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Chapter 7 Ocean Circulation
Essentials of Oceanography
7th Edition
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
Measuring surface currents
Direct methodsFloat meters
Intentional
Inadvertent
Propeller meters
Indirect methodsPressure gradients
Satellites
Doppler flow meters Figure 7B
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
Wind-driven surface currents
Figure 7-4
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
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
Ekman spiral
Ekman spiral describes the speed and direction of flow of surface waters at various depthsFactors:
WindCoriolis effect
Figure 7-6
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
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
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
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
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
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
Other types of upwelling
Equatorial upwelling
Offshore wind
Sea floor obstruction
Sharp bend in coastal geometry Figure 7-9
Equatorial upwelling
Antarctic surface circulation
Figure 7-13
Atlantic Ocean surface currents
Figure 7-14
North Atlantic Ocean circulation
Figure 7-15
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
Pacific Ocean surface currents
Figure 7-17
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
Normal conditions in the Pacific Ocean
Figure 7-18a
El Niño conditions (ENSO warm phase)
Figure 7-18b
La Niña conditions (ENSO cool phase; opposite of El Niño)
Figure 7-18c
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
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
Effects of severe El Niños
Figure 7-21
Indian Ocean surface currents
Figure 7-23
Northeast monsoon Southwest monsoon
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
Deep ocean characteristics
Conditions of the deep ocean:Cold
Still
Dark
Essentially no productivity
Sparse life
Extremely high pressure
Identification of deep currents
Deep currents are identified by measuring temperature (T) and salinity (S), from which density can be determined Figure 7-24
Atlantic Ocean subsurface water masses
Figure 7-25
Conveyer-belt circulation
Figure 7-27
End of Chapter 7
Essentials of Oceanography
7th Edition