We had an uncommonly cold winter that had our teeth chattering. Now, with spring in full swing, Hector Chikoore, an associate professor at the North-West University (NWU) who specialises in meteorology, climatology and climate change, looks back on the past season and gazes ahead at what the weather has in store for us.
1. Did we have an abnormally cold winter compared to previous years?
There are different ways of defining a season, and the period of the season differs based on these definitions. The meteorological definition generally groups four three-month periods together and defines these as autumn, winter, spring and summer. In line with this definition, the South African winter season is mostly defined as the period starting in June and ending in August. 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. The deviations from the long-term normal show an interesting mixed signal in which 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.
According to SAWS data, last winter (2019) was also characterised by daytime (maximum) temperatures that were mostly warmer than historical averages by at least 1 to 2 °C during June, July and August. One characteristic of a change is an increase in the character of extreme weather events, for example in terms of rainfall and temperature.
One decisive indicator of the extent of temperature anomalies is when long-term records are broken. Observations from the SAWS (from their website) show 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. Examples based on 25 years of observations include a lowest minimum temperature of ‑5,1 °C at Orania, breaking a previous record of ‑4,5 °C in August 2018. It remains to be seen whether these episodes of bitter cold (and snow) conditions had an impact on the spread of the coronavirus, for example by restricting the movements of people, thus encouraging them to stay at home.
Surface anticyclones are the major source of cold maritime air over the subcontinent. The frequency of ridging anticyclones from the South Atlantic and “blocking” systems may also lead to the anomalous advection of cold meridional air from the Southern Ocean. Cut-off low-pressure systems are also associated with extreme cold conditions, snowfalls and heavy rainfall, often disrupting transportation and infrastructure. Cold fronts, by definition, are baroclinic regions of strong temperature gradients, such that cold air pushes behind the band of cloud and thunderstorms. The frequency and duration of these systems may therefore account for temperature and rainfall anomalies. On a large scale and on longer time scales, several studies have found a link between anomalous winter temperatures over southern Africa and a negative phase of the Southern Annular Mode (SAM). The SAM is a remote phenomenon over the Southern Ocean associated with the location of the west-wind belt over the Southern Ocean. The SAM is also linked to the strength of a large anticyclone over the Indian Ocean (Mascarene High) and the location of mid-latitude storm tracks in winter.
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 247 mm of rainfall, which corresponds to 309% of the climatological normal.
2. 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?
In several regions, seasonal climate anomalies are often linked to anomalies in sea surface temperatures in nearby and remote oceans. The prospects for the upcoming summer rainfall season are predicted by climate scientists using numerical climate models that solve physics laws of both the ocean and the atmosphere. These models can simulate ocean processes such as the state of hot spots in the Indian Ocean and the remote Pacific Ocean that can have an impact on the South African climate. 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. This phenomenon accounts for nearly 40% of the variability of summer season rainfall and temperature over the subcontinent. 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 (~5%). El Niño conditions are mostly associated with offshore west-wind anomalies that tend to drive away moisture and tropical revolving systems.
It should be noted that the prediction of the upcoming summer rainfall is made on the basis of numerical climate models and is provided in probabilistic rather than deterministic terms. 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. In 1997, a severe El Niño developed and a widely expected severe drought in 1997/98 did not materialise, as near normal rainfall prevailed. Some studies found that the Angola Low was deeper than normal, allowing for moisture convergence and cloud bands to form. Cloud bands produce approximately half the summer rainfall in South Africa.
3. Have we been experiencing abnormal weather patterns over the last couple of seasons compared to past years? If so, can this be attributed to global warming?
Consistent with trends in many subtropical regions, the southern African region has experienced rapidly rising temperatures, particularly in the recent decade. 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 having been recorded as two of the warmest years according to SAWS records. 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. Drought events are usually accompanied by warmer than normal surface air temperatures and heat waves over the subcontinent, as the airflow is predominantly subsiding, warming adiabatically as it sinks. The higher temperatures have a downstream impact on evaporation from soils and open water, and therefore on rain-fed agriculture and water resources management.
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 – using thermodynamic arguments that a warmer atmosphere can hold more water vapour with higher convective available potential energy (the energy necessary to generate thunderstorms) – are projected to become more intense.
