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Water Mass Distribution

Water Mass Distribution. OEAS 604. Lecture Outline Thermohaline Circulation Spreading pathways in ocean basins T-S diagrams Mixing on T-S diagrams Chapter 9 – Knauss Chapter 14 – Talley et al. What is Thermohaline Circulation (THC)?. Circulation below the wind-driven currents

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Water Mass Distribution

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  1. Water Mass Distribution OEAS 604 • Lecture Outline • Thermohaline Circulation • Spreading pathways in ocean basins • T-S diagrams • Mixing on T-S diagrams • Chapter 9 – Knauss • Chapter 14 – Talley et al.

  2. What is Thermohaline Circulation (THC)? • Circulation below the wind-driven currents • Flow is determined by density distribution which is from temperature and salinity • Circulation forced by sinking of cold dense water at high latitudes • Return flow in upper water column • Referred to a Meridional Overturning Circulation (MOC) or Conveyor Belt Circulation

  3. North Atlantic and Southern Ocean NA – open to north, high latitude, limited seasonal sea ice, freshwater sources SO – bounded by continent, seasonal sea ice, no rivers, large polynyas

  4. Atlantic separated Into basins with little connections

  5. Indian Ocean Blocked to north Limited connection to Southern Ocean Complex bathymetry and limited openings to the east

  6. Pacific Ocean Western basin Deeper Opening to north constricted by Bering Strait

  7. Water Masses • Form in certain regions of the ocean • Advected by ocean currents • Form frontal regions were meet • Water mass – a body of water with a common formation history, having its origin in a particular region of the ocean • Source region – finite volume of ocean where water mass formation takes place

  8. Water Mass Formation Processes • Convection – surface density increased by cooling or evaporation causing sinking of water to depth of static stability • Subduction – combination of wind and cooling. Ekman transport pushes water down along isopycnal surfaces (Ekman pumping) and is injected into the ocean interior • Subsurface mixing - two or more water masses are brought together and mix to form distinctly different water mass – does not depend on air/sea exchange processes • Importation from marginal seas

  9. North Atlantic Deep Water Greenland Sea Water Labrador Sea Water Mediterranean Sea Water High salinity, low temp, high oxygen Antarctic Intermediate Water Low salinity, high oxygen Antarctic Bottom Water High salinity, low temperature, average oxygen Subtropical Underwater Low oxygen

  10. Antarctic Bottom Water Pathways Different varieties along western and eastern sides of basin Limited across basin exchange Flow on eastern side blocked by Walvis Ridge

  11. East-west section in South Atlantic NADW AABW AAIW

  12. West East T-S diagrams in East and West Atlantic – NADW, AAIW North Atlantic and South Atlantic Central Waters High salinity Mediterranean Sea Water

  13. Antarctic Circumpolar Current High speed current cores Subantarctic Front Polar Front Southern ACC Front Subtropical Front

  14. Bottom water formation

  15. Spreading and direction of AABW

  16. Intermediate and mode waters form along northern side of Polar and Subantarctic fronts

  17. T-S diagram from Southern Ocean

  18. AASW – Antarctic Surface Water WW – Winter Water SSW – Shelf Water HSSW – High Salinity Shelf Water ISW – Ice Shelf Water WS and RS – Bottom Water CDW - Circumpolar Deep Water

  19. T-S properties of water mass change around the Southern Ocean CDW – cools and freshens

  20. Spreading of Bottom water Limited exchange 90°E Ridge is a barrier

  21. T-S diagram for Indian Ocean AAMW – Australian Mediterranean Water

  22. Spreading of bottom water in Pacific Ocean

  23. North-south section in western Pacific

  24. T-S diagram from western Pacific

  25. T-S distribution for world oceans Pacific Atlantic Indian

  26. Water Mass Analysis • Water type - point in n-dimensional parameter space – it is a combination of temperature, salinity, nutrients, oxygen and other tracer values • Water mass – water formed in certain regions with particular temperature, salinity and other properties

  27. Mixing of water masses

  28. Two-point mixing • Determine the mixture along line between two source waters • f = fraction of water mass 1, f=1 all from water mass 1, f = 0 none from water mass 1 • Tmix = fT1 + (1-f)T2 • Smix = fS1 + (1-f)S2 • Know Tmix, Smix, T1, T2, S1, S, so solve for f

  29. Track water masses Core method Gives rates of mixing and transport direction

  30. Next Class • Continue water masses • Regional oceanography – water masses and currents

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