>> In this animation we're
going to look at the cycle
of El Nino and La Nina,
that is a pattern
that occurs regularly
in the Pacific Ocean
and has a huge effect
on climate and rainfall
and other kinds
of activity all around much
of the world,
especially
around the Pacific Ocean.
Now in order
to understand the development
of these two conditions--
El Nino and La Nina--
let's take a look
at what the normal situation
is in the Pacific Ocean.
Normally what happens is
--and you can see those reddish
arrows there--you get
these very strong
southeast trade winds blowing
from east to west
across the Pacific.
By the way,
this is North America
over here, South America.
This red line represents
the equator.
This is Australia
and New Guinea
and Asia would be out of view
on this side.
So as this,
as these trade winds blow the
air eventually rises
in low pressure systems
and creates a lot of rainfall
over on the west side
of the Pacific,
over near Indonesia.
And as that water,
and what happens is a
circulation cell called the
Walker Cell is set up here
that basically runs like that.
Well the effect of that is
to push the water away
from South America.
Ocean surface currents are
pushed this direction
and that takes warm water
and pulls it away to the west
and it lets cold water come
up from below.
Here we have warm water,
the thermocline,
the big change from warm
to cold and then cold water
down underneath, and as
that wind blows
and pulls the water away
from South America you tend
to get cold upwelling water
near South America.
Meanwhile,
a lot of very warm water
accumulates over here
in the Western Pacific.
It's called the West Pacific
Warm Water Pool.
Because that water takes a
long trip along the equator
and it gets very warm
in this area.
So this is a part
of the world that's very wet
and warm and rainy,
whereas this part
of the world tends to be cool
and drier in general.
Now the whole reason this can
happen is because these strong
trade winds push this water
from east to west.
But if those trade winds break
down what can happen is all
this warm water on this side
of the Pacific Ocean can kind
of slosh back across and pile
up on this side.
And that's really what happens
during an El Nino.
What happens during El Nino is
that the trade winds actually
weaken and break down.
It's called the Southern
Oscillation,
and what'll happen is
that water,
that warm water isn't,
there's nothing
to hold it back anymore
and it flows
across the Pacific and kind
of piles up over on this side
of the Pacific,
going all the way
up toward California,
all the way down to Peru
and Chile, and we don't get
upwelling any more over here
and the atmosphere pressure
patterns switch.
And so what happens now is
that that warm water fuels
rain and storms
and the trade winds are
actually reversed in direction
and this, this part
of the ocean now becomes the
rainy, stormy side
of the ocean.
And so during an El Nino we
typically have very high rain,
lots of storms and flooding
over on this side
of the Pacific from California
on down to South America.
Now what usually happens
within a year or so
after the El Nino is that we,
things kinds of,
it's like a pendulum swings
back the other way
and we get a La Nina.
La Nina really can be
considered to be a, sort of a,
an intensification
of the normal conditions.
Remember, this is the normal
situation where the trade
winds are blowing all the
water west and it's piling
up over here.
During La Nina the trade winds
blow even more intensely
and even more cold water gets
pushed west
and even more intense
upwelling happens along the
coast of South America.
So it really is just an
intensification
of the normal situation.
So the Pacific Ocean kind
of oscillates
between these two conditions,
the El Nino condition
in which the water sloshes
over to our side
of the Pacific,
bringing storms and rain
and warm water,
and the La Nina condition
in which all the water goes
back to the other side
of the Pacific
and cold upwelling becomes
even more intensified.
These cycles go back
and forth,
not particularly regularly
but anywhere from every two
to eight years is
pretty typical.