CAPE TOWN - Since April this year sea surface temperatures (SST’s) over the equatorial Pacific Ocean have been increasing, heralding the onset of an El Niño. While this in itself is not unusual, current predictions indicate that this could be a particularly strong event.
What exactly is El Niño?
El Niño occurs when ocean surface warming develops in the equatorial regions of the Pacific, along with a weakening of the easterly trade winds. These periods of higher than normal temperature anomalies last between nine months and two years.
La Niña on the other hand is the antithesis of El Niño, and occurs when the equatorial Pacific displays colder than normal sea surface temperatures along with enhanced easterly trade winds over the tropical Pacific Ocean.
This see-saw between El Niño and La Niña is known as the Southern Oscillation (referred to as ENSO - El Niño Southern Oscillation), and is a process that is continuous. Conditions are only considered ‘neutral’ when the SST’s are within 0.5ºC from the average temperature, and this usually takes place as a transitionary period between the two phases.
What makes the current event different?
Currently, SST’s are already more than 1ºC above normal for much of the ENSO region of the Pacific, with areas closer to the west coast of South America even warmer at over 2.5ºC from the norm (See figure below).
These numbers are the second highest ever recorded, only beaten by the unprecedented El Niño of 1997/98. The effects of the 97/98 event were felt globally, causing severe storms and flooding in some parts of the world and intense drought in other areas.
The infamous ice storm that crippled parts of Canada and the USA in January 1998 has also been attributed to that El Niño.
The latest forecasts, according to US’s National Oceanic and Atmospheric Administration (NOAA), mostly favour a further strengthening of this El Niño event and that it will continue until the end of 2015, perhaps lasting into the first half of 2016.
These forecasts are comprised of the output from several numerical models run by different agencies from around the world. Under any circumstances, each of these models indicate somewhat different outcomes although the majority currently agree that the warming trend of the SST’s will continue for the next few months. The agreement between models leaves little doubt that this El Niño will be a strong one and possibly close to the strength of 1997/98.
How does El Niño affect South Africa?
Both of the ENSO phases have been documented to have an effect on the eastern, summer rainfall parts of South Africa, with some correlation between El Niño and drier than normal conditions over this region.
For example, the severe drought that gripped the region in the early 1980’s coincided with another very strong El Niño event in 1983, although this is not a hard and fast rule since there are other factors that contribute to our seasonal rainfall patterns, and this was illustrated during the 97/98 event, which had little or no effect on South Africa.
What can be expected from this El Niño?
Almost all of central and eastern South Africa has already experienced below-normal rainfall over the last year (see figure below) but the North West and eastern KwaZulu-Natal are especially dry with only between 50-75% of their average annual rain having fallen. Water restrictions are already in place for several municipalities in KZN and the first restrictions in 32 years have recently been put into effect for Bloemfontein in the Free State.
If current predictions are correct and a strong El Niño does in fact materialise as the southern hemisphere enters spring and summer, the major concern is that it may, as it has in the past, stunt our summer rains and worsen the drought.
The most recent seasonal forecasts issued by the South African Weather Service only extends up to November, so for now it’s still too early to get an indication as to what the summer rainfall pattern may bring.