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Surface Ocean Circulation and Geostrophy
Fig. CO7
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Ocean currents
• Moving seawater
• Surface ocean currents– Transfer heat from warmer to cooler areas– Similar to pattern of major wind belts
because wind-driven– Affect coastal climates
Ocean currents and climate
August temperatures
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ZONAL ISOTHERMS
Basically zonal
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Measuring surface currents
Fig. 7.2
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Surface currents
• Frictional drag between wind and ocean• Wind plus other factors such as
– Distribution of continents– Gravity– Friction– Coriolis effect cause
• GyresGyres or large circular loops of moving water
Ekman spiral• Surface currents move at angle to wind• Ekman spiral describes speed and direction of seawater flow at
different depths• Each successive layer moves increasingly to right (N
hemisphere)
Fig. 7.5
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Ekman transport• Average movement of seawater under influence
of wind• 90o to right of wind in Northern hemisphere• 90o to left of wind in Southern hemisphere
Fig. 7.6
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Geostrophic flow• Ekman transport
piles up water within subtropical gyres
• Surface water flows downhill (gravity) and
• Also to the right (Coriolis effect)
• Balance of downhill and to the right causes geostrophic flow around the “hill”
Fig. 7.7
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Western intensification
• Top of hill of water displaced toward west due to Earth’s rotation
• Western boundary currents intensified
– Faster
– Narrower– Deeper
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Eastern Boundary Currents
• Eastern side of ocean basins
• Tend to have the opposite properties of Western Currents
• Cold
• Slow
• Shallow
• Wide
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orange arrows = wind
yellow arrows = water
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Ocean gyres
Fig. 7.3
Subtropical gyresSubtropical gyres Centered about Centered about
3030oo N or S N or S Equatorial currentEquatorial current Western Boundary Western Boundary
currentscurrents Northern or Northern or
Southern Boundary Southern Boundary currentscurrents
Eastern Boundary Eastern Boundary currentscurrents
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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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Climate effects of North Atlantic currents
• Gulf Stream warms East coast of U.S. and Northern Europe
• North Atlantic and Norwegian Currents warm northwestern Europe
• Labrador Current cools eastern Canada
• Canary Current cools North Africa coast
Atlantic Ocean circulation South Atlantic South Atlantic
Subtropical GyreSubtropical Gyre Brazil CurrentBrazil Current Antarctic Antarctic
Circumpolar Circumpolar CurrentCurrent
Benguela Benguela CurrentCurrent
South Equatorial South Equatorial CurrentCurrent
Fig. 7.14
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Other surface currents• Equatorial Countercurrents• Subpolar gyres
Fig. 7.4
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One Sverdrup = 1,000,000 cubic
meters/sec =1,000,000 tonnes/sec
• Wellborn road to Texas Ave to University to George Bush Drive to the height of the O&M Building in 2.75 minutes
• Peak flood flow of the Mississippi River is
0.03 Sv.
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Harald Sverdrup1888-1957
> 2 knots
< 2 knots
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Coastal upwelling
• Ekman transport moves surface seawater offshore (upwelling)
Fig. 7.11a
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07_12a-cS
Figure 7.10Equatorial upwelling
Other surface currents• Equatorial Countercurrents• Subpolar gyres
Fig. 7.4
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Low concentrations except in equatorial Pacificand high latitudes at the surface
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Satellite view of surface chlorophyll -purple areas are low values
red and yellow areas are high -
Note the large surface area with low chlorophyll
Purple areas are permanently stratified
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Antarctic circulation
Fig. 7.14
Antarctic Circumpolar Antarctic Circumpolar Current (West Wind Current (West Wind
Drift)Drift) Encircles EarthEncircles Earth
Transports more water Transports more water than any other currentthan any other current East Wind DriftEast Wind Drift
Antarctic DivergenceAntarctic Divergence Antarctic ConvergenceAntarctic Convergence
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Rings, or eddies, are important because they transfer heat, salt,plants and animals across strong boundaries.
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Fig. 7.16
Best Best studiedstudied
Meanders Meanders or loopsor loops
Warm-Warm-core ringscore rings
Cold-core Cold-core ringsrings
Unique Unique biological biological populationpopulationss
Gulf StreamGulf StreamP
Cold-coreRings
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S
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NorthAtlantic
Gyre
SouthAtlantic
Gyre
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One sverdrup = 1,000,000 cubic meters/sec
• Wellborn road to Texas Ave to University to George Bush Drive to the height of the O&M Building in 2.75 minutes
• Peak flood flow of the Mississippi River is
0.03 Sv.
Peak flood flowof the Mississippi River is .03 sv.
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Numbers are sverdrups (millions of cubic meters per sec)and the flow on the eastern boundary must equal
that on the west, therefore, slower broader flow on theeast means narrower, faster flow in the west.
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