Ocean Currents

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Ocean Currents

• http//oceanworld.tamu.edu/students/currents/index
  .html
• Ocean World

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What is a current?
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Ocean currents

• a steady horizontal movement of water or air in a
  definite direction.
• Ocean currents flow in complex patterns affected
  by wind, the water's salinity and heat content,
  bottom topography, and the Earth's rotation

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Topography of the ocean. Red and yellow hills
blue valleys
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Ocean currents

• Surface currents
• Upper kilometer wind driven
• Deeper Currents gravity driven
• Mixing moves very cold dense water up to the
  surface. The dense water is replaced by cold
  dense water that sinks to the bottom near
  Greenland, Norway and Antarctica. Deeper water is
  affected by long variability of climate.

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Wind

• Wind-driven currents affect about 20 of the
  ocean's total volume. These are the only currents
  that most people see. Sailors have charted
  patterns of the ocean's currents for hundreds of
  years.
• The sun ultimately creates winds in the
  atmosphere and ocean currents. Because the
  equator receives more direct rays from the sun
  throughout the year, this creates a temperature
  imbalance. Unequal heating of the atmosphere on
  land and the oceans creates winds and
  circulation.

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Wind energy is converted to water movements
called "currents" by friction between the wind
and the water surface. The surface currents
resemble the surface winds. Once these surface
currents are set in motion they are influenced by
three other factors Coriolis effect, presence of
coasts, and horizontal pressure gradients.
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Ekman spiral The Ekman transport combined with
the Coriolis force causes each layer of water to
change angle slightly creating a spiraling affect
in the water the spiraling is clockwise in the
northern hemisphere and counter-clockwise in the
southern hemisphere
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Gyres
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Trade winds

• originally named by sailors, the trade-winds are
  a group of winds which blow from the east.
• The trade-winds are part of the Hadley cells
  which lie between 30 degrees North and 30 degrees
  South. The easterly trade-winds are the surface
  component of Hadley cells which due to the
  Coriolis effect move from the Northeast north of
  the Equator and from the Southeast south of the
  equator.
• Where the two sets of winds meet along the
  Equator the lack of persistent winds result in
  relative calm seas, this area is known by sailors
  as the doldrums. Air masses that move from
  subtropical high pressure belts toward the
  equator. They are northeasterly in the Northern
  Hemisphere and southesterly in the Southern
  Hemisphere

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Salinity

• a measure of the quantity of dissolved salts in
  ocean water.
• This is influenced by the geologic formations
  underlying the area.
• Salinity is lower in areas underlain by igneous
  formations and higher in areas underlain by
  sedimentary formations.
• Higher salt concentrations are also more likely
  in arid regions where water evaporates leaving
  the same amount of salt in less water and thus
  increasing the salinity.

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Density

• the average mass per unit volume a measure of
  how much matter is squeezed into a given space
• the more closely packed the molecules, the higher
  the density of the material.
• Density in the ocean is determined by salinity
  and temperature

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Deep Ocean circulation

• the slow circulation of water at great depths is
  driven by density differences rather than by wind
  energy

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Upwelling

• a current of cold, nutrient-rich water rising to
  the surface.
• Upwellings are caused by strong seasonal winds
  moving surface coastal water out from the coast
  and leaving a space that the upwelling fills in.
• Many marine plants and animals live off this
  nutrient-rich water.

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Great Ocean conveyor belt
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The Global Conveyor Belt

• The global conveyor belt thermohaline circulation
  is driven primarily by the formation and sinking
  of deep water (from around 1500 meters to the
  Antarctic bottom water overlying the bottom of
  the ocean) in the Norwegian Sea.
• This circulation is thought to be responsible for
  the large flow of upper ocean water from the
  tropical Pacific to the Indian Ocean
• UNEP http//www.grida.no/climate/vital/32.htm

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Sea level changes

• Over the last 100 years, the global sea level has
  risen by about 10 to 25 cm.
• It is likely that much of the rise in sea level
  has been related to the concurrent rise in global
  temperature over the last 100 years.
• On this time scale, the warming and thermal
  expansion of the oceans may account for about 2-7
  cm of the observed sea level rise, while the
  observed retreat of glaciers and ice caps may
  account for about 2-5 cm..

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