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.