Transcript Surface Ocean Currents

OCEAN CURRENTS
Currents
• Surface currents in the oceans are driven by winds. The
major surface circulation patterns at sea are the result
of the prevailing winds in the atmosphere.
• Prevailing winds in the atmosphere drive surface
currents in the oceans in predictable patterns.
• Because the density of the water is about 1000 times
greater than the density of the air, the motion in the
water will continue even when there is no wind because
of the water’s inertia.
• Surface circulation is a response to the long-term
average atmospheric circulation.
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Gyres
• Large circular surface currents called gyres
dominate the wind-driven surface circulation
in each hemisphere.
• The major gyres in the Northern Hemisphere
rotate clockwise, while in the Southern
Hemisphere they rotate counterclockwise.
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Wind-driven transport
and resulting surface
currents in an ocean
bounded by land to
the east and west
Currents form large
oceanic gyres that
rotate clockwise in
the north and
counterclockwise in
the south
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Ocean Currents
• In the North Pacific the surface circulation is
dominated by a clockwise rotating gyre formed
by:
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–
–
–
a.
b.
c.
d.
the North Equatorial Current,
the Kuroshio Current,
the North Pacific Current, and
the California Current.
• This gyre is driven by the northeast trade
winds and the westerlies.
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Ocean Currents- Pacific
• Further north, in the North Pacific, there is a
smaller counterclockwise rotating gyre formed
by:
– a.
– b.
– c.
the North Pacific Current,
the Alaska Current, and
the Oyashio Current.
• There is very little exchange of water between
the Arctic Ocean and the North Pacific through
the Bering Strait.
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Ocean Currents- Pacific
• In the South Pacific, the surface circulation is
dominated by a counterclockwise-rotating gyre
formed by:
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–
–
–
a.
b.
c.
d.
the South Equatorial Current,
the East Australia Current,
the northern edge of the West Wind Drift, and
the Peru Current.
• This gyre is derived by the southeast trade winds and
the westerlies.
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Ocean Currents- Atlantic
• In the North Atlantic, there is a similar
clockwise rotating gyre formed by:
–
–
–
–
a.
b.
c.
d.
the North Equatorial Current,
the Gulf Stream,
the North Atlantic Current, and
the Canary Current.
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Ocean Currents- Atlantic
• In the South Atlantic, the counterclockwise
rotating gyre is formed by:
– a.
– b.
the South Equatorial Current,
the Brazil Current,
– c.
the northern edge of the West Wind
Drift, and
the Benguela Current.
– d.
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Ocean Currents- Indian
• Because the Indian Ocean is primarily a Southern
Hemisphere ocean, its surface circulation is dominated
by a counterclockwise rotating gyre formed by:
– a.
– b.
– c.
– d.
the South Equatorial Current,
the Agulhas Current,
the northern edge of the West Wind
Drift, and
the West Australia Current.
• The waters north of the equator in the Indian Ocean
move to the east in the summer and to the west in the
winter with the seasonal changes in monsoon winds.
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Ocean Currents- Polar
• Without any continents to block its flow, the West Wind
Drift is the only continuous current flowing around the
globe. It moves from west to east around Antarctica.
• Flow in the Arctic Ocean is dominated by a large
clockwise gyre. This gyre is not centered on the North
Pole. It is displaced toward the Canadian Basin
• Water flows out of the Arctic Ocean on either side of
Greenland into the North Atlantic.
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Eddies
• Currents do not generally flow in smooth
curves or straight lines. Current paths will
meander and sometimes close on
themselves to form eddies.
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Figure 8.7
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Satellite image
of warm and
cold eddies
spinning of
the Gulf
Stream
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Satellite view
of solar
reflection of off
eddies in the
Mediterranean
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Geostrophic Flow
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Sargasso Sea
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Patterns of global circulation
• Circulation patterns have changed with time in
response to changing configurations of
continents, climate patterns, and atmospheric
circulation.
• Studies of fossil organisms in marine sediments
show that the temperature of the North Atlantic
Ocean has varied during glacial and interglacial
periods.
• These changes have been related to
Milankovitch cycles.
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Milankovitch cycles
• The position and orientation of Earth with
respect to the sun change in a predictable
fashion. This changes the distribution and
intensity of solar radiation reaching the
surface of Earth that influences climate
and ocean circulation
• These cyclical changes are known as
Milankovitch cycles, named after a
Serbian mathematician who studied them
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Milankovitch cycles
• The Milankovitch cycles are driven by variations
that include:
– a.
– b.
– c.
changes in the shape of Earth’s orbit from
elliptical to more circular with a period of
about 100,000 years,
changes in the tilt of Earth’s axis of rotation
from 22° to 24.5° with a period of 41,000
years
the precession, or circular revolution, of
Earth’s axis of rotation with a period of about
23,000 years.
• There seems to be a time lag between the
occurrence of minimum levels of solar radiation
and minimum water temperatures.
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Milankovitch Cycle
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Milankovitch Cycle
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Milankovitch Cycle
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Recent Climate Change
• The recent history of climate changes and water
temperature fluctuations includes:
– a. minimum incoming solar radiation values about
23,000 years ago,
– b. maximum land ice and minimum water
temperatures about 17,000 years ago,
– c. a return to high solar radiation values about
12,000 years ago with a rapid warming of the water,
– d. the appearance of a mini-ice age, called the
Younger Dryas event, about 11,000 years ago in the
Northern Hemisphere that lasted about 700 years.
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Measuring Currents
• Currents can be measured using either
stationary instruments or instruments designed
to float and move along with the current.
• Moving water at any depth can be monitored
using specially designed buoys or floats whose
density can be changed to keep the instrument
at a specific depth. Data from the instrument
can then be relayed to a ship.
• Buoys can also be instrumented to measure
other physical properties of the water such as
temperature.
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El Niño
• The prevailing southeast trade winds that
create the nearly permanent zone of
divergence and upwelling along the west
coast of South America occasionally fail and
the upwelling ceases.
• This situation is called the El Niño and is
characterized by abnormally high sea
surface temperatures in the eastern Pacific.
• The El Niño is often followed by abnormally
cold periods called La Niña that can also
strongly affect atmospheric circulation.
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El Niño
• Along the western coast of South America off
Ecuador and Peru, the southeast trade winds
drive water away from the land, creating a
nearly constant region of divergence with
upwelling water rich in nutrients.
• These cold waters are extremely productive
with dense populations of plankton that
support a large and valuable food chain.
• Periodically, roughly every three to eight
years, the trade winds will break down and
the divergence will stop.
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a)Normal conditions
January 1997
b) El Niño conditions
November 1997
c) End of El Niño
and the beginning of
a normal cycle in
March 1998
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Changes in sea surface temperatures from normal off
the coast of Peru Higher values indicate an El Niño
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El Niño’s effects: (1) Huge areas of warm water drift east. (2)
Storms then follow, drenching California and more of South
America’s west coast. (3) The jet stream sometimes splits in two,
leaving the Pacific Northwest dry, with mild temperatures, and the
Northeast warm.
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Could El Niño be causing more frequent hurricanes and
other severe storms? Yes, say some researchers
Some scientists even link the recent onslaught of El Niños to the specter of global warming. In
the last 20 years, the world has experienced five El Niños—well above the historical norm of
one every four to seven years. Might the greenhouse effect be responsible?
El Niño acts as a kind of distributor of global warming. When carbon dioxide in the atmosphere
absorbs radiation from space, Graham’s global warming theory goes, it transfers this extra
energy to the oceans in the form of heat. As the equatorial Pacific warms, the extra energy
accelerates the “hydrologic cycle” of convection and precipitation—and speeds up the El Niño
rhythm by throwing more heat and moisture into the atmosphere. El Niño, in turn, spreads that
heat virtually across the globe, upsetting weather patterns in ways ranging from mildly amusing
(January sellouts of Bermuda shorts in the Northeast) to devastating (African droughts and44
famine).
For nine straight days in 1995, row after row of
sodden, gray clouds marched from the Pacific
Ocean into normally sunny California. They
deluged Los Angeles, San Francisco, and every
town in between with as much as 16 inches of
rain—more than the average annual total for
some locations. The devastation was widespread:
11 dead, hundreds of homes destroyed, and $1.3
billion in damages.
The 1982-’83 El Niño, the so-called El Niño of
the Century, wreaked an estimated $8.1 billion of
damage and destruction across five continents. It
wiped out delicate coral reefs and drove an
estimated 17 million island nesting birds from
their homes in the Pacific; caused drought that
devastated crops in India and led to paralyzing
dust storms and brushfires in Australia; spawned
a rash of cyclones that left 25,000 Tahitians
homeless; and dumped an incredible 100 inches
of rain in six months in some areas of Ecuador,
creating inland lakes where previously there had
been desert.
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La Niña
• Colder years often follow these periods of
abnormally warm temperatures.
• These cold periods have been called La
Niña, or “the girl.”
• The La Niña will cause a strengthening of
the trade winds with a decrease in
precipitation on the eastern side of the
basin and a corresponding increase on
the western side.
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