Deep Ocean Circulation

1
Deep Ocean Circulation

• Motion in the Ocean, Part 2, or
• Who wants to ride the Great Conveyor Belt?

2
Surface Circulation
3
How does the Deep Ocean respond to Surface
Circulation?

• The main gyres move heat and salt
• Resulting DENSITY variations lead to vertical
  flow (sinking)
• Formation of water masses, characterized by
  Temperature, Salinity

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Density Variation in Sea Water
Isopycnals constant density
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North Atlantic Circulation
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Density-Driven Water Flow

• Called Thermohaline Circulation, because
  temperature and salinity together determine
  density of seawaterThermo temperaturehaline
  salt

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Where does the Oceans Deepest Water Come From?

• The densest seawater is cold and salty
• This is formed at high latitudes in the North and
  South AtlanticNorth Atlantic Deep Water
  (NADW)Antarctic Bottom Water (AABW)

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Density of Sea Water
NADW


AABW
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Density Rules!
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Deep Water Masses
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Water Massesand oceanmixing
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Mediterranean Water
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Mediterranean Water
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Mediterranean Water
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Deep Atlantic Circulation
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Tracers in the Ocean

• Track the motion (direction and velocity)
• 14C, cosmic rays in the upper atmos
  (half-life is 5700 years)3H, nuclear weapons
  testing (half-life is 12.5 years)CFCs,
  chlorinated fluoro-carbons

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The Great Conveyor Belt
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Closing Isthmus of Panama and onset of the Ice
Ages?
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Consequences of Global Flow

• Ocean turnover is about 1500 years (time for a
  round trip on the conveyor belt)
• Deep water (made in the polar Atlantic) contains
  abundant O2 and CO2
• The high O2 content promotes oxidation of bottom
  sediments
• The CO2 content controls CCD (Carbonate
  Compensation Depth)

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Carbon Cycle and Global Warming

• The temperature of bottom water formation
  determines how much CO2 is dissolved in deep
  ocean water
• The rate of overturn of the oceans determines the
  burial rate of C from the atmosphere
• Organic C accumulates in sediments, depending on
  the O2 content of deep ocean

21
Carbon Cycle and Global Warming

• Organic C in sediments is reduced to CH4 (methane
  gas)
• Methane gas migrates upward and can be trapped as
  frozen gas hydrates near the ocean floor

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Gas Hydrates
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Gas Hydrates
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Gas Hydrates
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Gas Hydrates
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Climate Change Concerns

• What happens when sea level falls?(negative
  feed-back)
• What happens when deep water warms?(positive
  feed-back)
• Both effects liberate gas hydrates (CH4), which
  combines with O2 to form CO2, ultimately reaching
  the atmosphere

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Deep Ocean Circulation

• The ocean has an enormous capacity to absorb and
  release greenhouse gases
• So, the rate, temperature and composition of
  seawater circulating through the deep ocean is
  vitally important in assessing long term climate
  change