Surface Currents
When
you stir chocolate into a glass of milk, the milk
swirls
around the glass in a circle. This is similar to the way
an
ocean current moves. Ocean currents are a mass
movement,
or flow, of ocean water. Think of an ocean
current
as a river moving within the ocean.
A surface current is a current that moves water
on or near the surface of the ocean, sort of like a river in the ocean.
Surface currents
forces
the water in the ocean to move in huge, circular
patterns.
In fact, the currents on the ocean’s surface are
affected by the circulation of the winds on Earth. The prevailing winds,
such as the Trade Winds near the equator and the Westerlies near
40 degree N and 40 degree S of the equator, drive the major surface
currents. The Trade Winds come from the East and the Westerlies
come from the West.
Surface currents don’t affect the deep sections of the ocean.
The winds
move only the upper few hundred meters of seawater.
Some
seeds and plants are carried between continents by
surface
currents and sailors have relied on surface currents and
winds
to make sailing easier. You can see some surface
currents
in the figure below. The arrows show
the
circular direction that the currents follow. Some of
the
currents
are caused by warm winds and some are caused by cool winds.
1. Name one current that affects the oceans
around North America’s coasts
How do surface currents form?
Surface
ocean currents and surface winds are affected by
the
Coriolis (kor ee OH lus) effect. The Coriolis
effect is
the shifting of winds and surface currents from their
expected paths because of Earth’s rotation.
Earth
rotates toward the east. Because of this, winds in
the northern hemisphere turn to their right and winds in
the southern hemisphere turn to their left. These surface
winds
can cause water to pile up in certain parts of the
ocean.
When gravity pulls water off the pile, the Coriolis
effect
turns the water. This causes surface water in oceans to
spiral,
or circle, around the piles of water.
Look
again at the map of major surface currents. The
circular
patterns that you see are caused by the Coriolis
effect.
The currents north of the equator circle to their
right. Currents south of the equator circle to their left.
MARGIN NOTE: Gyre- Surface currents that are wind driven
and move in large circular patterns are called gyres.
continental deflection-when a current changes direction
because it is flowing towards a continent.
As the current strikes continent, it is forced to change direction.
What is the Gulf Stream?
Much
of what is known about surface currents comes
from
records that were kept by sailors in the nineteenth
century.
Sailors always have used surface currents to make
traveling easier. Sailors heading
west use surface currents
that
flow west. Sailors heading east use currents such as the
Gulf
Stream. The Gulf Stream is a 100-km-wide surface
current in the Atlantic Ocean.
When America was still a
colony
of England, sailors noticed that trips to England were
faster
than trips from England. Going eastward with the
Gulf Stream made
the journey quicker.
Margin Notes: The Gulf Stream is part of the North Atlantic Gyre.
The
North Atlantic Gyre carries warm water from the Equator across the Atlantic
over to Europe where it causes it to have a MUCH WARMER climate than it should
have!!!
2. How can surface currents be helpful to
ships?
How are surface currents tracked?
Items
that wash up on beaches, such as bottles, can provide
information
about ocean currents. One method used to track
surface
currents is to release drift bottles into the ocean. Drift
bottles are released from a variety of coastal locations.
Inside each bottle, a message and a numbered card state
where and when the bottle was released.
When the bottle
washes
ashore, the person who finds it may notice the card
inside.
The person will fill out the card with the information
about
when and where it washed ashore. The card is returned
to the research team and provides valuable information
about the surface currents that carried the bottle.
How do warm and cold surface currents
affect the climate?
Look
at the map of surface currents again. Notice that some
currents
start near the north and south poles, and other
currents
start near the equator. Currents on the west coasts of
continents begin near the poles where the water is colder.
The
California Current is an example of such a current. It
starts
near the north pole and is a cold surface current.
Currents on the east coast of continents start near the
equator where the water is warmer. The
Gulf Stream starts
in
waters near the equator and is a warm surface current.
As
a warm surface current flows away from the equator,
heat
is released to the atmosphere. The atmosphere is
warmed.
The transfer of heat helps determine climate.
3. What could scientists learn about currents
from a drift bottle’s trip?
Stop reading here, go to Classroom Google and
find the surface current activity!!
Upwelling
Remember
that surface currents carry water horizontally
or
parallel
to Earth’s surface. Water also travels vertically, from
the
bottom to the top of the ocean. Upwelling
is a vertical
circulation in the ocean that brings deep, cold water to the
ocean surface.
Along
some coasts of continents, wind blowing parallel to
the
coast carries water away from the land because of the
Coriolis
effect. Cold water from deep in the ocean rises up
to
replace it. The cold water is full of nutrients from
organisms that died, sank to the bottom, and decayed. Fish
are attracted to these nutrient-rich areas. Areas of upwelling
are
important fishing grounds. The picture below
shows upwelling off the coast of Peru.
4. What happens during upwelling?
5. Why does upwelling around Peru make
Peru a rich fishing ground?
Density Currents
Deep in the ocean, there is no wind to move the water.
Instead, differences in density cause water to circulate or
move. Cold water is more dense than warm water. Salty
water is more dense than less
salty water.
A
density current forms when a mass of seawater
becomes more dense than the surrounding water. Gravity
causes
this dense water to sink beneath less dense seawater.
The
deep, dense water spreads to the rest of the ocean.
Changes in temperature and salinity work together to
create density currents. A density current moves water
very slowly.
Where are density currents found?
One
important density current begins in Antarctica. In
winter,
the seawater there is more dense than at any other
time.
When seawater freezes, the salt is left behind in the
unfrozen water. This extra salt increases the seawater’s
density and causes it to sink. Slowly, the water begins to
spread along the ocean bottom toward the equator forming
a density current. In the Pacific Ocean, it could take up to
1,000 years for the water in this density current to reach
the equator.
6. Sequence of Events:
Number the events
to show the order in which a density current forms in Antarctica.
_____ unfrozen
seawater sinks
_____ dense
seawater spreads along ocean floor
_____ seawater
freezes
North Atlantic Deep Water
Another
density current starts in the North Atlantic Ocean. Around Norway, Greenland, and
Labrador, cold, dense waters form and sink. They form what is known as North
Atlantic Deep Water. This water covers the floor of the northern one-third to
one-half of the Atlantic Ocean. In the southern part of the Atlantic Ocean, this
current meets the density current from Antarctica. The Antarctic density
current is colder and denser. The North Atlantic Deep Water floats just above
it. Density currents circulate more quickly in the Atlantic Ocean than in the
Pacific
Ocean. In the Atlantic, a density current could
circulate in 275 years.
Do density currents affect other waters?
Density
currents also occur in the Mediterranean Sea. The
sea
connects to the Atlantic Ocean by a narrow passage
called
the Strait of Gibraltar. Warm temperatures and dry
air
in the Mediterranean region cause the seawater to
evaporate.
The salts remain behind. This increases the
salinity
and density of the sea. The dense, salty water travels
through
the Straits of Gibraltar into the Atlantic Ocean.
Because
it is much denser than water at the surface of the
ocean,
it sinks. However, it is not as dense as the very cold,
salty
water of the North Atlantic Deep Water. So, the water
from
the Mediterranean floats above it. It forms a middle
layer
known as the Mediterranean Intermediate Water. You
can
see the different water layers in the figure below.
7. Which
is more dense, the Antarctic
current or the
North Atlantic Deep Water?
Density activity time!! Go to Classroom Google
and find the density current activity.