Download Explaining ENSO

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
Pacific.
• 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
global).
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
waters.
• 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
Premier.
• 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
world.
• Examples of big events that can connect the
world:
– Volcanoes in Sumatra can spread ash around the
world.
– Krakatoa, 1883
– Dust storms in the Sahara cause poor visibility in the
Caribbean.
– 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
earth.
• 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
everywhere.
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
earth.
 Like blowing on your coffee –piles it up on the other side.
Atmospheric long distance
connections
• Weather is driven by long distance connections.
• Imagine the atmosphere is a balloon around the
earth.
• 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
No air out here, above the
atmosphere in space…
average
Less air
Earth (ground)
Colorado
Kansas
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
air.
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
pressure.
H
More air
L
Less air
Called barometric
pressure! Sometimes
measured in “mm”,
“inches”, or pounds
per square inch.
Nice weather, bad weather
• Generally:
H
= clear and dry
L
= cloudy and wet
Now you know how to interpret the weather map!
Winds
• 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!
H
Wind!
Less air
L
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
rapidly.
• 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.

Wet, cloudy.
• Rain forests have low pressure systems.
• These create a climate, not just daily weather.
Normal Pacific climate
• Generally have a relatively low pressure system sitting over western
equatorial Pacific.
L
H
That means trade winds
• Trade winds move east to west along the surface of the
water (south of the equator).
L
H
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.
spigot
Flip bathtub on its side
• That’s the equatorial Pacific.
• Trade winds “stir” the ocean.
Australia
Trade winds
Surface of ocean
Bottom of ocean
Pacific from side view
(as a cross section).
Peru
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.
Australia
Trade winds
Surface of ocean:
Peru
WARM
More warm water
More cold water
Bottom of ocean:
COLD
Normal climate
• Warm water causes evaporation; combines with low
pressure to create cloudy and wet climate in west
(Australasia).
• Cold water and high pressure creates clear and dry in
east (Peru).
West Pacific
Trade winds
Surface of ocean:
East Pacific
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.

Peru gets wetter.
2. Warm water stays in east.
3.
4.
5.
6.
7.
•
Therefore, high and low pressures start reversing.
Therefore, winds start moving the other way.
That pushes more warm water to the east.
High and low pressures reverse more strongly.
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
H
H
L
Normal South Pacific conditions
L
El Niño conditions
More seesaw
• The warm ocean waters slosh in the other
direction.
• Just like a bathtub sloshing from one side to the other.
East Pacific
winds
West Pacific
Surface of ocean: WARM
Bottom of ocean: COLD
Normal conditions
East Pacific
winds
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
L
H
El Niño climate
H
L
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
H
L
And atmospheric circulation
changes
• Typical winds circulate as shown
H
L
H
El Niño winds are different
• Reversed directions (in Northern Hemisphere too).
L
H
L
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
west.
Too far west.
H
L
El Niño
L
H
La Niño
La Niña seesaw
H
H
L
H
L
L
Normal South Pacific conditions
El Niño conditions
La Niña conditions
1.
2.
3.
Whoa, seesawed back too far!
Ok, waaay oversimplified again – but you get the idea.
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
off.
• 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.