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Deep Ocean Circulation

Deep Ocean Circulation. Motion in the Ocean, Part 2, or Who wants to ride the Great Conveyor Belt?. How does the Deep Ocean respond to Surface Circulation?. The main gyres move heat and salt Resulting DENSITY variations lead to vertical flow (sinking)

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Deep Ocean Circulation

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  1. Deep Ocean Circulation Motion in the Ocean, Part 2, or Who wants to ride the Great Conveyor Belt?

  2. 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

  3. Density Variation in Sea Water Isopycnals = constant density

  4. North Atlantic Circulation

  5. Density-Driven Water Flow • Called “Thermohaline Circulation”, because temperature and salinity together determine density of seawater“Thermo” = temperature“haline” = salt

  6. Where does the Ocean’s Deepest Water Come From? • The densest seawater is cold and salty • This is formed at high latitudes in the North and South Atlantic:North Atlantic Deep Water (NADW)Antarctic Bottom Water (AABW)

  7. Density of Sea Water NADW * * AABW

  8. Density Rules!

  9. Deep Water Masses

  10. Water Massesand oceanmixing

  11. Mediterranean Water

  12. Mediterranean Water

  13. Mediterranean Water

  14. Deep Atlantic Circulation

  15. 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

  16. The Great Conveyor Belt

  17. Closing Isthmus of Panama and onset of the Ice Ages?

  18. 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)

  19. 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

  20. 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

  21. 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

  22. 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

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