El Niņo – riding the climate roller coaster

Key text

El Niņo got its name from Spanish-speaking fishermen from Chile and Peru working in the Pacific. They noticed that their catches of anchovies sometimes suddenly declined around Christmas time (El Niņo means the Christ child). We now know that the change in the availability of fish off the Pacific coast of South America is just one small part of an enormous series of changes with effects in both the southern and northern hemispheres. Atmospheric scientists refer to these changes as the El Niņo Southern Oscillation (ENSO).

Southern Oscillation

The Southern Oscillation refers to the see-sawing change in average atmospheric pressure between the mid-Pacific and northern Australia (measured at Tahiti and Darwin). The Southern Oscillation Index measures the difference in atmospheric pressure between Tahiti and Darwin. When the pressure is persistently low over the mid-Pacific, it is high over Australia and the Indian Ocean. A persistent below average atmospheric pressure in the mid-Pacific is associated with an El Niņo and dry conditions. The opposite set of conditions to El Niņo, known as La Niņa, is frequently associated with heavy rains and flooding.

When an El Niņo event occurs, eastern Australia, parts of Asia and southern Africa may be plunged into severe drought, while parts of South America and the west coast of the USA may suffer unusually heavy rain and floods.

How El Niņo works

For most of the time, air over the Pacific Ocean circulates in a regular fashion. Hot, moist air rises over the wet, tropical Indonesian region and then travels eastwards at a height of about 10-15 kilometres. As it moves it cools and dries out, and finally it descends as cool, dry air near the Pacific coast of Peru. Consequently, this part of South America is usually dry. At the Earth's surface, the winds move in the opposite direction – from east to west – and this completes the circulation of air over the Pacific Ocean. This is known as the Walker circulation (Box 1: The Walker circulation and weather forecasting).

Related site: El Niņo – La Niņa animation Shows how changes in the Walker circulation affect rainfall patterns. (Bureau of Meteorology, Australia)

As the surface winds blow, they drag some of the surface waters of the ocean along with them. The result is that the Pacific Ocean is not quite level. The average sea level is usually slightly higher at the western (Australian) side than at the eastern (South American) side. The difference is very slight (less than a metre) but it can be detected.

For reasons that are still not well understood there is a breakdown of the Walker circulation that occurs every 2 to 7 years, leading to ENSO events lasting between 18 and 24 months. A possible trigger to the 1997 El Niņo may have been the unusually long duration of cyclone Justin, which occurred off the northern Australian coast in March. The easterly flow of surface winds was interrupted by 'westerly wind bursts' from the cyclone which may have initiated the movement of warm water in the western Pacific across to the eastern Pacific. Warmer sea surface temperatures in the eastern Pacific Ocean indicate an El Niņo; cooler temperatures, La Niņa.

The flow of warm water from the western to eastern Pacific causes noticeable changes in sea-level (up to 0.5 metres) and in some locations, for example off the coast of Peru, sea surface temperatures can rise very rapidly. Monitoring the changes in sea surface temperature along the equatorial Pacific is now one of the key diagnostic tools in tracing the development of El Niņo events.

Along with the temperature change, the moisture-laden east-west winds and their associated ocean currents slacken. The surface winds no longer blow from South America towards Indonesia carrying moisture picked up from the ocean. So the rains fail in Indonesia, Papua New Guinea and throughout much of eastern Australia too. The high altitude transport of air from Indonesia to Peru also decreases. The cool dry air doesn't arrive near Peru. The warm water now occurring there interferes with the cold, nutrient-rich Humboldt Current that normally travels northwards up that coast from Antarctica. As a consequence, the water is poorer in nutrients and fish numbers drop. But the warmer than usual water there means more evaporation and therefore a moister atmosphere. Heavy rain then occurs in that part of South America.

Impact of El Niņo

El Niņo events occur irregularly every 2 to 7 years and have major economic effects. In 1982-3 a major event occurred: the fisheries industry off the Pacific coast of South America lost about $290 million as catches declined. Countries like Peru and Ecuador had their heaviest ever recorded rains (northern Peru receiving about 340 times the average figure) and suffered considerable flooding, as did part of the western USA. Eastern Australia endured one of its worst ever droughts, resulting in a $2000 million loss in agricultural production, as well as bushfires and dust storms. Indonesia also had dry conditions, and the monsoon rains failed as far away as India.

Reliable forecasting of El Niņos would help to minimise such devastating effects. Unfortunately, it is not yet possible to predict when an El Niņo will start. Australian researchers are attempting to improve the forecasting of El Niņo events. Climate modelling is just one component of forecasting (Box 2: Modelling climate).

Box 1. The Walker circulation and weather forecasting

Australia experiences variable rainfall

The variability of the rainfall is a particularly important characteristic of the Australian climate. It has shaped Australia’s flora and fauna as well as its primary industry and way of life. The variability of rainfall is a consequence of Australia’s geographical location at the western edge of the largest ocean in the world, whose sheer size and water temperature distribution determine the nature of much of the Walker circulation. Fluctuations in the Walker circulation increase the variability of the Australian rainfall. The Walker circulation also has a major effect on the frequency and location of tropical cyclones and on annual rainfall pattern over the wider Pacific region.

Long-range weather forecasting

With satellite-based observations available, investigators have more closely studied the Walker circulation and the associated El Niño phenomenon. The approach to long-range weather forecasting has changed significantly over the past 25 years. Scientists now look at irregularities in the temperature of the surface of the ocean as a potential cause of the irregularities of the temperature of the atmosphere. At the same time, other scientists found that certain repetitive patterns of atmospheric flow are related to each other in different parts of the world.

These observations led to a better understanding about how the occurrence of unusually warm or cold ocean waters – and of unusually high or low atmospheric pressures – could be interconnected in worldwide climate systems. Knowledge about these links and about the behaviour of parts of these global systems helps forecasters to make better long-range predictions. This is partly because the features change slowly and with some regularity. This approach of studying interconnections between the atmosphere and the ocean may represent the beginning of a revolutionary stage in long-range forecasting.

In the last 10 years scientists have applied numerical weather prediction models to long-range forecasting. These models are not concerned with the predicting the details of weather 20 or 30 days in advance – but with predicting out-of-the-ordinary events in the global weather system. The reliability of these long-range forecasts, like that of short- and medium-range projections, has improved substantially in recent years.

Box 2. Modelling climate

Conditions on the surface of the Earth – such as temperature, humidity, wind speed and atmospheric pressure – as well as those at various heights in the atmosphere are entered into the computer. Through decades of observing and measuring the behaviour of the atmosphere, scientists have constructed mathematical equations that describe the known movements of heat into and out of the Earth and the atmosphere; how changes in atmospheric composition should cause these heat transfers to change; the rates of evaporation and precipitation of water; the major currents in the air and oceans and their distribution of heat; and the frequency of tropical cyclones.

The model of this land-ocean-atmosphere system is represented by a number of points, making up a grid that acts like a bare outline of a picture.

Just as the picture on a television screen is made of many changing dots organised in 625 lines in order to represent the reality that was filmed, so the changing parameters at each grid-point of a GCM represent the reality of Earth's climate system.

Current GCMs may contain one point every 500 kilometres of surface – whether on east-west or north-south transects – and between two and twenty points above each one on the surface to represent the height of the atmosphere. So, for example, they could have a total of 2000 points on the Earth's surface and 20,000 in the atmosphere. The solutions to a number of equations must be calculated for every one of these 22,000 points in order to move the model forward in time.

Each step of model-time could be an hour, or even less, and the model calculates how the conditions have changed during that time-step. Each point in the grid is given its starting conditions, using the chosen parameters of temperature and so on. Then a supercomputer solves the equations for each point. This creates a change in conditions, and the new information is used to run the equations all over again with the new numbers inserted.

Powerful supercomputers are capable of carrying out more than 500 million calculations per second. However, to solve all the equations necessary to run an Earth-atmosphere system for a model time of 1 year takes about 2 hours of computer time. As more computer power becomes available, the model can become more detailed, with points at closer intervals. The tighter the spacing of dots, and the smaller each time-step of the model, the more accurate can be the representation of reality, exactly as a finer-grained photograph or a greater number of frames per second in a moving film gives greater clarity.

The test for a computer model is to see how well it simulates known real events. Scientists can then see how well the model predicts the already known outcome. For example, you could feed into a GCM some of the temperatures, atmospheric pressures, wind and ocean movements that existed in the world at the end of 1982, which was the start of the strong El Niņo event that happened during 1982-82 – and then run the model for 2 years of model time. Would the outcomes given by the model resemble what we know really happened to temperature and rainfall around the world?

When compared with reality the best models have performed well; scientists therefore regard them as good representations of some aspects of reality, but of course they are not complete and cannot predict accurate outcomes for local regions, mainly because they still space the points of the grid too far apart.

Related sites

• Modelling climate (CSIRO Atmospheric Research, Australia)

• Climate modelling: General circulation models (Global Climate Change Student Guide, Manchester Metropolitan University, UK)

• Types of climate models (Met Office, UK)

Activities

• The Met Office, UK
  • Weather and design and technology – make a simple rain guage, thermometer screen, anemometer, wind vane, barometer and sundial.

• Bureau of Meteorology, Australia
  • Learn more about meteorology: students and teachers – provides a range of experiments and activities related to the weather (eg, El Niņo and rainfall, Meteorology – what is it?). The table of activities includes a description, suggested year level, learning outcomes, teachers lesson plan and student worksheet.

• Australian Broadcasting Corporation (Australia)
  • El Niņo – of droughts and flooding rains – student activities based on an article on El Niņo.

• Careers in Science, Australia
  • Meteorology, climate warming and/or El Niņo – students develop an appreciation for the relevance of scientific research to people’s lives.

• The Helix
  • Kitchen El Niņo – demonstrates what happens during an El Niņo event (August/September 1996, page 25).

• Xpeditions (National Geographic, USA)
  • The ocean and weather: El Niņo and La Niņa – students explore the weather phenomena El Niņo and La Niņa.

• National Aeronautics and Space Administration (USA)
  • Hills and valleys of the sea surface – students make a topographic model of the sea surface to explain El Niņo events.

• Science NetLinks (American Association for the Advancement of Science)
  • El Niņo – students learn how the atmosphere and oceans affect one another, and how a small change in sea surface height can have a large impact on weather.

Further reading

Useful sites

• Climate variability and El Niņo
  A good overview of what causes El Niņos and how they affect Australia's climate.
  http://www.bom.gov.au/climate/glossary/elnino/elnino.shtml

• El Niņo wrap-up
  Starts with a look at the current situation then briefly explains what El Niņo is. 'El Niņo education' gives more information about climate variability and El Niņo.
  http://www.bom.gov.au/climate/enso/

• El Niņo: "droughts and flooding rains"
  An article by Neville Nicholls describing the El Niņo-Southern Oscillation and how it affects Australia. Nicholls also discusses climate forecasting and the 1997 El Niņo.
  http://www.bom.gov.au/bmrc/clfor/cfstaff/nnn/nnn_el_nino.htm

The Long Paddock (Queensland Government, Australia)

• Climate management information for rural Australia
  Practical information to help people plan for climate variability. Information includes 'Seasonal climate outlook' and 'Queensland drought monitor'.
  http://www.longpaddock.qld.gov.au/

• El Niņo and the Southern Oscillation
  A good explanation of the El Niņo-Southern Oscillation.
  http://www.longpaddock.qld.gov.au/Help/ElNinoSouthernOscillation/

Australian Broadcasting Corporation

• Indian Ocean – influence on Australia's climate (The Science Show, 30 September 2006)
  Discusses how the Indian Ocean influences the climate over the whole of Australia.
  http://www.abc.net.au/rn/scienceshow/stories/2006/1751859.htm#

• Earth almost as hot as in ancient times (News in Science, 26 September 2006)
  Comments on the role of El Niņo in global warming.
  http://www.abc.net.au/science/news/stories/2006/1748930.htm?enviro

• Droughts and flooding rain (Scribbly Gum, August 2006)
  Describes El Niņo and La Niņa events that bring long dry spells and then periods of intense rain to south-eastern Australia.
  http://www.abc.net.au/science/scribblygum/august2006/

What are the potential contributions of El Niņo-Southern Oscillation research to early warning of potential acute food-deficit situations?

This paper by Neville Nicholls from the Internet Journal for African Studies discusses African El Niņo droughts which have led to devastating famines in Africa.
http://www.ccb.ucar.edu/ijas/ijasno2/nicholls.html

El Niņo and the Australian drought

Dr John Zillman, President of the World Meteorological Organisation, describes global patterns of climate, the Southern Oscillations Index and the 1997 El Niņo. (Beware: heavy graphics.)
http://www.atse.org.au/index.php?sectionid=383

National Oceanic and Atmospheric Administration, USA

• An El Niņo theme page
  An extensive site compiled by a number of different US government agencies. It explains El Niņo, discusses impacts, provides forecasts and has frequently asked questions. There are good illustrations and real-time graphics.
  http://www.pmel.noaa.gov/tao/elnino/nino-home.html

• NOAA La Niņa Page
  Includes general information, forecasts and impacts.
  http://www.elnino.noaa.gov/lanina.html

• The ENSO cycle
  Covers ocean temperatures, atmospheric circulation and rainfall patterns.
  http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensocycle/enso_cycle.html

El Niņo and La Niņa – tracing the dance of ocean and atmosphere (National Academy of Sciences, USA)

Explores how climatologists and oceanographers have joined forces to construct theoretical models to simulate and predict broad climate changes associated with ENSO.
http://www7.nationalacademies.org/opus/elnino.html

Glossary

Southern Oscillation Index (SOI). A measure for monitoring the Southern Oscillation. The index is compiled by measuring the atmospheric pressure differences between Tahiti and Darwin (monthly or seasonally) and comparing the result with the mean for that time of year. The index scale ranges between about +30 and -30. A strongly negative (more negative than -10) SOI for several months indicates an El Niņo event; a strongly positive (greater than +10) SOI for several months indicates a La Niņa. For more information about the SOI and how it is calculated see Southern oscillation index (Bureau of Meteorology, Australia).