simulated rainfall. The mechanisms of quasi-decadal variability are also shown to be similar to interannual forcing patterns : showing links to ENSO, the tropical and South Atlantic oceans, and the South West Indian Ocean. Rainfall correlation at decadal scales is dominated by the SW Indian Ocean, but the ‘dipole’ like forcing observed at interannual timescales is notable less coherent at quasi-decadal scales. Model-specific differences are explored, and though to be the result of the interaction between a weak (strong) decadal ENSO signal and a strong (weak) Southern Annular Mode influence at multi-decadal timescales. Using state of the art model simulations (Gonzales-Rouco et al., 2003 ; Tett et al., 2007) that incorporate anthropogenic forcings, the study also explores the impact of climate change on the quasi-decadal signals over the region. Results show that quasi-decadal variability is damped under anthropogenic forcing. The spatial structure appears unaltered, and many of the mechanisms are consistent with those in unforced simulations. A key change occurs in the tropical Indian Ocean, which substantially alters in relationship with the southern African rainfall. Implications for decadal predictability over the region are discussed and evaluated.


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