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University of Cape Town (2021)

Assessing dry spell and wet day frequencies over southern Africa during the summer rainy season

Thoithi, Wanjiru

Titre : Assessing dry spell and wet day frequencies over southern Africa during the summer rainy season

Auteur : Thoithi, Wanjiru

Université de soutenance : University of Cape Town.

Grade : Master of Science in Ocean and Atmospheric Science 2021

Rainfall over southern Africa experiences substantial temporal and spatial variability which heavily impacts poor rural populations in the region that rely on rainfed agriculture for their livelihoods. Instead of totals, seasonal rainfall is better characterised by wet and dry events occurring within rainy seasons as knowledge of the frequency of such events is able to inform agricultural activity. Dry spells (pentads having <5 mm) and moderate wet days (10-30 mm) over southern Africa were assessed using high resolution (0.05◦) Climate Hazards group Infrared Precipitation with Stations (CHIRPS) datasets over the period 1981/82-2018/19 during October-November (ON), December-February (DJF) and March-April (MA) using climatology, intensity-frequency and trend analysis. Correlations with SST over the tropical southeast Atlantic and climate modes namely, El Nino Southern Oscillation (ENSO), the Subtropical ˜Indian Ocean Dipole (SIOD) and Southern Annular Mode (SAM) were computed. These, together with regressed atmospheric and SST fields were used to identify possible mechanisms for changes in dry spell and moderate wet day frequencies during austral summer. Two strong gradients in dry spell frequency were found to be present during DJF, one diagonal along the western margins of the Kalahari desert and the other meridional, lying across 20-24◦S. Topographic influences on rainfall were observed near the Drakensberg and Chimanimani mountains, Mulanje massif and Madagascan highlands where dry spell frequency (DSF) (moderate wet day frequency (MWDF)) tended to be relatively lower (higher). A region which frequently experienced half of the season as dry was identified lying across 22-24◦S (18-25◦S) during DJF (MA), with a core in the central Limpopo River Valley where 85-100% (100%) of the seasons were dry for half the season. DSF and MWDF trends indicated that drying has occurred over central South Africa during ON whereas decreasing DSF and increasing MWDF trends pointed to a weakening diagonal and meridional gradient during DJF. Additionally, increasing MWDF trends over important agricultural areas have occurred during DJF. Trends over central South Africa, part of the diagonal gradient, were associated with changes in ENSO, SAM, the Botswana High and SST in the SE Atlantic whereas those in the western Botswana region, part of the meridional gradient, were associated with those in the SIOD, Mozambique Channel Trough and Mascarene High and SST in the eastern and western Pacific.


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