Download Ocean Circulation

CHAPTER 7
Ocean Circulation
© 2011 Pearson Education, Inc.
What is the temperature of the
water in the deepest oceans?
Chapter Overview
• Ocean currents are moving loops of water.
• Surface currents are influenced by major
wind belts.
• Currents redistribute global heat.
• Thermohaline circulation affects deep
currents.
• Currents affect marine life.
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Types of Ocean Currents
• Surface currents
– Wind-driven
– Primarily horizontal motion
• Deep currents
– Driven by differences in density caused
by differences in temperature and
salinity
– Vertical and horizontal motions
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Three-Cell Model of Atmospheric
Circulation
Remember this wind info from the last chapter???
Density
Waters of different densities form a
layered ocean
Global Ocean Currents
Measuring Surface Currents
• Direct methods
– Floating
device tracked
through time
– Fixed current meter
• Indirect methods
– Pressure gradients
– Radar altimeters
– Doppler flow meter
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Global Surface Current Flow
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Measuring Deep Currents
• Floating devices tracked through time
• Chemical tracers
– Tritium
– Chlorofluorocarbons
• Characteristic temperature and
salinity
• Argo
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Argo
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Surface Currents
•
•
•
•
Occur above pycnocline
Frictional drag between wind and ocean
Generally follow wind belt pattern
Other factors:
– Distribution of continents
– Gravity
– Friction
– Coriolis effect
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Subtropical Gyres
• Large, circular loops
of moving water
• Bounded by:
– Equatorial current
– Western Boundary
currents
– Northern or Southern
Boundary currents
– Eastern Boundary
currents
• Centered around
30 degrees latitude
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Five Subtropical Gyres
•
•
•
•
•
North Atlantic – Columbus Gyre
South Atlantic – Navigator Gyre
North Pacific – Turtle Gyre
South Pacific – Heyerdahl Gyre
Indian Ocean – Majid Gyre
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Subtropical Gyres and Currents
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Ekman Spiral
• Surface currents move
at an angle to the wind.
• The Ekman spiral
describes speed and
direction of seawater
flow at different depths.
• Each successive layer
moves increasingly to
the right in the
Northern Hemisphere
– Coriolis effect
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Ekman Transport
• Average movement of
seawater under
influence of wind
• 90 degrees to right of
wind in Northern
hemisphere
• 90 degrees to left of
wind in Southern
hemisphere
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Geostrophic Flow
• Ekman transport piles
up water within
subtropical gyres.
• Surface water flows
downhill and to the
right.
• Geostrophic flow –
balance of Coriolis
Effect and gravitational
forces
• Ideal geostrophic flow
• Friction generates
actual geostrophic flow
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GYRES
Ocean Currents and Climate
• Warm ocean currents warm the air at
the coast.
– Warm, humid air
– Humid climate on adjoining landmass
• Cool ocean currents cool the air at
the coast.
– Cool, dry air
– Dry climate on adjoining landmass
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Ocean Currents and Climate
August
February
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Upwelling and Downwelling
• Upwelling – Vertical movement of cold,
nutrient-rich water to surface
– High biological productivity
• Downwelling – Vertical movement of
surface water downward in water column
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Coastal Upwelling
• Ekman transport
moves surface
seawater offshore.
• Cool, nutrient-rich
deep water comes
up to replace
displaced surface
waters.
• Example: U.S.
West Coast
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Coastal Downwelling
• Ekman transport
moves surface
seawater toward
shore.
• Water piles up,
moves downward in
water column
• Lack of marine life
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Atlantic Ocean Circulation
• North Atlantic
Subtropical Gyre
– North Equatorial
Current
– Gulf Stream
– North Atlantic Current
– Canary Current
– South Equatorial
Current
– Atlantic Equatorial
Counter Current
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Gulf Stream
•
•
•
Best studied of all ocean currents
Meanders and loops
Merges with Sargasso Sea
–
–
Circulates around center of North Atlantic
Gyre
Unique biology – Sargassum
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Gulf Stream
•
Meanders or loops may
cause loss of water
volume and generate:
–
–
•
Warm-core rings –
warmer Sargasso Sea
water trapped in loop
surrounded by cool water
Cold-core rings – cold
water trapped in loop
surrounded by warmer
water
Unique biological
populations
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Climate Effects of North Atlantic
Currents
North-moving currents – warm
• Gulf Stream warms East coast of United
States and northern Europe
• North Atlantic and Norwegian Currents
warm northwestern Europe
South-moving currents – cool
• Labrador Current cools eastern Canada
• Canary Current cools north African coast
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La Niña Conditions
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La Niña – ENSO Cool Phase
• Increased pressure difference across
equatorial Pacific
• Stronger trade winds
• Stronger upwelling in eastern Pacific
• Shallower thermocline
• Cooler than normal seawater
• Higher biological productivity
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•
•
•
•
•
•
El Niño – Southern Oscillation
(ENSO)
High pressure in eastern Pacific weakens
Weaker trade winds
Warm pool migrates eastward
Thermocline deeper in eastern Pacific
Downwelling
Lower biological productivity
– Peruvian fishing suffers
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ENSO Conditions in the Pacific
Ocean
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La
Nina
El
Nino
Occurrence of ENSO Events
• El Niño warm phase about every
2–10 years
• Highly irregular
• Phases usually last 12–18 months
• 10,000-year sediment record of events
• ENSO may be part of Pacific Decadal
Oscillation (PDO)
– Long-term natural climate cycle
– Lasts 20–30 years
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La Nina
El Nino
ENSO Occurrences
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ENSO has Global Impacts
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Deep-Ocean Currents
• Thermohaline Circulation – deep ocean
circulation driven by temperature and
density differences in water
• Below the pycnocline
• 90% of all ocean water
• Slow velocity
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Conveyor Belt Circulation
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Power From Currents
• Currents carry
more energy
than winds
• Florida–Gulf
Stream Current
System
• Underwater
turbines
– Expensive
– Difficult to maintain
– Hazard to boating
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End of CHAPTER 7
Ocean Circulation
© 2011 Pearson Education, Inc.