Transcript CHAPTER 7 Ocean Circulation

CHAPTER 7
Ocean Circulation
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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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Global Surface Current Flow
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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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Subtropical Gyre Currents
Four main currents flowing into one another:
• Equatorial Currents
– North or south
– Travel westward along equator
• Western Boundary Currents – warm waters
• Northern or Southern Boundary Currents –
easterly water flow across ocean basin
• Eastern Boundary Currents – cool waters
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Gyres and Boundary Currents
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Other Surface Currents
• Equatorial Countercurrents – eastward
flow between North and South Equatorial
Currents
• Subpolar Gyres
– Rotate opposite subtropical gyres
– Smaller and fewer than subtropical gyres
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Activity:
• How do we measure both surface and deep
water currents? List and explain each
• What is ARGO? Give details, and why it is
important. Who will be interested in the data
• Diagram the Earth; label the prevailing wind
patterns, and then label the major surface
gyres. Explain the correlation. How different
would it look without the continents?
• Make a table similar to 7.1. just label the 5
major gyres, and include the 4 major
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
Ekman Spiral Animation
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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
Ekman Transport Animation
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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
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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
Ekman Spiral Animation
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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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Other Types of Upwelling
• Offshore winds
• Seafloor obstruction
• Coastal geometry
change
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Converging Surface Water
• Surface waters move
toward each other.
• Water piles up.
• Low biological
productivity
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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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Activity:
• Explain, “Western Intensification”. How does
coriolis factor into this
• Explain the effects of: western vs eastern
boundary currents
• Look at table 7.2; what is the main function of
each (big picture here) boundary current.
• Define the terms Upwelling & Downwelling.
Which one is more biologically productive and
why?
• List the 3 other causes of upwelling, and how
they work
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Pacific Ocean Circulation
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Atmospheric-Ocean Connections in the
Pacific Ocean
• Walker Circulation Cell – normal conditions
– High pressure in Eastern Pacific
– Strong SE trade winds
– Pacific warm pool on western side of ocean
– Thermocline deeper on western side
– Upwelling off the coast of Peru
• HUGE FISHING INDUSTRY HERE!!!
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Normal Conditions, Walker Circulation
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El Niño – Southern Oscillation (ENSO)
Walker Cell Circulation disrupted
• High pressure in E 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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Normal vs El Nino
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ENSO Conditions in the Pacific Ocean
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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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Normal vs. La Nina
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La Niña Conditions
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ENSO Occurrences
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ENSO has Global Impacts
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Activity:
• Compare El Nino to La Nina; major
differences and expected outcomes
• List the events that occurred in 1982/83
and 1997/98
• How often do these events occur?
• How long do the phases last?
• How has science proven these events
have always occurred?
• What is PDO?
– How has this affected climate change?
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– How long do they
occur?
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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Thermohaline Circulation
• Originates in high latitude surface ocean
• Surface waters sink
– Identifies deep water masses based on
temperature, salinity, and resulting density
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Thermohaline Circulation
• Deep-water masses include:
–Antarctic Bottom Water
–North Atlantic Deep Water
–Antarctic Intermediate Water
–Oceanic Common Water
• Cold surface seawater sinks at polar
regions and moves equatorward
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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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Activity:
• Complete Worksheet in class
• Becomes study guide
• Exam ??????
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End of CHAPTER 7
Ocean Circulation
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