World Meteorological Day took place on 23 March and commemorated the establishment of the World Meteorological Organization. It showcases the essential contribution of National Meteorological and Hydrological Services to the safety and wellbeing of society. The theme for this year is ‘The Sun, the Earth and the Weather’. It is based on the premise that the Sun delivers the energy that powers all life on Earth and it drives the weather, ocean currents and the hydrological cycle.
The Water Research Commission (WRC) hosts a Climate Change Flagship programme that funds various research and development partners to conduct research on aspects of weather and climate as it affects development, particularly for the water-linked sectors. Water is a critical determinant for development, economic growth and a better life. Its availability depends on weather and climate. Research commissioned shows how attributes of weather and climate affects planning and sustainability of development.
The oceans have a profound influence on the weather and climate of South Africa. Not only, most rainfall comes from condensation of water vapor originating from the flux of moisture from ocean to atmosphere but the temperature of the remote and surrounding oceans have an impact of the inter-annual variability of rainfall. A good example of this remote effect is the impact of the ocean during El Nino when the temperature of the Pacific and Indian Ocean is higher than normal.
This creates more rainfall above the higher sea surface temperature thus modifying the global Walker and Hadley circulation. Air usually rises in the equatorial regions, especially above the ocean creating rainfall which increases further the ascending motion of air. The air is eventually pushed poleward and then cools and sinks in the subtropical region where Southern Africa sits, creating high pressure and subsidence which is not favorable to rainfall. This is one of the teleconnection mechanisms linking remote oceanic region to South Africa. Little work has been done to connect streamflow to the El Nino Southern Oscillation previous.
A recently completed WRC study led by Prof Mathieu Rouault (UCT) looked at the tropical Atlantic Ocean, which is closer to South Africa although but smaller where a similar phenomenon to El Nino occurs, the Benguela Nino. Closer to South Africa, the Agulhas Current was known to impact the atmosphere above it due to high turbulent flux of moisture from sea to atmosphere. In this case, the results showed analyses of atmospheric dynamics associated with global ocean-atmosphere modes of variability which influence, seasonally and regionally, vital southern African rainfall receipts at decadal timescales. Such analyses helps to understand recurrent space-time evolution of rainfall and drought patterns across this dry region. It is also important to consider that, at the regional scale, anthropogenic climate change signals will be strongly modulated by natural climate variations, which are likely to induce variations at the decadal timescales.
On the other hand tropical weather systems occur infrequently over South Africa but nevertheless play a major role in the weather of southern Africa. They invade the northern parts of South Africa during the late summer months and are often associated with heavy rainfall and flooding. Tropical disturbances hardly ever occur between April and October, but rather have a peak between December and February. Due to the high frequency of heavy rainfall events that occur in summer over the eastern and north-eastern parts of South Africa, it is important to develop better forecasting techniques that can identify and predict tropical weather systems.
The results show that Continental Tropical Lows occur most frequently occur over southern Angola and Zambia. Through a study led by Dr Liesl Dyson (University of Pretoria), it is predicted that there will be a general increase in warm cored mid-tropospheric lows over South Africa towards the end of the century. The ensemble average of five CCAM climate projections shows that the number of warm lows over Zambia and Angola is expected to decrease with a slight increase in lows over Namibia, Botswana and north-eastern South Africa. Over the central coast of Namibia there is a predicted 40% increase in the number of lows towards the end of the century. This increase in the number lows could lead to an increase in the number of heavy rainfall events over northern Limpopo, especially the escarpment.
In another work led by Prof Francois Engelbrecht (formerly at CSIR) the climate change futures of eastern South Africa were explored, given the importance of this region for South Africa’s water security. Key areas of interest in this research included South Africa’s eastern escarpment, Lesotho and the mega-dam area. Of particular interest are the potential impacts of climate change on the hydroclimate of eastern South Africa, given the impact that such changes may have on water security in the country.
This stems from the fact that the Lesotho Highland water schemes and all of South Africa’s mega-dams are located in eastern South Africa. Moreover, Gauteng province of South Africa – the economic heartland of the country – has a critical dependence on the Lesotho Highland water schemes for both the quantity and quality of its water supply. It may be noted that the 2015/16 El Niño and associated drought brought water restrictions to Gauteng, given that it was the third season in a row associated with below-normal rainfall over eastern South Africa. A water crisis was looming, but good rains in the late summer of 2016/17, which occurred in association with a La Niña event, broke the drought.
Nevertheless, South Africa’s vulnerability to multi-year drought in the mega-dam region was demonstrated through this event. In this study, it was found that multi-year droughts may occur frequently over the mega-dam region as early as the mid-future period of 2046-2065, presumably in response to more frequent occurrence of El Niño events, and with detrimental implications for dam levels.
Despite the fact that the region projected to become generally drier, extreme convective rainfall events may occur more frequently over the mega-dam region. Both statistical and dynamic hydrological modelling are show the plausibility of increased streamflow over eastern South Africa. Drastic increases in temperature are likely to lead to increased evaporation in the mega dams and also in drastic reductions in soil moisture. Moreover, climate of the mega-dam region is projected to become more variable in terms of the annual anomalies of both temperature and rainfall.
These assessments and future projections advocates for a planning that considers future climate while adapting to the new normal which is characterized by extreme weather events. It is important to adequately plan to respond continually to both droughts and floods while increasing resilience. This will contribute to reducing the societal vulnerability while encouraging sustainable development.
Dr Brilliant Petja is the Research Manager for Climate Change at the Water Research Commission.