Topic 14 Density Driven Currents

1. Topic 14Density 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. DOWNWELLING Carries oxygen to the deep ocean UPWELLING Carries carbon dioxide and nutrients to the surface IMPORTANCE OF VERTICAL CIRCULATION

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 WaterDensity 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: • 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 http://maritime.haifa.ac.il/departm/lessons/ocean/

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 sinking sinking surface deep sinking 3 4 2 1 1-Weddell Sea 2-Ross Sea 3-Labrador Sea 4-Norwegian Sea

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

48. http://www2.fsg.ulaval.ca/giroq/now//dundas.jpg

49. Whales often use polynias for access to air.

50. Seawater: Temperature and Density