We had an uncommonly cold winter that had our teeth chattering. Now, with spring in full swing, we asked Prof Hector Chikoore, who specialises in meteorology, climatology and climate change, what the weather has in store for us.
Q: Did we have an abnormally cold winter compared to previous years?
A: Preliminary analyses of records – mainly for June and July – from the South African Weather Service (SAWS) suggest that the 2020 winter was one of the colder winter seasons, even though not necessarily the coldest since instrumental records in South Africa began in 1860.
Information on the SAWS website shows that, based on years of observations, several extreme temperatures were recorded during the 2020 winter, breaking long-term records of lowest minimum and lowest maximum temperatures.
Night-time (minimum) temperatures were generally lower over the central Highveld and the north-eastern interior than the long-term average, whereas daytime (maximum) temperatures were warmer (than normal), except for the north-eastern districts.
In addition to anomalous temperatures, the winter rainfall regions of South Africa also received significant amounts of rainfall, which should be positive for dam levels, agricultural activities and water supply.
Preliminary data from the SAWS shows most stations in the southern parts of the Western Cape experiencing more than 100% of normal rainfall for the month of July 2020. The station at Darling recorded 247mm of rainfall, which corresponds to 309% of the climatological normal.
Q: Will this have an effect on our spring and summer and is there a correlation between cold winters and the subsequent summer? What can the country expect regarding rainfall in the coming months?
A: The South African summer rainfall region is influenced by the El Niño Southern Oscillation (ENSO), a phenomenon linked to the state of sea surface temperatures (SSTs) in the equatorial Pacific Ocean. El Niño events occur when the SSTs in the equatorial Pacific Ocean are above average, while a La Niña event occurs when the SSTs are below average.
Most droughts in southern Africa occur during El Niño conditions, while La Niña tends to be linked to higher than average rainfall. Some studies have also found a higher likelihood of tropical cyclone landfall over the Mozambican coast during La Niña, even though there is a very small chance of landfall.
The latest ENSO forecasts from most global centres predict the development of La Niña conditions peaking in the summer, and also a higher probability of average to above-average seasonal rainfall over much of the subcontinent.
It must be emphasised that the atmosphere is a complex, non-linear dynamical system, and therefore there are other phenomena such as SSTs in the Indian and Atlantic Oceans that may act to oppose the ENSO signal.
Q: Have we been experiencing abnormal weather patterns over the last couple of seasons? If so, can this be attributed to global warming?
A: Consistent with trends in many subtropical regions, the southern African region has experienced rapidly rising temperatures, particularly in the recent decade.
The warmest year globally was 2016, which was enhanced by the occurrence of one of the most intense El Niño events in 2015/16. The increase in surface temperatures has also been accompanied by an increased frequency of extreme events such as heat waves and drought.
While the temperature trend is upwards and statistically significant, the long-term rainfall trend in several studies is mostly not statistically significant due to a high coefficient of variability.
Most studies on climate trends have found and projected a delay in the onset of the summer rainy season over southern Africa due to drying in the spring, which implies a longer dry season, with consequences for fire management.
In addition, heavy precipitation events have become and are projected to become more intense. (This is according to thermodynamic arguments that a warmer atmosphere can hold more water vapour with higher convective available potential energy – in other words, the energy necessary to generate thunderstorms.)
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The SAWS Annual State of the Climate of South Africa 2019 report indicated a statistically significant rising trend in surface air temperatures of 0,16 °C per decade, with the years 2015 and 2019 on record as two of the warmest years, according to SAWS.
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Prof Hector Chikoore is from the subject group Geography and Environmental Management in the School of Geo and Spatial Sciences on the Vanderbijlpark Campus.