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 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?

 

 

Stop reading here, go to Classroom Google and

find the surface current activity!!

 

Upwelling -the beginning of a density current!

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.