Lesson 11: El Niño Southern Oscillation(ENSO)

Physical Oceanography

We’ve learned a lot about physical forces in the oceans

What are two primary types of waves?

What is one major cause of tides?

What is the difference between a spring tide and a neap tide?

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Interactions between the air and the sea are important

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1. The El Niño Southern Oscillation (ENSO) is a disruption of the ocean-atmosphere system in the Tropical Pacific, the ocean basin located between New Guinea, Australia and the Americas

2. A key characteristic of ENSO is increased surface ocean temperature in the equatorial Pacific

3. The ENSO greatly affects global weather

4. Scientists use oceanic and atmospheric data to predict ENSO events

Oceans can affect the weather

Evaporation from the ocean can transfer heat and moisture to the atmosphere and thereby affect weather patterns

Currents can also transport heat from warm areas to cool areas and vice versa Example: The Gulf Stream transfers heat from the tropics to

Europe. Without it Europe would be much colder. Upwelling, the vertical transport of cold, nutrient-rich water

from the deep water to the surface can also cool nearby areas Example: Temperatures in the Galapagos are slightly cooler than

areas at similar latitudes due to upwelling in the region. ENSO is an example of a periodic interaction between the

ocean and atmosphere that can influence global weather and climate

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How does an El Niño occur?

Photo: NOAA / OAR / PMEL

Cool, nutrient rich water (blue and green) upwells along the west coast of North and South America to replace the water being pushed toward New Guinea

Strong trade winds (white arrows) blow from the Americas toward New Guinea and Australia

Under normal conditions…..

These winds “push” water that has been warmed by the sun (red) toward the coast of New Guinea and Australia

Heavy rains are concentrated in the Western Pacific Ocean

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How does an El Niño occur?

Photo: NOAA / OAR / PMEL

Upwelling along the coast of the Americas also decreases slowing the flow of cool, nutrient rich water to the surface

Trade winds weaken

Under El Niño conditions…..

As a result of weakening trade winds, warm water (red) moves eastward along the Equator, toward the Americas(white arrows)

Ocean heat alters the jet stream, causing rains to occur beyond their normal location

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Why do we care about El Niño ?

During El Niño (and La Niña) years, more destructive weather events tend to occur:– Droughts and brush fires– Intense hurricanes– Intense tropical storms– Severe coastal flooding– Decline of some marine species (to figure out

why, think about what happens during upwelling)

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How do you think scientists can predict El Niño events?

The red color means SST is warmer, blue is cooler

SouthAmericaAustralia

During an El Nino year, SST is warmer than normal

SouthAmerica

Australia

Photo:PMEL/NOAA

By looking for abnormal sea surface temperature (SST) – known as SST anomalies

By looking at the Southern Oscillation Index (SOI)

The SOI refers to an oscillation or “seesaw” in air pressure systems between the western and eastern Pacific that is linked to El Niño

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Which graph shows a normal year and which shows an El Niño year?

A.

B.

Australia

The color code corresponds with temperature: orange and red are relativelyWarm (27-31oC) and the blues are relatively cool (20-23 oC)

South America

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Were you right?

A. Normal

Australia

The color code corresponds with temperature: orange and red are relativelyWarm (27-31oC) and the blues are relatively cool (20-23 oC)

South AmericaB. El Niño

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La Niña

La Niña causes mostly the opposite effects of El Niño– For example: In the Southeastern U.S., El Niño would typically cause

cooler winter temperatures while La Niña would typically cause warmer winter temperatures

La Niña is characterized by unusually cold ocean temperatures in the eastern equatorial Pacific

La Niña is thought to occur due to increases in the strength of the normal patterns of trade wind circulation

These trade winds increase upwelling off the coast of South America, bringing cool water to the surface

Impacts include increased rainfall in the Western tropical Pacific

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Decide whether each graph represents normal, El Niño or La Niña conditions

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A. December 1998

B. December 1993

C. December 1997

La Niña: relatively cool SST

El Niño: relatively warm SST

Normal

Class activity

Test your skills at predicting El Niño!

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