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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S

S