What’s behind the wet weather in Gauteng?

Gauteng is having a very wet December. Flash floods, water damage and people’s homes and infrastructure have been destroyed. But is this rainfall normal? How does it compare with December in previous years? Sarah Roffe, a climate researcher at the Agricultural Research Council, helps unpack it for us.

Are we in a particularly bad rainy season?

I don’t think it is easy to say whether we are having a good or bad rainy season. Compared to previous years, the current rainy season has been characterised by above-normal rainfall totals over some areas and, according to forecasts from the South African Weather Service, we will probably continue experiencing above-normal rainfall until April 2023. 

This can be viewed as a good rainy season for crop and livestock production and for our water resources. But if we think about the distribution of rainfall and the fact that flooding events are happening ( in Gauteng many areas have and are experiencing flooding), then perhaps we could view the rainy season as a bad rainy season. So really the answer here depends on the perspective you take, and I think it is better to consider both aspects.

Is the climate crisis changing our weather patterns?

Over Southern Africa, there are certainly detectable changes in our weather patterns. However, it can be difficult to say with certainty whether these are because of human-induced climate changes. The Sixth IPCC [Intergovernmental Panel on Climate Change] Assessment report has shown that there is high confidence that temperature changes since the 1950s can be attributed to human-induced climate change. Conversely, the report shows that there is little confidence that human activities have contributed to rainfall changes experienced over Southern Africa since the 1950s.

Despite this, over recent decades scientists have reported changes in both rainfall and temperatures and characteristics thereof. For temperatures, Southern Africa is characterised by increasing trends, with central regions experiencing the largest warming trends, while coastal regions have experienced warming to a smaller degree. Among the temperature trends reported, the most notable are those for heat extremes.

Recent research has shown that most regions of Southern Africa are characterised by trends of increasing frequency and magnitude of heat extremes. Trends for rainfall are more difficult to detect and are associated with much more uncertainty because rainfall can vary substantially from one year to the next, making it harder to statistically detect a trend. 

Since roughly the 1950s, many regions of Southern Africa have experienced declining annual rainfall totals. Although it is valuable to consider changes in annual rainfall totals, it is more relevant to consider how the distribution of rainfall events has changed and how rainfall during the wet season has changed.

If we consider those regions predominantly experiencing summer rainfall (all of Southern Africa, excluding the western coast and southwestern Cape as well as the southern coast) during the October-March/April months, then it would be important to highlight that most regions have experienced declining trends.

Despite this, many regions are characterised by increasing trends for the frequency and magnitude of individual rainfall events. But there are also increased frequency and intensity of dry spells (consecutive dry days) happening between rainfall events.

We’re in a La Niña event. What is it, how long does it last and what are the effects of it on rainfall over Southern Africa?

A La Niña event, or more specifically phase, represents one of the phases of the El Niño Southern Oscillation (ENSO). ENSO is a natural cycle occurring in the tropical Pacific Ocean temperatures, winds and clouds. This cycle naturally swings between three phases, being El Niño, Neutral and the current phase we’re experiencing being La Niña. One of these phases typically lasts for about a year, but this can extend up to three years, as we are seeing with the current La Niña phase.

We are currently in an unusual three year spell. This happened during 1973-1976 as well as 1998-2001, when La Niña extended to a similar length as the current one.

During Neutral phases, representing “normal” conditions, the naturally occurring tropical trade winds blow from east to west as is typical, and they pile up warm water over the Indonesian Island chain region ( the western Pacific Ocean), leaving cooler water to pile up off the coast of South America (the eastern Pacific Ocean). This naturally causes air to rise over the warmer water region and sink over the cooler water region creating a large horizontal circulation cell known as the Walker Circulation.

 The La Niña phase is similar to the Neutral phase except the trade winds blow harder and the warm and cool water contrast is larger, making the Walker Circulation even stronger. The El Niño phase can be viewed as the opposite of the Neutral and La Niña phases. The east-to-west flowing trade winds either blow much slower or even in a different direction causing the warm and cool water to pile up to switch. Each of these phases globally influences weather and climate, but their influence differs across regions. The La Niña phase promotes wetter weather over Southern Africa, but over Southern USA it can promote drier conditions.

It is important that we understand ENSO but the effects on Southern African climate and weather are perhaps most important to understand. Moisture for our summer rainfall frequently comes from the southwest Indian Ocean from the generally east to west flowing winds transporting moisture inland over southern Africa. These winds are generally strong during a La Niña phase, which is a major reason why we often experience wetter than normal conditions during the summer wet season. 

Because of the switch in winds during El Niño, we have less moisture flowing inland and often experience drier conditions as a result. It is important to note my mention of “often”, because these typical conditions do not always happen during El Niño and La Niña phases.

Specifically considering the La Niña effects over Southern Africa, this is typically wetter and cooler than normal (about a 30-year average) conditions during all of the extended summer months of October to March/April, but its strongest effect on rainfall is generally during the mid-summer months of November to February. Above normal rainfall can sound like a good thing, but this can also cause flooding if perhaps rainfall is concentrated over fewer, larger events.

Research has highlighted that flooding is often more frequent during the La Niña phase summers. This can, for example, be damaging to infrastructure. It can even be damaging to lives as seen by the recent lives lost with the flooding in Johannesburg this month. Also concerning is that such heavier rainfall events can influence optimal planting time and cause crop damages, ultimately influencing food security. Many other negative effects can arise, such as water-borne diseases during flooding episodes.

There is a shift to more La Niña conditions and this might be linked to human-induced warming.

When will El Niño come and will there be a drought?

At present, there is limited certainty on when the next El Niño will occur. Several climate services track the current status of the ENSO and also provide forecasts on the status of it. Currently, the forecasts suggest that the ongoing La Niña is expected to persist until early 2023. From February to April 2023, there is a reported 71% chance of ENSO-neutral conditions, meaning that it is likely the current La Niña will break. 

Typically, ENSO-neutral conditions following a La Niña should lead to an El Niño, but this might not always the case – during June and July 2021 the La Niña broke, but from August La Niña conditions again persisted – and the length of time the ENSO-neutral conditions will hold is also uncertain; this could be as short as one month and as long as three years.

Despite this, there is certainty that an El Niño will happen in the future and with El Niño there is always a concern of drought, particularly during Southern Africa’s extended summer wet season of October to March/April. 

This is because ENSO has a stronger effect on Southern Africa’s weather and climate during summer (particularly November to February), and El Niño more typically brings drought,) as well as hotter than normal conditions, which typically promotes drier conditions. It is, however, always important to acknowledge that this is not always, and while most El Niño’s are generally associated with drought to varying degrees, the summer of 1997-98 represents an example of when the El Niño did not cause drought. 

The worry regarding drought is heightened across Southern Africa because drought events significantly affect our food and water resources, and in line with changing rainfall patterns over Southern Africa, it is expected that drought events could be more devastating, potentially leading to further chances of “Day Zero” events such as that which almost occurred in Cape Town during 201517.

In 2015-16 the El Niño caused drought-like conditions where the Vaal Dam’s capacity fell to almost 20%, which would compromise the province.