(Revised version: Feb 2004)

Here is some information on the most talked-about topic in climate circles.

What is El Niño?
What is La Niña?
What about ENSO?
The scientific definition for ENSO
What happens in the ocean during ENSO?
What happens in the atmosphere during ENSO?
The impact of ENSO on global climate
The impact of ENSO on South Africa
Does El Niño cause drought in South Africa?
Can ENSO be forecast?
Useful links to other ENSO sites

What is El Niño?

El Niño 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 Niño 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 Niño 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 Niño 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 extend.

What is La Niña?

La Niña 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 Niña, Spanish for "the girl", is the opposite of El Niño. 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 Niño/La Niña", 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 Niño and La Niña.

ENSO = El Niño Southern Oscillation

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 (see map of Nino regions ) are greater than or equal to 0.5°C, there is a good chance of an El Niño event taking place. When the anomalies are smaller than or equal to -0.5°C, there is a good chance of a La Niña 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.

Graph of Nino 3.4 anomalies for 1980 - 2003
(click on thumbnail for enlarged version)

What happens in the ocean during ENSO?

The Pacific Ocean is a huge mass of water which can control many climate features in its region, since changes in the ocean result in characteristic changes in the atmosphere which, in turn, alter climate and weather patterns across the globe.

During normal years (non-ENSO years) relatively cold water occurs along the west coast of South America , an effect increased by upwelling of cold water along the Peruvian coast. The cold water then flows westward along the equator to Australia and is heated by the tropical sun. These normal conditions make the western Pacific about 3°C to 8°C warmer than the eastern Pacific.

Normal conditions in the Pacific Ocean and the atmosphere (click on thumbnail for enlarged version)

During La Niña years, the upwelling off the Peruvian coast is enhanced and the SSTs in the Nino regions become cooler than usual.

During El Niño years, the area of warm water (usually over the western tropical Pacific near Australia ) cools down and the warm water is displaced eastward to the central Pacific. The upwelling off the Peruvian coast is suppressed and the SSTs in this region become warmer than usual.

Conditions in the Pacific Ocean and the atmosphere during El Niño (click on thumbnail for enlarged version)

Measuring the ocean : Sea-surface temperature (SST) anomalies are (amongst other) used to measure the state of the global oceans. The long-term mean for each location over the oceans is calculated from a long record of SST data for these specific locations. The mean is then subtracted from the current value. If the ocean is warmer than usual, it will have a higher SST value than the mean and therefore a positive anomaly. On the other hand, a colder-than-usual ocean surface will give rise to a negative anomaly. El Niño events are associated with positive SST anomalies, while La Niña events are associated with negative SST anomalies.

Map of Nino regions
(click on thumbnail for enlarged version)

Source:

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/nino_regions.html

What happens in the atmosphere during ENSO?

The atmospheric circulation between high and low pressure regions in the tropical Pacific is known as the Walker Circulation. The easterly trade winds are part of the low-level component of the Walker circulation. During normal years (non-ENSO years), the trade winds move over the warmer sea bringing warm, moist air towards the Indonesian region. This moist air rises to high levels and travels eastward before sinking over the eastern Pacific Ocean . The rising air is associated with a region of low air pressure, towering cumulonimbus clouds and rain. High pressure and dry conditions accompany the sinking air. (Figure: Normal conditions in the Pacific Ocean and the atmosphere .)

During La Niña years, the Walker Circulation operates in the same way as described for normal years, but because of the larger area of colder water off the South American coast and the displacement of warmer water to the west, the atmospheric pattern also shifts accordingly.

During El Niño years, the Walker Circulation is altered due to the changes in the Pacific Ocean . The lifting and sinking of air - and therefore rainy and dry conditions - move with the warmer and colder SSTs to form the pattern depicted in the figure Conditions in the Pacific Ocean and the atmosphere during El Niño .

Measuring the atmosphere : The Southern Oscillation Index (SOI) gives a simple measure of the strength and phase of the difference in sea-level pressure between Tahiti (in the mid-Pacific) and Darwin (in Australia ). This difference is given in terms of an index. The typical Walker circulation has an SOI close to zero, while a strong negative value usually indicates that the oscillation has entered an El Niño phase. A strong positive value usually indicates a La Niña phase.

The Southern Oscillation Index for 1980 - 2003

(click on thumbnail for enlarged version)

The impact of ENSO on global climate

ENSO influences both rainfall and temperature patterns worldwide. The following figures show the changes in climate and weather patterns due to El Niño and La Niña. Please note that these figures are only applicable for December to February (summer in the southern hemisphere) when ENSO's impact on southern Africa is most profound. There is very little evidence that El Niño or La Niña effect winter rainfall over our region.

The global impact of El Niño during December

to February

(click on thumbnail for enlarged version)

The global impact of La Niña during December

to February

(click on thumbnail for enlarged version)

The impact of ENSO on South Africa

Although the southern part of Africa generally receives below-normal rainfall during El Niño years and La Niña 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 Niño 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 Niño always cause drought in South Africa? No. Although most El Niño years have been associated with below-normal rainfall, the impact of El Niño is often reduced by the sufficient groundwater and soil moisture content carried over from previous seasons.

The influence of ENSO on rainfall over the summer rainfall regions of South Africa are shown in the following cases for 1952/53 to 1999/2000:

North-eastern part of South Africa:

During El Niño years (red bars), the observed rainfall indices for October-November-December show normal to below-normal rainfall, while the observed rainfall indices for January-February-March vary between above-normal, normal and below-normal for this region.

During La Niña years (blue bars), the observed rainfall indices for October-November-December show normal to above-normal rainfall, while the observed rainfall indices for January-February-March vary between above-normal, normal and below-normal for this region.

North-eastern part of South Africa

(click on thumbnail for enlarged version)

Central part of South Africa :

During El Niño years (red bars), the observed rainfall indices for September-October-November vary between above-normal, normal and below-normal, while the observed rainfall indices for December-January-February show normal to below-normal rainfall for this region.

During La Niña years (blue bars), the observed rainfall indices for September-October-November was mainly normal, while the observed rainfall indices for December-January-February vary between above-normal, normal and below-normal rainfall for this region.

Central part of South Africa

(click on thumbnail for enlarged version)

Can ENSO be forecast?

Yes. SSTs 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 Niño/La Niña forecast is NOT a rainfall forecast.

More about ENSO

Links to other ENSO sites:

World Meteorological Organisation (WMO)

http://www.wmo.ch/nino/ninoi.html

Climate Prediction Center (CPC)

www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensocycle/enso_cycle.html

www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/cold_impacts.html

www.cpc.ncep.noaa.gov/products/analysis_monitoring/impacts/warm_impacts.html

International Research Institute for Climate Prediction (IRI)

http://iri.ldeo.columbia.edu/climate/ENSO/index.html

Glossary

Anomaly : The deviation from the mean. To calculate SST anomalies, the long-term mean for a specific point in the ocean is subtracted from the current value. A negative value indicates that the current value is cooler (smaller) than usual, while a positive value indicates that the current value is warmer (larger) than usual. For example:

Cumulonimbus : Exceptionally dense and vertically developed clouds, occurring either as isolated clouds or as a line or wall of clouds with separated upper portions. These clouds appear as mountains or huge towers, at least a part of the upper portions of which are usually smooth, fibrous, or striated, and almost flattened. This part often spreads out in the form of anvil (incus) or vast plume. Its precipitation is often heavy and always of a showery nature.

Index: An index is calculated by dividing the anomaly with the standard deviation. Value - Mean (i.e. anomaly) / Standard Deviation. This has the effect that an index does NOT have a unit. For example, the SST anomaly of a certain point can be 2.4°C, but the index may be 1.3.

SOI: Southern Oscillation Index

SST : Sea-surface temperature

Three-month running mean : is calculated by getting the mean of three consecutive months' values, for example: