Browsing by Author "Chikoore, Hector"

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    Drought in Southern Africa: structure, characteristics and impacts
    (University of Zululand, 2017) Chikoore, Hector; Jury
    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.
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    Vegetation feedback on the boundary layer climate of South Africa
    (2005) Chikoore, Hector; Jury, R.
    Upward feedbacks from the land surface and vegetation to the atmospheric boundary layer over southern Africa are investigated. Satellite derived rainfall and vegetation data and model evaporative fluxes for the period 1981-2000 are used to reveal spatial and temporal inter-relationships via Principal Components Analysis. Seasonal rainfall and NDVI exhibit distinct unimodal seasonal cycles maximizing during the austral summer over the Zambezi valley and interior plateau. Spectral analysis indicates major cycles of intraseasonal rainfall events at approximately 40 and 20 days. The 40-day oscillation reflects the Madden-Julian Oscillation and is partially phase-locked to the seasonal cycle. The spatial loadings are focused on a region along the eastern edge of the Kalahari (± 23°E), extending from the western Zambezi toward central South Africa, spanning 20° of latitude. The loading pattern is consistent with tropical-temperate troughs and associated northwest (NW) cloud bands. This 40-day mode connects two 20-day modes over the Agulhas current and Angola, hence NW cloud bands are a slower terrestrial harmonic of the faster modes at either end. NDVI also exhibits intraseasonal cycles operating at approximately 40 days with spatial loadings co-located with the rainfall mode except in the south. Vegetation has a 1-2 dekad lagged correspondence with rainfall. It is hypothesised that an earlier rainfall event and subsequent 'greening' results in an evapo-transpiration flux that affects the next rainfall event. Boundary layer structure is studied along west-east transects and compared with vertical and horizontal fluxes of moisture. The sensitivity of vegetation to rainfall is most pronounced over the eastern Kalahari due to high evaporative losses during the intervening dry spells. Since vegetation-rainfall interactions can be a result of moisture convergence, surface evapotranspiration, or convection, this study focuses on the vertical moisture flux from the land surface and attempts to separate this from the dominant large-scale horizontal moisture convergence through a budget analysis. The boundary layer deepens but does not diminish even in the absence of external forcing. A most significant finding is the agreement between vegetation and low level velocity potential. A sharp increase in vegetation appears to draw airflow towards itself, in a self sustaining way. Results of this study contribute to the understanding of land-atmosphere interactions and their role in the climate system of southern Africa.
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    Vegetation feedback on the boundary layer climate of Southern Africa
    (2005) Chikoore, Hector
    Upward feedbacks from the land surface and vegetation to the atmosphere boundary layer over southern Africa are investigated. Satellite derived rainfall and vegetation data and model evaporative fluxes for the period 1981-2000 are used to reveal spatial and temporal inter-relationships via Principal Compnents Analysis. Seasonal rainfall and NDVI exhibit distinct unimodal seasonal cycles of intraseasonal rainfall events at approxemately 40 and 20 days. The 40-day oscillation reflects the Madden-Julian Oscillation and is partilly phase-locked to the seasonal cycle. The spatial loadings are focused on a region along the eastern edge of the Kalahari, extending from western Zambezi towards central South Africa.