Counter Currents

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Counter Currents

• Equatorial currents are typically accompanied by
  counter currents flowing on the surface in the
  opposite direction form the main current.
• Counter currents can exist beneath surface
  currents, and are sometimes referred to as
  udercurrents.

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Counter Currents

• The first of these was discovered in 1951 in the
  central Pacific by Townsend Cromwell and is
  called the Cromwell Current. It is an equatorial
  counter current in the Pacific.
• The Pacific Equatorial Undercurrent or Crowmwel
  current has an average velocity of 5 km (3 miles)
  per hour.

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Counter Currents

• The Cromwell current is found at a depth of 100
  to 200 meters and is about 300 kilometers wide.
• It carries a volume of water equivalent to about
  half of the Gulf Stream.
• It has been traced over 14,000 kilometers from
  New Guinea to Equador.

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Counter Currents

• Oceanographers are uncertain about what causes
  undercurrents, but near the equator (where the
  Coriolis effect is negligible) some water is not
  deflected north or south when it reaches the west
  side of the basin.
• Instead it stay on the equator and underflows the
  surface current back to the east.

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Exceptional Surface Currents

• Another exception to expected surface flow is
  seen in the lesser current circuits at higher
  latitudes.
• Some of these such as the Greenland and Labrador
  currents in the North Atlantic and the Kamchactka
  and Alaska currents in the North Pacific move in
  apparent contradiction to the influence of the
  Coriolis, gravity and friction.

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Exceptional Surface Currents

• These currents form when larger currents are
  split and deflected by collision with a
  continent.
• The polar easterly winds at these latitudes keep
  the water moving westward.
• The size of these high-latitude currents on the
  map is exaggerated.

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Effects of Surface Current on Climate

• Surface currents distribute tropical heat
  worldwide.
• Warm water flows to higher latitudes, transfers
  heat to the air and cools, moves back to low
  latitudes, absorbs heat again, and repeats the
  cycle.
• The greatest amount of heat transfer occurs at
  the mid-latitudes.

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El Nino

• Surface winds over most of the tropical Pacific
  normally blow from east to west.
• The trade winds blow from the normally
  high-pressure area over the eastern Pacific (near
  Central and south America) to the normally stable
  low pressure over the western Pacific (near
  Australia).

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El Nino

• For reasons still unknown these pressure areas
  change places at irregular intervals of roughly
  three to eight years.
• Winds over the tropical Pacific reverse
  direction and blow form west to east.
• This change in atmospheric pressure is called the
  Southern Oscillation.

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El Nino

• Ten of these oscillations have occurred since
  1950.
• This reversal causes a eastward flow of warm
  water. The eastward flow of warm water usually
  arrives along the coast of South America around
  Christmas.
• In the 1890s Peruvian fisherman started using
  the expression Corrente del Nino to describe
  this flow.

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El Nino

• Normally, a current of cold water, rich in
  upwelled nutrients, flows north and west away
  from the South American continent.
• When the propelling trade winds falter during an
  El Nino/Southern Oscillation (ENSO) event, warm
  equatorial water that would normally flow
  westward in the equatorial Pacific backs up to
  flow eastward.

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A Non El Nino Year
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El Nino Year
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Equatorial Upwelling

• Because the meteorological equator lies about 5
  degrees north of the geographical equator the
  South Equatorial currents of the Atlantic and
  Pacific straddle the geographical equator.
• Though the Coriolis effect is weak near the
  geographical equator water moving in the currents
  on either side of the geographical equator is
  deflected poleward.

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Equatorial Upwelling

• Equatorial upwelling occurs in these
  westward-flowing equatorial surface currents.
• Upwelling is an important process because this
  upwelled water coming from within and below the
  pycnocline is often rich in nutrients.

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Equatorial Upwelling

• The long, thin band of upwelling and biological
  productivity extending along the equator westward
  from South America is clearly visible in the
  following satellite image.

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Coastal Upwelling

• Wind blowing parallel to the shore or offshore
  can cause coastal upwelling. The friction of the
  water blowing along the ocean surface causes the
  water to begin moving, the Coriolis effect
  deflects it to the right and the resultant Ekman
  transport moves it offshore.

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Langmuir Circulation

• Winds that blow steady across the ocean, and the
  small waves that are generated can induce long
  sets of counter-rotating cells in the surface
  water.
• These slow twisting cells, or vortices, align in
  the direction of the wind.
• It takes abut an hour for a particle in a cell to
  complete one revolution.

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Langmuir Circulation

• Streaks of foam, seaweed, or debris, known as
  windrows, collect in areas where adjacent
  vortices converge, while regions of divergence
  remain clear.
• Langmuir circulation rarely disturbs the ocean
  below a depth of about 29 meters (66 feet) .

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• The End