1. Topic 14 Density Driven Currents
GEOL 2503
Introduction to Oceanography

2. 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?

3. IMPORTANCE OF VERTICAL CIRCULATION
DOWNWELLING
Carries oxygen to the deep ocean
UPWELLING
Carries carbon dioxide and nutrients to the surface

4. Driven by surface processes
Recall atmospheric circulation
These zones control evaporation and precipitation patterns
Latitude controls surface temperature

5. Recall patterns of rising and falling air

6. Precipitation and Evaporation Patterns
Average ocean salinity is 35 ‰
Tropics—rising air—heavy rain
Oceans: lower salinity (34.5 ‰)
Land: tropical rain forests
30° N+S—falling air—evaporation
Oceans: higher salinity (36.7 ‰)
Land: deserts

7. Precipitation and Evaporation Patterns
50-60° N+S—rising air again—heavy rain again
Oceans: cool and lower salinity (34.0 ‰)
Land: northern hemisphere forests

8. Another Important Factor
Polar latitudes—seasonal influences
Winter—sea ice forms—salinity increases
Summer—sea ice melts—salinity decreases

9. Ocean surface salinity

10. Ocean Surface Water Density Summary
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

11. Controls on Salinity Increased by: Decreased by: evaporation
sea ice formation
Decreased by:
precipitation
sea ice melting
inflow of river water

12. Many combinations of salinities and
temperatures produce the same density

13. 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

14. 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

15. Same for Temperature, Salinity
Thermocline—zone with a rapid change in temperature with depth
Halocline—zone with a rapid change in salinity with depth
Pycnocline—zone with a rapid change in density with depth
All occur between about 100-1,000 m

19. There is no pycnocline (or halocline or thermocline) in high latitudes because of rapidly sinking water

20. 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

21. Upwelling and Downwelling
Vertical motions of water up or down:
Sinking water—downwelling
Rising water—upwelling
Continuity of flow is basic concept

22. Another way to drive upwelling and downwelling
Push surface water together or apart
Has nothing to do with density
Processes called surface convergences and divergences

23. Divergence zones—surface water pushed apart by winds and surface currents
Convergence zones—surface water pushed together by winds and surface currents

24. Ocean surface convergence and divergence driven by global wind patterns

25. Remember the El Niño Story

26. Antarctic Convergence is the northern limit of the Southern Ocean

28. Antarctic Convergence
Also known as the Polar Front
It’s a line encircling Antarctica where cold, northward-flowing Antarctic waters sink beneath the relatively warmer waters of the sub-Antarctic.
It is a dynamic boundary, meaning it's precise location may shift, but is generally located between 48º S and 61º S latitude

29. Coastal Upwelling/ Downwelling

30. All driven by differences in density

31. 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

33. The Mediterranean Seafloor

34. Mediterranean outflow water

35. Map view
Side view

37. Southern Ocean Water Masses and Circulation
Schematic depth-latitude diagram showing the major circulation and water masses of the Southern Ocean.
The following water masses are highlighted:
(1) Antarctic Bottom Water flowing along the abyssal ocean,
(2) Circumpolar Deep Water upwelling into the Antarctic Divergence Zone,
(3) Antarctic Intermediate Water in the temperature range 4-6°C, and
(4) Subantarctic Mode Water in the upper ocean north of the Subantarctic Front (SAF).

38. 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

42. Deep water forms only in a few places
surface
deep
sinking 3 4 2 1
1-Weddell Sea Ross Sea Labrador Sea Norwegian Sea

43. 1 3 2 4
1-Weddell Sea Labrador Sea
2-Ross Sea Norwegian Sea

44. Could it happen?

45. A. thermocline
B. mixed layer
C. layer with the highest density
D. curve for low latitudes
E. curve for high latitudes

46. A. thermocline and pycnocline
B. warmest water
C. seasonally warm water
D. Antarctic Bottom Water
E. Antarctic Intermediate Water
F. North Atlantic Deep Water
G. upwelling

47. Polynya (polynia) An area of open water surrounded by sea ice
Now used as geographical term for areas of sea in Arctic or Antarctic regions which remain unfrozen for much of the year.
The term derives from the Russian word for a hole in the ice
Adopted in the 19th century by polar explorers to describe navigable portions of the sea.

49. Whales often use polynias for access to air.

50. Seawater: Temperature and Density

51. Seawater: Ice Formation