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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) ↓
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
Rotation on a Globe
Speed of Rotation
Apparent Deflection
Coriolis on a Cylinder
• Coriolis impact on fast moving object
minimal
• Impact on slow moving wind/ocean
current greater
• Current flowing 0.5 m/s @45o lat, travels
1800 m/hr, deflected 300 m from original
path
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
Surface 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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•
•
•
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
Ekman Transport
In Northern Hemisphere
•
•
•
•
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
Note Equatorial Upwelling
Peru Upwelling
California Upwelling Spring &
Summer
High Productivity
Diatomaceous sediments accumulate, preserved in basins
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
California Current
•Most studied-separates from east flowing North Pacific Current
•Flow from shelf break to 1000km offshore
•Movement down to 500m, speeds to .5m\sec
•Interacts with sea floor
•Eddy produces counter current
California Current
Colder water transported southward
Changes in current strenght,position impacts sediment accumulation
Changes associated with El Nino/La Nina events, glacial/interglacial period
Santa Barbara Basin Core-Monterey
Rocks
• Laminated
•
•
diatomaceous
sediment
Changes in forams
distinct at 1925 and
later. More
tropical-subtropical
forms
Low oxygen level
preserve layers
(c) 2005 David Field
www.sakura.cc.tsukuba.ac.jp
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
Temperature Variations
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