How do ocean currents work? - Jennifer Verduin
In 1992,
a cargo ship carrying bath toys
got caught in a storm.
Shipping containers washed overboard,
and the waves swept 28,000 rubber ducks
and other toys into the North Pacific.
But they didn’t stick together.
Quite the opposite–
the ducks have since washed up
all over the world,
and researchers have used their paths
to chart a better understanding
of ocean currents.
Ocean currents are driven
by a range of sources:
the wind, tides, changes in water density,
and the rotation of the Earth.
The topography of the ocean floor
and the shoreline modifies those motions,
causing currents to speed up,
slow down, or change direction.
Ocean currents fall into
two main categories:
surface currents and deep ocean currents.
Surface currents control the motion
of the top 10 percent
of the ocean’s water,
while deep-ocean currents mobilize
the other 90 percent.
Though they have different causes,
surface and deep ocean currents
influence each other
in an intricate dance that keeps
the entire ocean moving.
Near the shore,
surface currents are driven
by both the wind and tides,
which draw water back and forth
as the water level falls and rises.
Meanwhile, in the open ocean, wind is the
major force behind surface currents.
As wind blows over the ocean,
it drags the top layers
of water along with it.
That moving water pulls on
the layers underneath,
and those pull on the ones beneath them.
In fact, water as deep as 400 meters
is still affected by the wind
at the ocean’s surface.
If you zoom out to look at the patterns
of surface currents all over the earth,
you’ll see that they form
big loops called gyres,
which travel clockwise
in the northern hemisphere
and counter-clockwise
in the southern hemisphere.
That’s because of the way
the Earth’s rotation
affects the wind patterns that
give rise to these currents.
If the earth didn’t rotate,
air and water would simply
move back and forth
between low pressure at the equator
and high pressure at the poles.
But as the earth spins,
air moving from the equator to the
North Pole is deflected eastward,
and air moving back down
is deflected westward.
The mirror image happens
in the southern hemisphere,
so that the major streams of wind
form loop-like patterns
around the ocean basins.
This is called the Coriolis Effect.
The winds push the ocean beneath
them into the same rotating gyres.
And because water holds onto heat
more effectively than air,
these currents help redistribute
warmth around the globe.
Unlike surface currents,
deep ocean currents are driven primarily
by changes in the density of seawater.
As water moves towards the North Pole,
it gets colder.
It also has a higher
concentration of salt,
because the ice crystals that form
trap water while leaving salt behind.
This cold, salty water is more dense,
so it sinks,
and warmer surface water takes its place,
setting up a vertical current called
thermohaline circulation.
Thermohaline circulation of deep water
and wind-driven surface currents
combine to form a winding loop
called the Global Conveyor Belt.
As water moves from the depths of
the ocean to the surface,
it carries nutrients that nourish the
microorganisms
which form the base of many
ocean food chains.
The global conveyor belt is the
longest current in the world,
snaking all around the globe.
But it only moves a few
centimeters per second.
It could take a drop of water
a thousand years to make the full trip.
However, rising sea temperatures are
causing the conveyor belt
to seemingly slow down.
Models show this causing havoc with
weather systems
on both sides of the Atlantic,
and no one knows what would happen if it
continues to slow
or if it stopped altogether.
The only way we’ll be able to forecast
correctly and prepare accordingly
will be to continue to study currents
and the powerful forces that shape them.
