Topic 14 Density Driven Currents

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Topic 14Density Driven Currents

• GEOL 2503
• Introduction to Oceanography

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THERMOHALINE CIRCULATION

• VERTICAL circulation driven by DENSITY
• Density of sea water mainly controlled by
  temperature and salinity
• Temperature is more important
• Warm water has low density, and oceans are heated
  from the top. So how does water sink?

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IMPORTANCE OF VERTICAL CIRCULATION

• DOWNWELLING
• Carries oxygen to the deep ocean

• UPWELLING
• Carries carbon dioxide and nutrients to the
  surface

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Evaporation and Precipitation Patterns

• These zones control evaporation and precipitation
  patterns

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Evaporation and Precipitation Patterns

• Average ocean salinity is 35
• Tropicsrising airheavy rain
• Oceans lower salinity (34.5 )
• Land tropical rain forests
• 30 NSfalling airevaporation
• Oceans higher salinity (36.7 )
• Land deserts

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Evaporation and Precipitation Patterns

• 50-60 NSrising air againheavy rain again
• Oceans cool and lower salinity (34.0 )
• Land northern hemisphere forests

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Another Important Factor

• Polar latitudesseasonal influences
• Wintersea ice formssalinity increases
• Summersea ice meltssalinity decreases

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Ocean surface salinity
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Density Controls

• Temperature and Salinity are the major controls
  on density of ocean surface water
• As salinity increases, density increases
• As salinity decreases, density decreases
• As temperature increases, density decreases
• As temperature decreases, density increases

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Controls on Salinity

• Increased by
• evaporation
• sea ice formation
• Decreased by
• precipitation
• sea ice melting
• inflow of river water

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Many combinations of salinities and temperatures
produce the same density
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Surface Processes

• Less-dense water stays at surface
• Warmer, less saline
• Denser water tends to sink
• Colder, more saline
• The result is a density-layered ocean

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Changes with Depth

• Well-mixed surface layer to about 100 m
• Increasing density to about 1,000 m
• Constant density below 1,000 m
• Region between 100-1,000 m is called a pycnocline
• Pycno- refers to density

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Same for Temperature, Salinity

• Thermoclinezone with a rapid change in
  temperature with depth
• Haloclinezone with a rapid change in salinity
  with depth
• Pycnoclinezone with a rapid change in density
  with depth
• All occur between about 100-1,000 m

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There is no pycnocline (or halocline or
thermocline) in high latitudes because of rapidly
sinking water
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Density-Driven Circulation

• Vertical circulation in the oceans is driven by
  density
• Density is mostly controlled by surface changes
  in temperature and salinity
• Because of this, vertical circulation is called
  thermohaline circulation

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Upwelling and Downwelling

• Vertical motions of water up or down
• Sinking waterdownwelling
• Rising waterupwelling
• Continuity of flow is basic concept

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Additional processes cause upwelling and
downwelling

• Surface convergences
• Surface divergences
• Water moving together or apart
• Water moving toward or away from a continent

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Surface Convergence

• Waters driven together
• Water driven against a continent
• Downwelling occurs

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Surface Divergence

• Waters driven apart
• Water driven away from a continent
• Upwelling occurs
• Remember the El Niño story

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All driven by differences in density
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Water Masses

• Water with similar temperature and density
  characteristics
• Acquire characteristics at the surface
• Thermohaline circulation moves masses throughout
  the ocean depths
• Named by
• Where from
• Where found

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Water masses
AC Antarctic convergence AD Antarctic
divergence STC subtropical convergence
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Map view?
Side view?
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T-S Curves

• Temperature and Salinity (T-S) plotted on graphs
• Remember, water masses have characteristic
  patterns of temperature and salinity
• So T-S curves delineate water masses

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