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What is El Nino?
El Nino is the warming of sea-surface temperatures in the equatorial Pacific Ocean which influences atmospheric circulation, and consequently rainfall and temperature in specific areas around the world.
El Nino is translated from Spanish as the boy child. Peruvian anchovy fishermen traditionally used the term – a reference to the Christ child – to describe the appearance of a warm ocean current off the South American coast around Christmas. Over the years the term El Nino has come to be reserved for the sequence of changes in the circulation across the Pacific Ocean and Indonesian archipelago when warming is particularly strong. Approximately 14 El Nino events affected the world between 1950 and 2003. Amongst them was the 1997/98 event, by many measures the strongest thus far this century, although South Africa escaped the impact of it to some extent.
What is La Nina?
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.
La Nina, Spanish for the girl, is the opposite of El Nino. SSTs in the equatorial Pacific become cooler than normal, giving rise to the term cold event. This situation is reflected by negative SST anomalies. The Walker circulation intensifies and the SOI consequently becomes positive during this event.
What about ENSO?
The changes in the Pacific Ocean are represented by the term El Nino/La Nina, while changes in the atmosphere are known as the Southern Oscillation. Because these two cannot be separated, the term ENSO is often used. ENSO refers to both El Nino and La Nina.
The scientific definition for ENSO
A scientific definition was recently developed to help scientists to identify ENSO events. When the three-month running mean of the SST anomalies in the Nino 3.4 region are greater than or equal to 0.5°C, there is a good chance of an El Nino event taking place. When the anomalies are smaller than or equal to -0.5°C, there is a good chance of a La Nina event taking place. Take note, however, that strong ENSO events (which are more likely to affect our seasonal climate) have a larger SST anomaly and normally last for a period much longer than three months.