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Surface Currents
Origin of Currents
• Ocean surface currents are wind driven
• Air movement due to less dense air rising and more dense
air sinking
• Horizontal air flow along Earth’s surface is wind
• Air circulating in this manner is convection currents
Convection Currents
• Air becomes less dense when:
– It is warmed
– Atmospheric pressure ↓
– Water vapor (humidity) ↑
• Air becomes more dense when:
– It is cooled
– Atmospheric pressure ↑
– Water vapor (dry air) ↓
Warm and Cold Fronts
• Caused by eastward moving air masses
– Warm air rises over cold, dense air – condenses and precipitates
– Steeper cold front pushes warm air up – cools, condenses and
precipitates
Wind Movement
Non-rotating Earth
• Simple wind pattern
– Warm air rises at equator, flows toward poles
– Air cools at poles, sinks, and flows toward equator
• Winds named by direction from which they blow
– North-blowing winds = southerly winds
– South-blowing winds = northerly winds
Wind Movement
Rotating Earth
• At equator, warm air rises
– Zone of low pressure
– Clouds and precipitation
– Reaches troposphere and
moves poleward
– As it spreads, it cools
• 30° N&S, cool air sinks
– Area of high pressure
– Dry conditions
– Location of world deserts
• 60° N&S, air masses meet
– Form Polar Front
– Air masses rise, diverge and
sink @ 90° and 30° N&S
Wind Movement
• At equator, warm air rises, condenses and precipitates
• At 30° and 90°, cool air sinks
• Air that sinks does not flow back in a straight north-south path –
it curves (Coriolis Effect)
Rotation on a Globe
Speed of Rotation
Apparent Deflection
Wind Movement
Coriolis Effect
• Deflected winds due to
movement over spinning
object
– Produce wind bands
• In Northern Hemisphere:
– Winds are deflected to the right
– Travel clockwise around high P
• In Southern Hemisphere:
– Winds are deflected to the left
– Travel counter-clockwise around
high P
Assume water-covered Earth
High and Low Pressure
In Northern Hemisphere
• Cyclones
– Low pressure air
– Converge
– Ascend
• Anticyclones
– High pressure air
– Diverge
– Descend
High and Low Pressure Areas
• Land masses modify wind bands
Variations in Wind
Daily Coastal Areas
• Daytime
– Warmer land; cooler sea
– Onshore flow
• Evening
– Cooler land; warmer sea
– Offshore flow
Variations in Wind
Seasonal
July
January
• In summer:
– Air rises over warmed land, condenses
– Sudden and large volume of precipitation
– Winds accumulate more H2O vapor from ocean
• In winter have opposite effect
Surface Current Circulation
Ekman Transport
In Northern Hemisphere
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•
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Winds over H2O set ocean surface currents in motion
Surface H2O is deflected 45° by Coriolis Effect
Deflection increases with depth
Net water movement 90° to right of wind direction in N. Hemi.
Upwelling and Downwelling
• West Coast of N. America
– Northerly winds  summer upwelling
– Southerly winds  winter downwelling
Divergence and Convergence
• Convergence
– Wind-driven currents collide
– Downwelling
• Divergence
– Currents move away
– Upwelling
• Equatorial region
– Created by SE and NE
trade winds
– Divergence
– Upwelling
Warm and Cold Currents
• Uneven solar heating produces ocean temperature pattern
• Ocean currents redistribute heat and influence climate
• Clockwise rotation in Northern Hemisphere
Surface Currents and Coriolis Effect
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•
•
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Ocean currents are driven by wind
Moving water deflected by Coriolis Effect
Wind deflection creates ocean circulation gyres
Water “piles up”, gravity causes H2O to flow down slope
Geostrophic Currents
Gyres
gyre
gyre
gyre
gyre
gyre
• Convergence thickens surface layer – builds a dome
• Circular current systems in major ocean basins: Gyres
• Boundary currents parallel to ocean margins
Pacific Currents
• Equatorial and boundary currents  Convergence
– Warm currents in West Pacific
i.e. Kuroshio, Australia
– Cold currents in East Pacific
i.e. California, Peru
Western Intensification
• Gyres displaced to west by Earth’s rotation
• Western Intensification
– Faster, narrower currents
• Eastern Currents
– Slower, more diffuse currents
El Niño Year
• Factors producing El Niño year
– Trade winds weaken
– Warmer H2O moves east
– Upwelling ceases along western S. America
– Thermocline deepens
– Rainfall shifts from eastern to western Pacific
• May result from atmospheric pressure changes
El Niño Impact
• Leads to death of cold-water organisms
• Warm-water organisms migrate north and south