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What are El Nino and La Nina?

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How does El Nino and La Nina affect our weather patterns?

el-nina-la-nina-explained“Even though the strong El Nino event of 2015/2016 has fully dissipated, food producers will continue to face pressure in the face of the worst drought in South Africa in 30 years as recent weather reports show a weakening of La Nina patterns. This does not bode well for South Africa’s hopes for good rains this summer.”

But what are El Nino and La Nina, and how do they affect our rainfall patterns?

What is El Nino?

El Nino, a Spanish word used by Peruvian anchovy fishermen that means ‘boy child’ – a reference to Christ child. El Nino is the WARMING of ocean currents off the South American coast around Christmas.
Increased sea-surface temperatures (SST’s) in the equatorial Pacific Ocean influence atmospheric circulation, and consequently rainfall and temperature in specific areas around the world.

What is La Nina?

La Nina, Spanish for ‘the girl’, is the opposite of El Nino. La Nina is the COOLING of sea-surface temperatures in the equatorial Pacific Ocean which influences atmospheric circulation, and consequently rainfall and temperature in specific areas around the world.

What is ENSO?

“ENSO” refers to the El Nino/Southern Oscillation, a naturally occurring phenomenon that involves fluctuating ocean temperatures in the equatorial Pacific. The warmer waters essentially slosh, or oscillate, back and forth across the Pacific, much like water in a bath tub. This results in a somewhat periodic variation between below-normal and above-normal sea surface temperatures and dry and wet conditions over the course of a few years.

The impact of ENSO on South Africa

Although the southern part of Africa generally receives below-normal rainfall during El Nino years and La Nina usually brings normal or above-normal rainfall, it cannot be accepted as a rule! Southern Africa can be divided into numerous rainfall regions, each region having a different correlation with ENSO. Also, ENSO explains only approximately 30% of the rainfall variability, which means that other factors should also be taken into account when predicting seasonal rainfall. For example: The 1997-98 El Nino was the strongest on record, but not all of South Africa received below-normal rainfall. Some regions had an abundance of rain because of moist air that was imported from the Indian Ocean. One should be careful not to make a general rule for rainfall and temperature changes in ENSO years over southern Africa.

Does El Nino always cause drought in South Africa?

No. Although most El Nino years have been associated with below-normal rainfall, the impact of El Nino is often reduced by the sufficient groundwater and soil moisture content carried over from previous seasons.

Can ENSO be forecast?

Yes. SST’s are used to measure the state of the ocean (and ENSO) and can be forecast up to 9 months ahead with good skill. Computer models are used for this and the first indication of ENSO influencing the October-to-March (summer) rainfall season can be forecast as early as the preceding May.

IMPORTANT: An El Nino/La Nina forecast is NOT a rainfall forecast.

Ref: South African Weather Service 

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