Browsing by Author "Mahlalela, Precious"

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    Revisiting the links between the Southern Annular Mode and rainfall over the Western Cape region of South Africa
    (2018) Mahlalela, Precious; Reason, Christopher; Blamey; Ross
    The winter rainfall region of South Africa displays considerable interannual variability and prevalence to prolonged dry periods. Although not completely understood, a wide range of factors have been highlighted to contribute to this interannual variability. The relatively poor understanding of rainfall variability in this region is of concern considering the low rainfall received in 2015-2017, resulting in the City of Cape Town enforcing severe water restrictions due to dam levels falling dangerously low. The focus of this thesis is on the influence of the Southern Annular Mode (SAM) on rainfall over the region, the possible influence of El Niño Southern Oscillation (ENSO) is also considered. To achieve this, a correlation analysis was conducted using the Marshal (2003) SAM index and station rainfall anomalies over the region for the period 1979 to 2016. The results show that five (three) of the six driest (wettest) years were associated with a positive (negative) SAM phase. However, the relationship is found to be statistically insignificant at a 95% significance level. The relationship is also found to show spatial variability, with strong negative correlations over the West Coast, while a weak positive correlation is observed over the South Coast. Furthermore, a decadal analysis in the relationship found it to be statistically insignificant (at the 95 th significance level) for most of the study period, with an exception of the early winter over the West Coast which shows a strong negative correlation after 2015. A composite analysis showed that dry (wet) winters tend to be associated with a positive (negative) SAM pattern superimposed with a wave number 3 anomaly. In addition, there are La Niña (El ivNiño) – like SST anomalies in the tropical Pacific. These circulation and SST patterns are more or less observed during the generally dry 2015-2017 winters except that winter 2015 shows an El Niño SST anomaly.
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    Variability and trends in rainy season characteristics of the Eastern Cape
    (2023) Mahlalela, Precious; Reason, Christopher; Blamey, Ross
    Forming part of south-eastern South Africa, the Eastern Cape province has been prone to extreme weather events such as floods and droughts. The region also displays considerable interannual rainfall variability with a tendency towards prolonged dry periods in recent decades. There is generally a poor understanding of the factors contributing to this rainfall variability. This is concerning considering the recent prolonged (2015 - 2020) drought, that has had major socio-economic effects particularly on the large impoverished rural population as well as on some urban areas where supplied water services have broken down in several cases. Even with some rainfall relief in the province during 2022, water shortages persist, particularly in the largest metropolitan area of Nelson Mandela Bay. The region is influenced by both midlatitude and tropical systems leading to a complex regional meteorology that hitherto has not been much studied compared to other parts of South Africa. Here, variability and trends in rainfall characteristics for the Eastern Cape are examined. Focus is placed on the spring (September-November) and summer (December- February) as these seasons contribute the largest proportion to annual totals. The spring season contributes between about 25-35% of the annual rainfall total, while the summer season contributes about 40-45%. Due to limited available station data, the Climate Hazards Infrared Precipitation with Stations (CHIRPS) data set is used. Comparisons with the available station data, provides confidence in the CHIRPS-derived results. On interannual time scales, the results indicate that dry (wet) springs over the Eastern Cape are associated with a cyclonic (anticyclonic) anomaly southeast of South Africa as part of a shift in the zonal wavenumber 3 pattern in the midlatitudes. Over the landmass, a stronger (weaker) Botswana High is also apparent with increased (decreased) subsidence over and near the Eastern Cape which is less (more) favourable for cloud band development and hence reduced (enhanced) rainfall during dry (wet) springs. The summer season shows significant El Niño Southern Oscillation (ENSO) and Southern Annular Mode (SAM) influences as well as from the Botswana High. Composites show that dry (wet) summers tend to be associated with a negative (positive) SAM pattern superimposed with a wave number 4 anomaly. According to CHIRPS data, the spring season has shown a significant decreasing trend in total rainfall as well as the number of light, moderate, and heavy rainfall days over most of the province since 1981. The summer signal is less consistent, with a significant increase in rainfall in some inland areas but a decrease in rainfall near the coast which is not found to be statistically significant. The observed summer trends are attributed to an increase in light and moderate rainfall days inland and a decrease in heavy rainfall days near the coast. An examination of the regional rainfall features suggests that the observed decrease in spring rainfall may be related to an observed decrease in the number of cloud bands during the spring while, the observed increase in rainfall inland during the summer might be associated with increased ridging along the south coast. Analysis of mid-century (2040-2060) CMIP5 rainfall projections suggests that there may be a flattening of the annual cycle over the Eastern Cape with the winter becoming wetter and parts of the summer drier. There is a large spread in CMIP5 model projections over the region with the multi-model mean projecting a very slight drying in both seasons. It is suggested that existing climate models may find representing the Eastern Cape region particularly challenging given its sharp gradients in land surface and ocean conditions and its complex meteorology