Unpacking the climate cycle of the El Niño and the impact it has on South African climate.
The 2015-2016 drought has been attributed to the El Niño phenomenon. However, many do not understand exactly how it works or why we experience drought in South Africa.
South Africa is one of the most water scarce countries in the world. It receives only half of the worlds average rainfall, which needs to supply the country’s population and various industries with water.
In general, South Africa has a dry, semi-arid climate, receiving a low annual rainfall of about 460mm per year, versus the world average of 860mm. In addition, there is a great variability of rainfall within the country. This uneven distribution is owed to an array of factors that affect the South African climate such as latitude, ocean currents and topography.
There are various ways to categorise the climatic zones in South Africa. The Köppen climate classification is the most widely used by climatologists across the globe, but it is quite complex. Instead, climatologist Dr Mark Tadross from CSAG, offers the following simplified climate regions for the country:
What is an El Niño?
Another global player that affects rainfall patterns and the hydrology in South Africa is the El Niño. The El Niño is a complex climatic phenomenon that starts in the Pacific Ocean and influences climate around the world. Over 2015 and 2016 it has had a powerful impact on South African weather, leading to the strongest El Niño event recorded since 1997-1998, according to NASA.
To understand how an El Niño works, it helps to know what happens during “normal” conditions. Usually the ocean water at the equator receives more energy from the sun and is therefore warmer. This warmer water has a direct impact on the humidity and atmosphere above it.
As the Earth spins, the trade winds over the ocean tends to blow from east to west. These gusts (called prevailing winds) continuously move warm water and push it across the Pacific Ocean in the direction of Australia and Asia (refer to the diagram below). As a result, water near the Americas is cooler, as illustrated by the thermocline (line indicating changes in temperature) showing warmer water moving towards Australia.
An El Niño event occurs when the trade winds strength blowing off the South American coast is weakened. For reasons that remain uncertain, these winds are weaker so there is a decrease of warm water being moved from east to west. As a result, the warm water builds up along the west coast of Peru and changes the rainfall patterns on a local and global scale. This can happen every two to seven years, says WeatherSA.
How the El Niño affects the global climate is variable. According to the Scientific American Journal the warmer waters along the American coast leads to increased evaporation of warm moist air over a larger area in the tropics. Because of the increased size and energy, the deep atmosphere and upper-air currents are affected which leads to a global shift of “short term weather systems” towards the east.
This usually leads to drought conditions in Africa, Australia and Indonesia as weather systems that usually bring water shift away. On the other hand, there is increased precipitation along the West Coast of the Americas. In the tropics is a drop of large storms across the Atlantic but a spike of hurricanes in the Pacific Ocean.
Effects of the El Niño in South Africa
In South Africa, an El Niño event is associated with increased temperatures and lower rainfall, according to climatologist Tristan Hauser, member of CSAG. However, Pierre-Louis Kloppers, data manager at CSAG, says that the drought effect the El Niño has in South Africa is variable, and cannot always be predicted in terms of timing or severity.
“The severity of the El Niño does not determine the amount of drought we experience (in South Africa),” says Kloppers. “It just means that there is a higher chance of a drought occurring.”
“Take the El Niño in 1997 for example. Everyone threw up their arms and said it would be the strongest El Niño, there is going to be a drought; and didn’t work out like that,” elaborates Tadross.
The El Niño’s severity is measured by the Oceanic Niño Index (ONI) which gives an indication of the El Niño strength. This is done through measuring the sea surface temperature in the middle of the Pacific Ocean. According to Johnston this severity has little impact on the strength of the drought experienced in South Africa.
Climatologist Peter Johnston avoids referring to the recent El Niño as the sole reason that the drought South Africa experienced was so severe. “We know that an El Niño increases the chances of a drought happening in South Africa by 60%,” he says. Yet, Johnston explains that the recent El Niño severity could be linked to other factors, such as natural variability, climate change and other oceanic influences over the South Indian or South Atlantic Ocean.
Still, the El Niño is still associated with drought conditions in Southern Africa and the La Nina with wetter conditions. An El Niño has a particular effect on South Africa’s rainfall. As the event is instigated in the tropics, it affects South Africa’s tropical eastern coast and interior, which is a summer rainfall region, more than the winter rainfall region along the West Coast, says Kloppers. This is why the drought was experienced more severely to the east and interior of the country than in the west and south.