chapter 7 ocean circulation essentials of oceanography 7 th edition
TRANSCRIPT
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Chapter 7 Ocean Circulation
Essentials of Oceanography
7th Edition
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Ocean currents
Surface currentsAffect surface water within and above the pycnocline (10% of ocean water)Driven by major wind belts of the world
Deep currentsAffect deep water below pycnocline (90% of ocean water)Driven by density differencesLarger and slower than surface currents
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Measuring surface currents
Direct methodsFloat meters
Intentional
Inadvertent
Propeller meters
Indirect methodsPressure gradients
Satellites
Doppler flow meters Figure 7B
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Surface currents closely follow global wind belt pattern
Trade winds at 0-30º blow surface currents to the east
Prevailing westerlies at 30-60º blow currents to the west
Figure 7-3
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Wind-driven surface currents
Figure 7-4
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Current gyres
Gyres are large circular-moving loops of waterSubtropical gyres
Five main gyres (one in each ocean basin):North PacificSouth PacificNorth AtlanticSouth Atlantic Indian
Generally 4 currents in each gyre
Centered at about 30º north or south latitude
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Current gyres
Gyres (continued)Subpolar gyres
Smaller and fewer than subtropical gyres
Generally 2 currents in each gyre
Centered at about 60º north or south latitude
Rotate in the opposite direction of adjoining subtropical gyres
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Ekman spiral
Ekman spiral describes the speed and direction of flow of surface waters at various depthsFactors:
WindCoriolis effect
Figure 7-6
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Ekman transport
Ekman transport is the overall water movement due to Ekman spiral
Ideal transport is 90º from the wind
Transport direction depends on the hemisphere
Internet visualization
Figure 7-6
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Geostrophic flow and western intensification
Geostrophic flow causes a hill to form in subtropical gyres
The center of the gyre is shifted to the west because of Earth’s rotation
Western boundary currents are intensified
Figure 7-7
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Western intensification of subtropical gyres
The western boundary currents of all subtropical gyres are:
FastNarrowDeep
Western boundary currents are also warmEastern boundary currents of subtropical gyres have opposite characteristics
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Currents and climate
Warm current warms air high water vapor humid coastal climate
Cool current cools air low water vapor dry coastal climate Figure 7-8a
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Upwelling and downwelling
Vertical movement of water ()Upwelling = movement of deep water to surface
Hoists cold, nutrient-rich water to surface
Produces high productivities and abundant marine life
Downwelling = movement of surface water downMoves warm, nutrient-depleted surface water down
Not associated with high productivities or abundant marine life
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Coastal upwelling and downwelling
Ekman transport moves surface water away from shore, producing upwelling
Ekman transport moves surface water towards shore, producing downwelling
Figure 7-11
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Other types of upwelling
Equatorial upwelling
Offshore wind
Sea floor obstruction
Sharp bend in coastal geometry Figure 7-9
Equatorial upwelling
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Antarctic surface circulation
Figure 7-13
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Atlantic Ocean surface currents
Figure 7-14
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North Atlantic Ocean circulation
Figure 7-15
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The Gulf Stream and sea surface temperatures
The Gulf Stream is a warm, western intensified currentMeanders as it moves into the North AtlanticCreates warm and cold core rings
Figure 7-16
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Pacific Ocean surface currents
Figure 7-17
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El Niño-Southern Oscillation (ENSO)
El Niño = warm surface current in equatorial eastern Pacific that occurs periodically around ChristmastimeSouthern Oscillation = change in atmospheric pressure over Pacific Ocean accompanying El NiñoENSO describes a combined oceanic-atmospheric disturbance
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Normal conditions in the Pacific Ocean
Figure 7-18a
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El Niño conditions (ENSO warm phase)
Figure 7-18b
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La Niña conditions (ENSO cool phase; opposite of El Niño)
Figure 7-18c
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The 1997-98 El Niño
Sea surface temperature anomaly map shows warming during severe 1997-98 El Niño
Internet site for El Niño visualizations
Current state of the tropical Pacific
Figure 7-19a
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El Niño recurrence interval
Typical recurrence interval for El Niños = 2-12 yearsPacific has alternated between El Niño and La Niña events since 1950
Figure 7-20
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Effects of severe El Niños
Figure 7-21
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Indian Ocean surface currents
Figure 7-23
Northeast monsoon Southwest monsoon
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Deep currents
Deep currents:Form in subpolar regions at the surfaceAre created when high density surface water sinksFactors affecting density of surface water:
Temperature (most important factor)Salinity
Deep currents are also known as thermohaline circulation
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Deep ocean characteristics
Conditions of the deep ocean:Cold
Still
Dark
Essentially no productivity
Sparse life
Extremely high pressure
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Identification of deep currents
Deep currents are identified by measuring temperature (T) and salinity (S), from which density can be determined Figure 7-24
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Atlantic Ocean subsurface water masses
Figure 7-25
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Conveyer-belt circulation
Figure 7-27
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End of Chapter 7
Essentials of Oceanography
7th Edition