Drought in Southern Africa: structure, characteristics and impacts
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Date
2017
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University of Zululand
Abstract
Drought is a complex, slow onset phenomenon which is a recurring and inevitable feature of the regional climate of southern Africa. This thesis focuses on the structure and characteristics of meteorological droughts in southern Africa and their impacts on surface soil moisture, agricultural yields and surface hydrology. In exploring drought characteristics in southern Africa (15-28°S, 22-32°E) monthly satellite datasets and reanalysis models are employed for the period from the austral summer of 1979-80 to that of 2011-12. Drought frequency and severity are determined using a Precipitation minus Evapotranspiration anomaly index which is related to the self-calibrating Palmer Drought Severity Index. It is found that sensible heat flux is strongly correlated with potential evapotranspiration and may be a useful drought indicator. Seven droughts are identified using the drought index with most occurring in the early part of the study period (1979-1995), whilst the 1992 drought was the most severe. It is shown that rain trends in the study region are neutral but sensible heat flux exhibits upward trend suggesting reduced available water for evaporation from the land surface. Changes in evaporation due to warmer temperatures may become more important than changes in precipitation in the surface water balance over southern Africa. While rainfall, outgoing long wave radiation and soil moisture composite anomalies is greatest over Zimbabwe, potential evapotranspiration, air temperature and vegetation anomalies maximize over the western Limpopo valley. The droughts identified in this study are among the 10 hottest seasons during the study period and area associated with increased probability of heat waves. The immediate cause of meteorological drought over southern Africa is the establishment and persistence of the mid-tropospheric Botswana High which intensifies displacing the tropical rain belts equator ward whilst cloud bands shift to the warm ocean east of Madagascar. The jet stream is strengthened and displaced equator ward as the Angola Low weakens at the surface. The low-level moisture flux from the Indian Ocean is reduced and westerly wind anomalies become dominant. The vertical structure of zonal and meridional winds and vertical motion from the surface to 100 hPa exhibits enhanced subsidence in the area bounded by 18-38°E and between 10-30°S during drought seasons. This is a key finding as it shows that drought tends to be widespread over southern Africa covering an area extending from South Africa’s KwaZulu-Natal to northern Malawi. The El Niño Southern Oscillation signal is the dominant mode of variability particularly over the eastern sector of southern Africa. Five of seven droughts identified occurred during an El Niño event while some were enhanced by a positive phase of the Indian Ocean Dipole. There is a strong link between upper divergence and the Indian Ocean Dipole, where upper convergence over central South Africa and Botswana results in sinking motion. Fluctuations in sea-level pressure over the adjacent tropical Atlantic Ocean due to Benguela Niños act to modulate the Angola Low and moisture convergence over southern Africa such that sometimes the west coastal margins experience a wet anomaly as the rest of the region endures drought. This thesis also investigates relationships between drought and environmental and socio-economic indicators such as soil moisture, stream flow, lake levels, vegetation indices, maize yields and agricultural productivity. Strong negative soil moisture anomalies occur over the region during drought with a maximum over Zimbabwe affecting maize yields there. The El Niño impact on maize, Lake Kariba reservoir levels and vegetation is comparable to the impact on rainfall. Successive drought periods have had greater impact on livelihoods and economies of southern Africa which are heavily dependent on rain-fed agriculture.
Description
A thesis submitted to the Faculty of Science & Agriculture in fulfillment of the requirements for the Degree of Doctor of Philosophy in the Department of Geography and Environmental Studies at the University Of Zululand, South Africa, 2017
Keywords
drought --climate changes --Southern Africa