Earth Science 16.1 Ocean Circulation

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Earth Science 16.1 Ocean Circulation
Ocean
Circulation
Earth Science 16.1 Ocean Circulation
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The waters of the world’s oceans are
in constant motion, powered by many
different forces.
Winds, for example, generate surface
currents which strongly influence the
climates of the coastlines.
Winds also produce waves.
Some waves carry energy from
powerful storms to distant shores
where their impact erodes the land.
In other parts of the ocean, density
differences create deep-ocean
circulation of currents.
These deep-ocean currents are
important for ocean mixing and
recycling of nutrients.
Earth Science 16.1 Ocean Circulation
Surface Circulation:
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Ocean currents are masses of ocean
water that flow from one place to
another.
The amount of water can be large or
small.
Deep ocean currents
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Ocean currents can be at the very
surface or deep below the surface.
The forces that create and govern
these currents can be simple or
complex.
In all cases, the currents that are
generated involve masses of water in
motion.
Earth Science 16.1 Ocean Circulation
Surface Currents:
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Surface currents are movements
of water that flow horizontally in
the upper part of the ocean’s
surface.
Surface currents develop from
friction between the ocean and
the wind that blows across it’s
surface.
Surface currents
Earth Science 16.1 Ocean Circulation
Surface Currents:
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Some of these currents do not
last long and affect only small
areas.
Other surface currents are more
permanent and extend over large
portions of the oceans.
These major movements of
surface waters are closely related
to the general circulation patterns
of the atmosphere.
Surface currents
Earth Science 16.1 Ocean Circulation
Gyres:
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Huge circular-moving current
systems dominate the surfaces
of the oceans.
These large whirls of ocean
water within an ocean basin are
called gyres (gyros = circle).
There are 5 main ocean gyres:
 North Pacific Gyre
 South Pacific Gyre
 North Atlantic Gyre
 South Atlantic Gyre
 Indian Ocean gyre
Earth Science 16.1 Ocean Circulation
Gyres:
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Although wind is the force that
generates surface currents,
other factors also influence the
movement of ocean currents.
The most significant of these
factors is the Coriolis effect.
The Coriolis effect is the
deflection of the currents away
from their original course as a
result of the Earth’s rotation.
Earth Science 16.1 Ocean Circulation
Gyres:
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Because of Earth’s rotation,
currents are deflected to the
right (clockwise) in the
Northern Hemisphere and to
the left (counterclockwise) in
the Southern Hemisphere.
As a result, gyres flow in
opposite directions in the two
different hemispheres.
Earth Science 16.1 Ocean Circulation
Ocean Currents and Climates
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Ocean currents have an
important affect on climates.
When currents from lowlatitude equatorial regions move
into higher latitudes, they move
heat from the warmer tropics to
the cooler regions of the polar
areas.
The Gulf Stream, a warm water
current, is an excellent example
of this process.
Warm waters of the Gulf Stream shown
In red moving north clockwise into the
North Atlantic
Earth Science 16.1 Ocean Circulation
Ocean Currents and Climates
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The Gulf Stream brings warm
water from the equator up to
the North Atlantic Current
which is an extension of the
Gulf Stream.
This current allows Great
Britain and much of
northwestern Europe to be
warmer during the winter than
would be expected for this
latitude.
Warm waters of the Gulf Stream shown
In red moving north clockwise into the
North Atlantic
Earth Science 16.1 Ocean Circulation
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These northern areas of Europe are as far north as Alaska,
Newfoundland and southern Greenland. Because of these
currents, warmth moves north and carries heat far inland
warming northern Europe substantially in the winter months.
Earth Science 16.1 Ocean Circulation
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The effects of these warm
ocean currents are felt mostly
in the middle latitudes in
winter.
In contrast, the influence of
cold currents is mostly felt in
the tropics or during summer
months in the mid-latitudes.
Cold currents begin in cold
high-latitude regions near the
poles.
As cold currents travel
towards the equator, they
help cool down and moderate
the warm temperatures of the
adjacent land areas.
North Atlantic currents
Earth Science 16.1 Ocean Circulation
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Ocean currents play a major
role in maintaining Earth’s
heat balance.
They do this by transferring
heat from the tropics, where
there is an excess of heat, to
the polar regions, where less
heat exists.
Ocean water movement
accounts for about a quarter
of this heat transport.
Wind currents transport the
remaining three-quarters.
North Atlantic currents
Earth Science 16.1 Ocean Circulation
Upwelling:
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In addition to producing surface
currents, winds can cause vertical
water movements.
Upwelling is the rising of cold water
from deeper layers to replace
warmer surface water.
Upwelling is a common wind-induced
vertical water movement.
One type of upwelling; called coastal
upwelling, is most common along the
west coasts of continents; most
notably along California, western
South America, and West Africa.
Coastal upwelling currents
Earth Science 16.1 Ocean Circulation
Upwelling:
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Coastal upwelling occurs in these
areas when wind blow toward the
equator and parallel to the coast.
Coastal winds combined with the
Coriolis effect cause surface water to
move away from the shore.
As the surface water moves away
from the coast, it is replaced by
water that “upwells” from below the
surface.
This slow upward movement of water
from depths of 50 to 300 meters
brings water that is cooler than the
surface waters and lowers the
surface temperatures near shore.
Coastal upwelling currents
Earth Science 16.1 Ocean Circulation
Upwelling:
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Upwelling currents bring greater
concentrations of dissolved
nutrients, such as nitrates and
phosphates, to the ocean surface.
These nutrient-rich waters from
below promote the growth of
microscopic plankton, which in turn
support extensive populations of
fish and other marine organisms.
In the NASA image at right, we see
a high chlorophyll concentration
along the California coast as a result
of phytoplankton growth from the
nutrients provided by these
upwelling currents.
Chlorophyll concentrations
Earth Science 16.1 Ocean Circulation
Deep Ocean Circulation:
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In contrast to the largely horizontal movements of surface currents,
deep-ocean circulation has a significant vertical (up-down) movement
of ocean waters.
Deep-ocean circulation accounts for the thorough mixing of deepocean waters.
Earth Science 16.1 Ocean Circulation
Density Currents:
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Density currents are vertical
currents of ocean water that
result from density differences
among water masses.
Denser water sinks and slowly
spreads out below the surface.
An increase in water density can
be caused by either
 A decrease in temperature or
 An increase in salinity of the
water
Examples ocean density current layers
Earth Science 16.1 Ocean Circulation
Density Currents:
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Processes that increase the
salinity of water include
evaporation and the formation of
sea ice.
Processes that decrease the
salinity of water include
precipitation, runoff from land,
icebergs melting, and sea ice
melting.
Density changes due to salinity
variations are important in very
high latitudes polar regions where
water temperatures remain low
and relatively constant.
Examples ocean density current layers
Earth Science 16.1 Ocean Circulation
High Latitudes:
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Most water involved in deep-ocean
density currents begins in high
latitudes at the surface.
In these regions, surface waters
become cold, as it’s salinity
increases as sea-ice forms.
When this water becomes dense
enough, it sinks, initiating deep
ocean currents.
When sea-ice forms; the water beneath
the ice increases in salinity
Earth Science 16.1 Ocean Circulation
High Latitudes:
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Once this water sinks, it is removed
from the physical process that
increased it’s density in the first
place.
It’s temperature and salinity remain
relatively unchanged during the time
it is in the deep ocean.
Because of this, oceanographers can
track the movements of density
currents in the deep ocean.
By knowing the temperature,
density, and salinity of a water mass;
scientists are able to map the slow
circulation of the water mass
through the world’s oceans.
Earth Science 16.1 Ocean Circulation
High Latitudes:
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Near Antarctica, surface
conditions create the highest
density of water in the world.
This cold salty water sinks to the
sea floor, where it moves
throughout the ocean basins in
slow currents.
After sinking from the surface
of the ocean, deep waters will
not reappear at the surface for
an average of 500 to 2000 years.
Earth Science 16.1 Ocean Circulation
Evaporation:
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Density currents can also
result from increased
salinity of ocean water due
to evaporation.
In the Mediterranean Sea,
conditions exist that lead to
the formation of a dense
water mass at the surface
that eventually flows into
the Atlantic Ocean.
Mediterranean Currents
Earth Science 16.1 Ocean Circulation
Evaporation:
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Climate conditions in the
eastern Mediterranean
include a dry northwest wind
and sunny days.
These conditions lead to an
annual excess of evaporation
compared to the amount of
precipitation (rain).
When seawater evaporates,
salt is left behind, and the
salinity of the remaining
water increases.
Mediterranean Currents
Earth Science 16.1 Ocean Circulation
Evaporation:
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The surface waters of the
eastern Mediterranean Sea
have a salinity of about 38
PPT (parts per thousand) or
3.8%.
In the winter months, this
water flows out of the
Mediterranean Sea into the
Atlantic Ocean.
At 3.8%, this water is more
dense than the Atlantic
Ocean surface waters so it
sinks as it moves into the
Atlantic.
Mediterranean Currents
Earth Science 16.1 Ocean Circulation
Conveyor Belt Model:
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A simplified model of ocean
circulation is similar to a
conveyor belt that travels
from the Atlantic Ocean
through the Indian and
Pacific Oceans and back
again.
In this model, warm surface
waters flow toward the
poles.
When the water reaches the
poles, it’s temperature drops
and salinity increases making
it more dense.
Earth Science 16.1 Ocean Circulation
Conveyor Belt Model:
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Because the water is now
dense, it sinks and moves
toward the equator.
It returns to the equator as
cold deep water that
eventually “upwells” to
complete the circuit
warming again.
As this conveyor belt moves
around the globe, it
influences global climates by
converting warm water to
cold water and releasing
heat into the atmosphere.
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