WEBVTT

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In 1992,

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a cargo ship carrying bath toys 
got caught in a storm.

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Shipping containers washed overboard,

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and the waves swept 28,000 rubber ducks 
and other toys into the North Pacific.

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But they didn’t stick together.

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Quite the opposite–

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the ducks have since washed up 
all over the world,

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and researchers have used their paths

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to chart a better understanding
of ocean currents.

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Ocean currents are driven 
by a range of sources:

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the wind, tides, changes in water density,

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and the rotation of the Earth.

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The topography of the ocean floor 
and the shoreline modifies those motions,

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causing currents to speed up,

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slow down, or change direction.

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Ocean currents fall into 
two main categories:

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surface currents and deep ocean currents.

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Surface currents control the motion

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of the top 10 percent 
of the ocean’s water,

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while deep-ocean currents mobilize 
the other 90 percent.

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Though they have different causes,

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surface and deep ocean currents 
influence each other

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in an intricate dance that keeps 
the entire ocean moving.

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Near the shore,

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surface currents are driven 
by both the wind and tides,

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which draw water back and forth 
as the water level falls and rises.

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Meanwhile, in the open ocean, wind is the 
major force behind surface currents.

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As wind blows over the ocean,

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it drags the top layers 
of water along with it.

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That moving water pulls on 
the layers underneath,

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and those pull on the ones beneath them.

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In fact, water as deep as 400 meters

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is still affected by the wind 
at the ocean’s surface.

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If you zoom out to look at the patterns 
of surface currents all over the earth,

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you’ll see that they form 
big loops called gyres,

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which travel clockwise 
in the northern hemisphere

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and counter-clockwise 
in the southern hemisphere.

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That’s because of the way 
the Earth’s rotation

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affects the wind patterns that
give rise to these currents.

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If the earth didn’t rotate,

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air and water would simply 
move back and forth

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between low pressure at the equator

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and high pressure at the poles.

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But as the earth spins,

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air moving from the equator to the 
North Pole is deflected eastward,

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and air moving back down 
is deflected westward.

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The mirror image happens 
in the southern hemisphere,

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so that the major streams of wind

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form loop-like patterns 
around the ocean basins.

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This is called the Coriolis Effect.

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The winds push the ocean beneath 
them into the same rotating gyres.

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And because water holds onto heat 
more effectively than air,

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these currents help redistribute 
warmth around the globe.

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Unlike surface currents,

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deep ocean currents are driven primarily 
by changes in the density of seawater.

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As water moves towards the North Pole,

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it gets colder.

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It also has a higher 
concentration of salt,

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because the ice crystals that form 
trap water while leaving salt behind.

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This cold, salty water is more dense,

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so it sinks,

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and warmer surface water takes its place,

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setting up a vertical current called 
thermohaline circulation.

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Thermohaline circulation of deep water 
and wind-driven surface currents

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combine to form a winding loop 
called the Global Conveyor Belt.

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As water moves from the depths of 
the ocean to the surface,

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it carries nutrients that nourish the 
microorganisms

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which form the base of many 
ocean food chains.

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The global conveyor belt is the
longest current in the world,

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snaking all around the globe.

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But it only moves a few 
centimeters per second.

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It could take a drop of water 
a thousand years to make the full trip.

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However, rising sea temperatures are 
causing the conveyor belt

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to seemingly slow down.

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Models show this causing havoc with 
weather systems

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on both sides of the Atlantic,

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and no one knows what would happen if it 
continues to slow

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or if it stopped altogether.

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The only way we’ll be able to forecast
correctly and prepare accordingly

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will be to continue to study currents 
and the powerful forces that shape them.