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 are powered by wind blowing over the water. The wind
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?
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
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?
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
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
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.