The El Niño Southern Oscillation enso background



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The El Niño Southern Oscillation - ENSO

Background

ENSO is an atmosphere and ocean phenomenon. It is associated with quasi-periodic, sea-saw like changes in the distribution of sea surface temperature, elevation of sea surface level, sea surface level atmospheric pressure that results in large scale perturbations to the mean, averaged atmospheric and oceanic circulation. The phenomenon is primarily confined to the Equatorial Pacific Ocean, although its influence upon the state of the atmosphere and ocean can be felt around the planet.

The existence of ENSO was first recognized as a reoccurring atmospheric oscillation in atmospheric pressure differences between Darwin, Australia and Tahiti, French Polynesia at the end of the 19th century. The pressure difference is now expressed through the Southern Oscillation Index (SOI). It sea-saws between values of about +30 and –30 (Figure 1). At about the same time, reports of reoccurring, quasi-periodic changes in sea surface temperature, i.e. an unusual warming off the South American coast became known to western scientists as El Niño.

More recently, scientists found that the atmosphere and ocean phenomena independently discovered and observed are really a result of one and the same coupled ocean-atmosphere process, hence, the term ENSO. It is now known as the largest global anomaly of the climate system. The discovery of this link between the ocean and atmosphere sparked major international research efforts. Soon it was recognized that El Niño is quite often followed by an event that results in an unusual cooling of the eastern Pacific Ocean and an associated warming in the west. In some way, the Pacific Ocean behaves like a huge bath tab, in which water sloshes from one end to the other quasi-periodically, raising sea surface level and temperature at one end and lowering both at the other end. The unusual cooling in the east is now referred to as La Niña. One cycle of warm, normal, cold, normal and warm conditions in sea surface temperature takes about 3-4 years to complete. El Niño and La Niña are both extreme phases of ENSO and are often also referred to as the ENSO warm and ENSO cold phase. During the warm event, the SOI value is positive and during the cold event it is negative.




Figure 1: A time series of the Southern Oscillation Index based upon the sea level pressure difference (anomaly) between Tahiti and Darwin for the period 1970 to 2000. The index oscillates between values of about –30 and +30 with a period of about 3-4 years. Strongly negative values are associated with an unusual cooling (La Niña or ENSO cool phase) and strongly positive values are associated with warming (El Niño or ENSO warm phase) of the eastern equatorial Pacific Ocean. A very distinct, clear-cut example is the warm event in 1987-88 which was followed by a cold event in 1988-1989.
Activity 1: Build your own El Niño

What You Need

Rectangular clear container (glass or plastic--approx.18"x4"x4")

Cold water

Cooking oil

Blue food coloring

Hair dryer

Blank newsprint paper to set the container on

Markers for making map

Handout: world map showing the Pacific Ocean
What You Do

1. Make your Pacific Ocean—

a. Place the container in the center of the blank piece of newsprint and in the margins mark the coast of South America on the east side and Indonesia/Australia on the west side.

b. Fill container half full of water.

c. Mix in a few drops of blue food coloring.

2. Create your “warm water” layer

a. Pour cooking oil over the surface of the water until you have about 1 inches of oil. (It’s OK if it seems to mix. Just let it rest until everything is separate again. )

3. Let the tradewinds blow!

a. Plug in your hair dryer (Keep it away from any water, including spills. If you DO happen to drop it into your ocean, DON’T fish it out; unplug it.)

b. Turn on your blow-dryer and blow the air stream across the water from East (South America to West (Australia/Indonesia)



Questions:

What do you see in terms of the "warm" and "cold" water.

What happens when you stop the tradewinds?
What’s Going On?

The container represents a slice of Pacific Ocean between Indonesia and South America. The blue is the cooler nutrient-rich water and the oil layer is the layer of warm water created naturally when the sun’s rays hit the surface. Where the two layers meet is the thermocline. The hair dryer represents the trade winds.


As the air blows over the surface of the water, the hot water will move to the other end of the container. The cold water will rise to the surface replacing the hot water. This represents the usual winds that drive the warm water to Australia. Turn off the hairdryer and look at the slope between the hot and cold water. Does the water return to El Nino conditions now that the wind has stopped? Of course, the ocean's water is not exactly half hot, half cold. The warm layer is really a thin surface layer. While this model helps you understand the processes at work, it doesn't give an accurate representation of El Niño.
When the tradewinds blow you should notice that the “warm water” (oil) piles up in the West as it is blown by the trade winds. This is the normal condition for the equatorial Pacific Ocean.

Use your red markers to show the normal location and direction of the warm water and air in the Pacific (along the eastern Australian coast and around Indonesia). Label these “normal conditions.”




Figure 2a shows the actual winter pressure zones and winds for normal conditions. Note the high pressure cell in the eastern Pacific off South America driving the south easterly trade winds.


Figure 2b shows normal Walker circulation in the Equatorial Pacific
Discuss what happens to the air above the warm water in terms of how much moisture the air can hold. Notice how the blue water moves upwards and mixes up more towards the surface at the east end. This mix-up is called upwelling and it’s essential for bringing up nutrient-rich water from the bottom to the surface. Discuss how plankton feed on the nutrients, and in turn fish feed on the plankton, so these areas tend to be rich in fish and other sea life.

Isn’t it interesting that food chains begin with weather conditions.



When the tradewinds stop the layer of warm water flows across the ocean from West to East. This is the warm water part of the El Niño condition. Notice that the upwelling that you saw with the tradewinds has stopped. Now a thick layer of warm water (oil) covers the surface in the East, choking off the cool, nutrient-rich waters from rising up to the surface. Take out the maps and use blue markers to show the El Niño location and direction of the warm water and air in the Pacific (across the Ocean from Australia to South America.). Label them “El Niño conditions.” Show the patterns of El Niño in the world's oceans using different colors to represent warmer and cooler water, and arrows to represent the direction in which the water is moving.


Figure 2c shows El Niño conditions in the Equatorial Pacific




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