Thermohaline Circulation

1. Thermohaline Circulation Readings: Kennett, Chapter 8 Ocean Circulation - Open Univ. Chapter 6

3. Subsurface Water Masses • Convection - Vertical movements of water masses • Origin - Density differences • Temperature • Heat loss • Salinity • Evaporation • Sea Ice formation

4. Water Column Stability/Instability • Generally the density of ocean water increases with depth. In this case, the water column is stable. • A stable water column does not mix vertically unless acted on by outside forces (for example, winds).

5. Water Column Stability/Instability • If the density of water decreases with depth, the water column is unstable. • Usually an unstable water column occurs because surface water cools and becomes more dense. Less commonly, evaporation causes increased salinity and thus increased density of surface water. • An unstable water column does not last long. The dense water will sink, and the less dense water will rise, resulting in turbulence and vertical mixing.

6. The density of seawater is controlled by its salinity and its temperature. • Processes that increase the salinity of seawater include evaporation and the formation of sea ice. • When ice crystals form, salt is rejected from their structure. The water surrounding the crystals becomes saltier. The resulting brine seeps out of the forming ice. This brine rejection continues for a year or more; old sea ice is less salty than new sea ice. • Processes that decrease seawater salinity are precipitation at the sea surface, land runoff (from rivers, glaciers, etc.), or melting of sea ice.

7. The Deep Ocean Properties • Properties in the subsurface are displayed as lines of constant value, for example: • Isopycnals are lines of constant density. They show the position of water layers that have uniform density. • Isotherms are lines of constant temperature. They show the position of water layers of uniform

9. Water Masses • Layers of ocean water with characteristic temperature, salinity and density (and, in some cases, other properties such as nutrient concentrations) are called water masses.

10. Water Masses • North Atlantic Deep Water • 1.8 to 4°C; 34.89 -35.0‰ • Antarctic Bottom Water • 0.5 to 2.0°C; 34.74 to 34.89 ‰ • Antarctic Intermediate Water • 4 to 5°C; 34.2‰

11. Density • The density, , of seawater is function of temperature, salinity, and pressure. It increases with increasing salinity and pressure, and decreases with increasing temperature. The density is expressed in units of kg/m3, or sometimes g/cm3. Oceanographers use a number of different ways to express the density of seawater, so you may see the terms density anomaly, potential density, (pronounced sigma-theta), specific volume, specific volume anomaly or others. The most commonly used of these are defined below.

12. Sigma  Sigma, is a short-hand for seawater density, where 1000 kg/m3 has been subtracted. So for = 1024.32 kg/m3, = 24.32 kg/m3. While all the versions of have units of kg/m3, it is often reported without units, which is a throwback to when the definitions included a ratio of the seawater density to the density of freshwater, thus rendering the variable dimensionless. Sigma-t, is density of seawater calculated with in situ salinity and temperature, but pressure equal to zero, rather than the in situ pressure and 1000 kg/m3 is subtracted. Sigma  density of the water at the surface

13. Sigma  Sigma  density of the water at the surface

14. Note that the Pressure term is ignored in the Sigma T graph. AABW appears less dense than NADW. The numbers for each station represent depth in 100’s of meters. E.g., 50 at the bottom graph = 5000m

15. DEEP-WATER FORMATION SITES-places where large volumes of sea water sink • NORTH ATLANTIC - NORWEGIAN AND GREENLAND SEAS • North Atlantic Deep Water (NADW) • ANTARCTIC - WEDDELL SEA • Antarctic Bottom Water (AABW)

17. Bottom Water sources

18. INTERMEDIATE -WATER FORMATION SITES • AAIW - SOUTHERN OCEAN - NORTH OF THE ANT. DIVERGENCE • NORTH PACIFIC - INTERMEDIATE DEPTH WATER

19. Intermediate Water Sources

20. Salinity driven circulation • tends to occur in semi-isolated seas in areas of net evaporation in the sub-tropics • Mediterranean - • Red Sea -

21. Thermohaline Model

22. Thermohaline Model

23. Coriolis Force • Deflection of moving objects to the right in the northern hemisphere and to the left in the southern hemisphere. • This is due the rotation of the Earth.

24. Circulation Tracers • Conservative • Those properties that are changed only at the margins of the boundaries of the ocean • Non-Conservative • Those properties that can be altered by physical, chemical or biological processes

25. Non-Conservative TracersO2, PO4 etc.

26. Nonconservative Properties - Dissolved O2 Biological processes: photosynthesis (light) organic materials + O2 CO2 + H2O inorganic N (NO3-, NH4+) inorganic P (PO43-) respiration Physcial processes: O2dissolved O2atmosphere

27. Nonconservative Properties - Dissolved O2 O2 [O2]  atmosphere sea surface mixed layer O2 net photosynthesis organics pycnocline oxygen minimum sinking  depth O2 deep water rising water net respiration organics burial sediments

28. Dissolved Oxygen

30. PO4 distribution in ocean

32. Atlantic Thermohaline Circulation

33. The North Atlantic Denmark Strait Iceland Faeroe Faeroe Bank Channel Charlie-Gibbs FZ

34. The North Atlantic

35. The North Atlantic

36. GREAT SALT PUMP • Fresh water carried from the Atlantic to the Pacific - mainly by trades across central America. Creates a salt inbalance between the N. Atlantic & Pacific • Deep flow - NADW and AABW transport excess salt to the Pacific • Surface return flow - via the Pacific - Indian Ocean - S. Atlantic - Gulf stream

39. The Southern Hemisphere

40. The Southern Hemisphere

42. Sea Ice Formation • SEA ICE - ice that forms from seawater freezing at the oceans surface (-2°C) • - typically less than 10 meters thick • 3 step Formation Process: • 1. starts as tiny needles (spicules) 1 to 2 cm long forming frazel ice • 2. Slush - eventually start to coalesce • 3. pancake ice • Freezing process excludes salt - Not pure freshwater, however. Some seawater is trapped. Salinity, is usually less than 10 ppt.

43. Sea ice. As sea ice forms, it takes up freshwater, and excludes most of the salt from the salty seawater. The water that is left behind is dense because it is so salty.

44. Implications for Deepwater 1. Cooling - initially as sea ice forms - latent heat of freezing is lost to the atmosphere, cooling the water, warming the atmosphere. Later, it prevents further cooling. 2. Salt exclusion - salt is excluded from the ice - elevating salinity of surface water -convection begins as less dense waters rise in fingers to replace sinking waters sea ice expansion is an important process in the formation of deep waters at polar latitudes

45. Polynyas • Coastal Polynyas - Open areas (leads) in the sea-ice along the coast • created by persistent offshore winds which push ice away from the coast • Open Ocean Polynyas - • Open areas (leads) in the sea-ice created by winds or warm currents beneath

47. Antarctic Intermediate Water

48. Antarctic Intermediate Water

49. MedWater

50. Mediterranean Water