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  • >> 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.

>> In this animation we're

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A2 US pacific water nino el nino warm water nina

El Nino - La Nina

  • 125 10
    Wayne Lin posted on 2015/07/29
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