Explaining ENSO - Regis

Explaining ENSO - Regis

Explaining ENSO - Regis


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Explaining ENSO

El Niño Southern Oscillation

What is El Niño?

• A global-scale and periodic shift in climate that causes

localized changes in weather, initiated in the South


• Impacts depend on location – localized drought, flooding, heat,

cold, wind, lack of wind, etc.

• Often severe.

• Every ~3 to 7 years.

• Usually (more or less) the same impacts each time it happens.

• Sometimes far more severe or far milder.

• Weather – short day to day variations in atmospheric conditions at

a particular place.

• Climate – variations in atmospheric conditions over longer time

scales (e.g., seasons) and larger spatial scales (e.g., regional or


Why’s it named El Niño?

• Named by Peruvians who noticed that the

observed effects start around Christmas

time. “The Child” or “Christ Child”.

• Fishing industry collapses.

• Damaging floods.

• Droughts in the year following an El Niño.

How’s an El Niño Happen?

• Well, that’s the point of this lecture.

• In short, caused by a massive shift in Pacific ocean


• Lecture outline:

• First describe global-scale phenomena.

• Then describe basic weather.

• And then describe the El Niño climate details.

WARNING! Oversimplified.

• I’m a geophysicist by training.

• But not everyone is.

• My intended audience is non-specialists.

• So in the interest of clarity, I’m going to WAAAY

oversimplify some concepts.

• Will leave out many details.

• Will exaggerate to make a point.

• Still, this description of ENSO will be

fundamentally sound.

• I’ll give you all the right ideas, and I’ll get you up to speed so

that you can consult other resources for details.

Earth is a small place…

• We make the common mistake of

believing that the Earth is huge.

• Reasoning:

– Takes forever to walk across the Earth.

– If I jump up and down on my bed, then mom complains;

but I don’t get an annoyed phone call from the Chinese


• But really it’s a matter of scale.

A matter of scale: Consider walking

from Denver to Boulder.

Takes freakin’ forever.

Takes a day.

Takes minutes.

Denver Boulder

Big events can be impact the entire


• Examples of big events that can connect the


– Volcanoes in Sumatra can spread ash around the


– Krakatoa, 1883

– Dust storms in the Sahara cause poor visibility in the


– Earthquakes in the Canary Islands (North Africa)

could cause tidal waves in NYC.

How does the planet “connect”

over long distances?

• Air • Atmospheric circulation – winds move the air around the


• Water

• Oceanic circulation – water sloshes around.

• Think Gulf Stream.

• Think tides.

• Earth • Rings like a bell

– Hammer one side hard enough and the entire Earth rings.

Ringing like a bell

• Hit it in one place, and it oscillates


This is what earthquakes do.

Oceanic long distance connections

• Imagine a bath tub.

• If splash around a lot, the water will slosh from side to side.

• Ocean is a giant bathtub. How’s it slosh?

• Moon pulls water from one side to the other.

– Tides!

• Wind blows water from one side to the other.

– Waves!

– A steady wind can keep the water “piled up” on one side of the


Like blowing on your coffee –piles it up on the other side.

Atmospheric long distance


• Weather is driven by long distance connections.

• Imagine the atmosphere is a balloon around the


• Squeeze the balloon on one side.

• Bulges outward everywhere else.

• Or more accurately, if we pile up more air in one

place, then somewhere else must have less air.

• Conservation of mass.

• Which brings us to basic weather.

More air, less air


More air

Earth (ground)

No air out here, above the

atmosphere in space…

Less air



Warning: Waaay oversimplified! Air density and temperature also

play key roles. But this is a good (if rough) place to start.

High and low pressure

• Imagine that the atmosphere is made of lead

instead of air.

• All that air above you would be VERY heavy.

• Places with more air would be heavier than places with less


And they are heavier! We just don’t notice because we

are constantly wandering around with that weight on us.

And air isn’t as heavy as lead, but it still has weight.

• These are high and low pressure systems.

• You see these on weather maps as a big H and L.

The weight of the atmosphere

• High pressure would be heavier than low



More air


Less air

Called barometric

pressure! Sometimes

measured in “mm”,

“inches”, or pounds

per square inch.

Nice weather, bad weather

• Generally:

H = clear and dry


= cloudy and wet

Now you know how to interpret the weather map!


• Winds are associated with changes from

high to low pressure.

• Air wants to escape high pressure regions and

move to low pressure regions.

More air



Wind! Less air

Winds try to equalize


the pressure, by moving

more air to places with

less air.

Long term H and L

• Most high and low pressure systems move through


• Bad weather in Denver today (L).

• Good weather in Denver tomorrow (H).

• Some H and L hang around for years.

• Generally have high pressure systems hanging out over deserts.

Dry, clear.

• Rain forests have low pressure systems.

Wet, cloudy.

• These create a climate, not just daily weather.

Normal Pacific climate

• Generally have a relatively low pressure system sitting over western

equatorial Pacific.


That means trade winds

• Trade winds move east to west along the surface of the

water (south of the equator).


Winds push the water

• Remember the coffee cup?

• Blow on the coffee and it pushes it to one side.

• The trade winds push Pacific water from

the east to the west.

Imagine giant bathtub

• Give bathtub a good stir…

• Water rotates around the tub.

View from above, looking down

on the bathtub.


Flip bathtub on its side

• That’s the equatorial Pacific.

• Trade winds “stir” the ocean.

Trade winds

Australia Peru

Surface of ocean

Bottom of ocean

Pacific from side view

(as a cross section).

Piles up water

• Water is about a half meter higher in the

western Pacific.

• Wind blows the water higher.

• Remember those world-wide connections?

• And it’s warm water!

• The sun warms the ocean surface.

• That warmer water gets moved west.

Warm water in west

• There’s more warm water in the west.

• Upwelling cold water near Peru.

More warm water

Trade winds

Australia Peru

Surface of ocean: WARM

Bottom of ocean: COLD

More cold water

Normal climate

• Warm water causes evaporation; combines with low

pressure to create cloudy and wet climate in west


• Cold water and high pressure creates clear and dry in

east (Peru).

Trade winds

West Pacific East Pacific

Surface of ocean: WARM

Bottom of ocean: COLD

Now let’s mess things up.

• What if I turn off the trade winds?

1. Warm water stops accumulating in west.

Less storms, drier.

2. Warm water stays in east.

Peru gets wetter.

3. Therefore, high and low pressures start reversing.

4. Therefore, winds start moving the other way.

5. That pushes more warm water to the east.

6. High and low pressures reverse more strongly.

7. Etc!

• Get massive self-perpetuating feedback loop!

• Everything gets reversed!

Stopping the trade winds

• Only needs to be momentary to mess up

the system.

• What would trigger this?

• Hard to pin down.

So many interacting systems…

• It’s a chaotic system.

A butterfly flaps its wings in Brazil, and…

What do you get? El Niño!

• Think of a windy day.

• Wind comes and goes in gusts.

• Sometimes it gets momentarily calm.

For whatever reason…

• Oops, in that momentary calm, get El Niño!

• Stop the trade winds for a mere moment.

• Everything reverses.

• Takes about a year for the system to sort itself

out and get back to normal.

El Niño Climate

• It’s a giant seesaw!

ENSO – El Niño Southern Oscillation



Normal South Pacific conditions


El Niño conditions


More seesaw

• The warm ocean waters slosh in the other


• Just like a bathtub sloshing from one side to the other.

East Pacific


Surface of ocean: WARM

Bottom of ocean: COLD

Normal conditions

West Pacific


East Pacific West Pacific

Surface of ocean: WARM

Bottom of ocean: COLD

El Niño conditions

So what’s that do to climate?

• El Niño reverses the climate.

West Pacific East Pacific

Normal conditions

East Pacific West Pacific

El Niño conditions

Normal Climate


El Niño climate



Warning: This exaggerates the seesaw to make a point.

Really, the changes are more subtle with the L moving

eastward but not all the way across the Pacific.

So how come it’s world-wide?

• This oscillation stirs up the weather everywhere.

• Think of the coffee cup.

• If I blow on one side, it piles up on the other.

• But some of the coffee also blows back around the edges of

the cup.

• El Niño warm waters also “blow” around the

edges of the continents.

• i.e., creep up and down along the coasts

Warm water moves along coasts



And atmospheric circulation


• Typical winds circulate as shown




El Niño winds are different

• Reversed directions (in Northern Hemisphere too).




What’s the result of El Niño?

• Oh, that’s a full lecture.

– Read “Late Victorian Holocausts: El Niño Famines and

the Making of the Third World” by Mike Davis.

– One of the most well-known impacts is the shutdown

of the Peruvian fisheries.

– Cold upwelling water carries nutrients.

– El Niño turns that off and the fish die off.

– Obvious implications for famine.

• But I’ll also mention a few local impacts.

El Niño impacts in US

• The change in wind directions and the

increasingly warm water along the coast causes:

– Increased hurricane activity and flooding in the Gulf

and Caribbean.

– Increased flooding in Southern California.

– Milder winters in north-central states.

– Increased snowfalls in the Colorado/New Mexico

Rocky Mountains.

• Sheeesh. We have it easy compared to much of the world.

La Niña?

• So if that’s El Niño, what’s La Niña?

• Same thing, but other extreme.

• Follows an El Niño.

• The warm water sloshes too far back to the





El Niño La Niño

Too far west.


La Niña seesaw


Normal South Pacific conditions


El Niño conditions




La Niña conditions

1. 2. 3.

Ok, waaay oversimplified again – but you get the idea.

Whoa, seesawed back too far!

La Niña effects

• Generally the opposite of El Niño.

• If you were plagued by a drought, now you are

plagued by floods.

• And vice-versa.

And the future?

• Add climate change to the mix and all bets are


• Some believe we could enter a more permanent El Niño.

• Will we aggravate the problem?

• Will we dwarf the problem?

• Can we learn from ENSO history, and prevent

the same kind of disasters from climate change?

Dave Bahr, Tad Pfeffer,

and Neil Humphrey in

Greenland. Drilling ice

cores to extract info about

climate change.

Neil Humphrey, Tad Pfeffer, Joel Harper, Dave Bahr and others drilling bore holes in an Alaskan glacier

to learn how glaciers respond to climate change.

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