These deep-ocean currents are driven by differences in the water's density, which is controlled by temperature (thermo) and salinity (haline). This process is known as thermohaline circulation. In the Earth's polar regions ocean water gets very cold, forming sea ice.
1. Chapter 5 Surface Ocean/ Deep Ocean Circulation
2. Winds also cause large gyres (circular patterns) in the surface ocean…
6. Get downwelling at the centers of gyres low productivity
7. Phytoplankton patterns reveal circulation patterns: (Note the large gyres in Pacific Ocean)
8. Major Ocean Surface Currents
9. Gulf Stream Western edge of North Atlantic gyre Brings warm tropical waters north Transfers heat to Northern Europe Waters in the Gulf Stream are salty (because of evaporation)
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12. What causes deep ocean circulation? • Surface ocean circulation can lead to density differences in surface water • What makes water dense? (Hint: 2 factors)
13. What makes water dense? • Cold temperatures* • High salt content Cold, salty waters sink below warm or fresher waters This drives circulation in deep waters *Except, curiously, below 4oC
14. Seawater density vs. T and S
15. Annual mean surface water salinity Units: Parts per thousand by mass http://en.wikipedia.org/wiki/Seawater#Salinity
16. Seawater temperature vs. depth • On average, seawater temperature decreases with depth • This difference is less pronounced, though, at high latitudes http://www.windows.ucar.edu/tour/link= /earth/Water/temp.html&edu=high
17. Seawater density vs. depth • On average, seawater density increases smoothly with depth • Slight differences exist, though, between different ocean basins http://www.windows.ucar.edu/tour/link=/earth/ Water/density.html
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19. Salty waters from the Gulf Stream mix with cold waters in the North Atlantic
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21. Salty waters from the Gulf Stream mix with cold waters in the North Atlantic The result is very cold and very salty water.
22. Salty waters from the Gulf Stream mix with cold waters in the North Atlantic The result is very cold and very salty water. This North Atlantic water sinks, and begins to move along the bottom of the Atlantic Ocean.
23. Salty waters from the Gulf Stream mix with cold waters in the North Atlantic The result is very cold and very salty water. This North Atlantic water sinks, and begins to move along the bottom of the Atlantic Ocean. Eventually it warms, and returns to the surface, creating a return flow.
24. The Atlantic Conveyor • Because it is driven by differences in temperature and salinity, this is termed the thermohaline circulation
25. Radiocarbon Age of Deep Water Ref: Broecker and Peng, Tracers in the Sea (1982), p. 269
26. How Carbon-14 is made • 14N: 7 p, 7 n • 14C: 6 p, 8 n • C-14 production: 14 N + n 14C + p • C-14 decay: 14 C 14N + e (Beta decay)
27. Preview of global warming/effect on thermohaline circulation • Question: Has the Atlantic Conveyor Belt ever shut down? – Answer: We think it did shut down for almost 1000 years at the end of the last Ice Age. This is called the Younger Dryas Period
28. • Question 2: Could this happen again in the relatively near future? – Answer: Maybe! Global warming causes increase in rainfall in the N. Atlantic surface water becomes fresher and, hence, less dense – This effect is augmented by runoff of glacial meltwater from Greenland – Thermohaline circulation shuts down about 75 years from now in some climate model simulations!
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