APES U4 El Nino lesson

El NiñoSouthern Oscillation
(ENSO)

Physical Oceanography

Interactions between the air and the sea are important

2

1.

The El NiñoSouthern 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

3

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

4

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

5

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)

6

How do you think scientists can predict El
Niño events?

The red color means SST is
warmer, blue is cooler

During an El Nino year, SST is
warmer than normal

South
America

South
America

Australia

Photo:PMEL/NOAA

7

Australia

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 relatively
Warm (27-31oC) and the blues are relatively cool (20-23 oC)

South America

8

Were you right?

A. Normal

Australia

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

South America

B. El Niño

9

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 Pacific10

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

11

A. December 1998

B. December 1993

C. December 1997

La Niña:
relatively
cool SST

El Niño:
relatively
warm SST

Normal

Multiple Choice

During an El Nino year, the rainfall in (Eastern) Australia would likely be:

• Greater than normal

• Less than normal

• About the same as a non El Nino year

• It is impossible to determine.

Multiple Choice

Typically there are trade winds that blow along the equator in the Pacific Ocean from East to West. In an El Nino year the trade winds

• increase in strength

• weaken

• do not change

Multiple Choice

What is upwelling?

• the rising of a cold, nutrient rich current from deep in the ocean

• a surge in fish populations like Peruvian anchovy

• a temperature cell in the atmosphere that rises rapidly

• a hurricane that forms rapidly

Multiple Choice

During El Nino, upwelling in the Pacific ocean off the coast of South America

• increases

• shows no change

• decreases

• Impossible to determine

Note, the image is showing chlorophyll (productivity)

​

​Which year shows an El nino event?

Multiple Choice

How could La Niña affect weather during the summer in Georgia?

• It could decrease wind.

• It could increase rainfall.

• It could decrease rainfall.

• It could increase temperatures.

Multiple Choice

During a(n) ____ event, trade winds are much stronger than usual.

• El Nino

• La Nina

• Normal

• Partly sunny

Multiple Choice

The picture represents a(n) ___ event.

• El Nino

• La Nina

• Non-El Nino (normal conditions)

Multiple Choice

Why do the fisherman of Peru and South America not welcome a strong El Nino?

• El Nino poisons the fish

• El Nino causes strong winds and currents which move fish westward

• El Nino bring cold water to the surface which kills the base of the food chain

• El Nino shuts off upwelling and nutrients needed for the marine food chain do not rise to the surface

Multiple Choice

Does this image show normal or El Nino conditions?

• normal

• El Nino

Multiple Choice

Which region would likely experience a drought during this ocean circulation pattern?

• South America

• Indonesia

Multiple Choice

One consequence of El Nino is _______ and increased rainfall in South America..

• flooding
• stronger Tradewinds
• strong winter storms
• drought