Ocean Circulation

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Chapter 9

• Ocean Circulation

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Types of Ocean Currents

• Surface currents
• Only involve 10 of the water in the ocean
• Thermohaline circulation
• Slow, deep water circulation driven by density
  differences

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Surface Currents

• Surface currents are driven by 3 factors
• Wind drag
• Pressure gradients
• Coriolis deflection

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Wind Currents

• Friction caused by wind blowing across the ocean
  pushes surface currents

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• Surface currents move at a 45 angle to wind
  direction
  
• Net water transport is at a right angle to wind
  direction

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Pressure Gradients

• The net transport of water causes it to pile up
  in a hill in the middle of the ocean
  
• Gravity pulls the water down the hill
• The location of the hill is to the left of center
  because of earths rotation

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• Because of atmospheric circulation patterns,
  there is a high pressure area in the center of
  the large ocean currents
  
• This also causes the water to mound up in the
  middle

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Coriolis Deflection

• Currents in the northern hemisphere rotate in a
  clockwise direction
  
• Currents in the southern hemisphere rotate in a
  counter-clockwise direction

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• These large currents are called geostrophic gyres

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Eastern vs. Western Currents

• Western currents
• Narrow
• Fast
• Deep

Eastern currents Wide Slow Shallow
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Eastern vs. Western Currents

• Western currents
• Warm
• Low productivity
• Eastern currents
• Cold
• High productivity

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Other currents

• Transverse currents
• The northern and southern portions of the gyres
• Equatorial currents stronger than high latitude
  currents
  
• West wind drift (Antarctic circumpolar current)
• Largest of all surface currents, caused by strong
  Antarctic west wind
  
• Never deflected by a continent

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Countercurrents and Undercurrents

• Countercurrents
• Currents flowing in the opposite direction of the
  main current
  
• Backward flow is because of the lack of wind, and
  pile-up of water on the western edge of the ocean
  basin
  
• Undercurrents
• Countercurrents that flow under surface currents
• Can be very large (as big as the current above
  sometimes)

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Effects on Climate

• Cold ocean currents (eastern currents) usually
  cause cooler, dryer weather
  
• The cool water doesnt have enough heat in it for
  much water to evaporate out of it, so the air is
  dry
  
• Warm ocean currents (western currents) usually
  cause warmer, more humid weather
  
• The warm water causes a lot of evaporation, so
  the air is humid
  
• Therefore, on most continents, the western coast
  is dryer than the eastern coast

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Upwelling and Downwelling

• If wind is blowing parallel to shore, the water
  will move 90 to the right
  
• If this moves water away from the coastline,
  upwelling will occur
  
• This happens with a northerly wind on the west
  coast
  
• If this moves water towards the coastline,
  downwelling will occur
  
• This happens with a southerly wind on the west
  coast
  

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• Upwelling also occurs along the equator
• Because surface water is removed by the
  geostrophic gyres and surface winds
  
• Upwelling and downwelling can occur at high
  latitudes
  
• Because there is no pycnocline

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El Niño/La Niña

• Under normal circumstances, there is a cold water
  current flowing from east to west at the equator
  
• During an El Niño year, the currents are backed
  up, and the equatorial current becomes warmer
  than usual

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• El Nino can seriously impact marine life off of
  the west coast of the Americas because of the
  lack of nutrients in the warmer water
  
• Normally arid regions on the west coast of the
  Americas receive higher than usual rain

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Thermohaline Circulation

• Caused by density differences (salinity and
  temperature)

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