Theses and Dissertations

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Browsing Theses and Dissertations by Author "Chikoane, H."

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    Heat waves in South Africa: Observed variabilty, structure and trends
    (2017-05-18) Mbokodo, Innocent Lifa; Nethengwe, N. S.; Chikoane, H.; Bopape, M. M.
    Heat waves are warm extreme temperature events that have environmental and socio-economic impacts in many regions across the world. Negative impacts of warm extreme temperatures over South Africa necessitate the need to study the nature of heat waves. Observations and satellite datasets are analysed in the investigation of the nature and trends of heat waves over South Africa in the present (1983-2012) and future (2010-2039, 2040-2069, 2070-2099) climates. Case study and composite analysis of National Centers for Environmental Prediction datasets were done using the Grids Analysis and Display Systems to get an in-depth understanding of the structure of heat waves in South Africa. Future climate model output obtained from the Conformal Cubic Atmospheric Model was used for future heat wave trends in South Africa. The simulations were made using the Representative Concentration Pathways 4.5 and 8.5. Heat waves are unusual events in the present climate (1983-2012) over much of the country, with 20 of the selected 24 stations experiencing an average of less than one heat wave per season. Heat waves are also more frequent and last longer during warm phase of El Niño-Southern Oscillation (ENSO) than in cool phase of ENSO with the north-east being the most prone region. Composite analysis of 500 hPa omega indicates subsidence over the interior of South Africa in both phases of ENSO. Heat waves in South Africa are localized and associated with a middle level high pressure system that persists over the interior inducing anticyclonic flow and subsidence. The anticyclonic circulation over a region experiencing heat wave weakens with decreasing height over land areas which may be due to frictional forces at the surface and the high is placed further south-east at the surface. Advection of dry continental northerly winds also contributes to high maximum temperatures during heat waves in the interior. Maximum temperatures are expected to increase drastically from the present-day climate to the 2070 – 2099 period, with an average increment of about 8°C during DJF in much of the central interior. As a result, heat wave occurrences are expected to be higher in the future warmer climates when climate change signal is higher. Most increases are expected for heat waves lasting for a week than those lasting for over 2 weeks. CCAM outputs also indicated that heat waves in South Africa are expected to last longer and become more intense during the future warmer climates. Longer lasting and more intense heat waves are expected over the Karoo than in other parts of the country.
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    Impact of spatio-temporal variability of the Mascarene High on weather and climate over Southern Africa
    (2017-05) Xulu, Nkosinathi Goodman; Nethengwe, N. S.; Chikoane, H.; Bopape, M. M.
    Subtropical anticyclones locate and modulate weather and climate over subtropical belts for both the Northern and Southern Hemispheres. This study investigates the spatio-temporal variability of the Mascarene High over the South Indian Ocean on (anomalous) weather and climate over southern Africa at intraseasonal, seasonal, interannual, multidecadal and event time-scales. The Mascarene High is located 25-35°S, 40-110°E, playing a vital role in day-to-day weather and climate patterns conditions over southern Africa. Spatio-temporal characteristics of the Mascarene High investigated in this study span the period 1985-2014 and 2071-2100, using NCEP-NCAR reanalysis datasets for present-day climate observations and the Conformal-Cubic Atmospheric Model (CCAM) for future projections. The Mascarene High is analysed using mean sea level pressure (MSLP) extracted from ECMWF ERA-interim monthly reanalysis data. The Mascarene High is also subjected to Principal Components Analysis, depicting eastern displacements of the weather system to be dominant for weather and climate fluctuations over southern Africa. The Mascarene High migrates south (north) during austral summer (winter) and is centred over the eastern Indian Ocean in summer in connection with the Indian Ocean Subtropical Dipole. Event scale analysis is also employed for investigating Mascarene High blocking and induced anomalous weather. Mascarene High blocking leads to anomalous rainfall events over southern Africa associated with tropical cyclones, cut-off lows and cloud bands. There is also a vital geographical variability of the Mascarene High development, distribution and movement in the South Indian Ocean at the different time-scales. Projections of the Mascarene High indicate a shift in mean location as a result of future expansion and intensification. This projected expansion and intensification is expected to shift tropical cyclone trajectories equatorward, with the baroclinic structure of cold fronts expected to shift poleward affecting changes in the weather and climate of southern Africa. This finding is important as it projects changes in weather and climate conditions over southern Africa in a changing climate due to increased greenhouse gas emissions.