How do ocean
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 per cent
of the ocean’s water, while deep ocean currents mobilise the other 90 per cent.
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 metres 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 centimetres 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.
Source: TED Ed – Created by Jennifer Verduin