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Ocean circulation Arnaud Czaja 1. Ocean and Climate 2. Key observations 3. Key physics

Ocean circulation Arnaud Czaja 1. Ocean and Climate 2. Key observations 3. Key physics. Part I Ocean and Climate (heat transport and storage). Net energy loss at top-of-the atmosphere. =. +. Poleward energy transport. Ha. Ho. Imbalance between and

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Ocean circulation Arnaud Czaja 1. Ocean and Climate 2. Key observations 3. Key physics

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  1. Ocean circulationArnaud Czaja1. Ocean and Climate2. Key observations3. Key physics

  2. Part IOcean and Climate(heat transport and storage)

  3. Net energy loss at top-of-the atmosphere = + Poleward energy transport Ha Ho Imbalance between and = energy (heat) storage

  4. Poleward heat transport and storage are small… Energy exchanged at top-of-atmosphere : Planetary albedo Solar constant

  5. SeasonalHeat storage Q5

  6. Heat transport: a long history of measurements… Bjerknes’ (1964) monograph. Data from Sverdrup (1957) & Houghton (1954) Ha+Ho Ha Northward heat transport Ho Equator Pole

  7. Ha+Ho Ha Northward heat transport Ho 70N 50N 10N 30N Vonder Haar & Oort, JPO 1973. GERBE approved!

  8. NB: 1PW = 10^15 W “Across the same latitude, Ha is 1.7PW. The ocean therefore can be considered to be more important than the atmosphere at this latitude in maintaining the Earth’s budget”. Hall & Bryden, 1982; Bryden et al., 1991.

  9. GERBE approved! (ask more to Chris D.!) Trenberth & Caron, 2001

  10. GERBE approved! Ha+Ho Ho Ha Wunsch, JCl. 2005.

  11. Ganachaud & Wunsch, 2003

  12. Sometimes effects of heat storage and transport are hard to disentangle • Is the Gulf Stream responsible for “mild” European winters?

  13. WARM! COLD! Eddy surface air temperature from NCAR reanalysis (January, CI=3K) “Every West wind that blows crosses the Gulf Stream on its way to Europe, and carries with it a portion of this heat to temper there the Northern winds of winter. It is the influence of this stream upon climate that makes Erin the “Emerald Isle of the Sea”, and that clothes the shores of Albion in evergreen robes; while in the same latitude, on this side, the coasts of Labrador are fast bound in fetters of ice.” Maury, 1855. Lieutenant Maury “The Pathfinder of the Seas”

  14. Model set-up (Seager et al., 2002) • Full Atmospheric model • Ocean only represented as a motionless “slab” of 50m thickness, with a specified “q-flux” to represent the transport of energy by ocean currents Atmosphere

  15. Q3 Seager et al. (2002)

  16. Heat storage and Climate change The surface warming due to +4Wm-2 (anthropogenic forcing) is not limited to the mixed layer… How thick is the layer is a key question to answer to predict accurately the timescale of the warming. Ho = 50m Ho = 150m Ho = 500m NB: You are welcome to download and run the model : http://sp.ph.ic.ac.uk/~arnaud

  17. Q1 Ensemble mean model resultsfrom the IPCC-AR4 report

  18. Strength of ocean overturning at 30N (A1B Scenario + constant after yr2100) Q4

  19. Part IISome key oceanic observations

  20. World Ocean Atlas surface temperature ºC

  21. Thermocline

  22. World Ocean Atlas Salinity (0-500m) psu

  23. The “great oceanic conveyor belt”

  24. The ocean is conservative below the surface (≈100m) layer • Temperature Not changed by absorption/emission of photons. • Salinity. No phase change in the range of observed concentration.

  25. Salinity on 1027.6 kg/m3 surface Conservative nature of the ocean Spatial variations of temperature and salinity are similar on scales from several hundreds of kms to a few kms. 10km 2km 50km Ferrari & Polzin (2005)

  26. Matsumoto, JGR 2007

  27. “Circulation” scheme

  28. “Circulation” scheme Two “sources” of deep water: NADW: North Atlantic Deep Water AABW: Antarctic Bottom Water Williams & Follows (2009)

  29. In – situ velocity measurements Amplitude of time variability Location of “long” (~2yr) currentmeters Depth NB: Energy at period < 1 day was removed From Wunsch (1997, 1999)

  30. Moorings in the North Atlantic interior (28N, 70W = MODE) (ask more to Ute and Chris. O.!) 1 yr NB: Same velocity vectors but rotated Schmitz (1989)

  31. Direct ship observations NB: 1m/s = 3.6kmh = 2.2mph = 1.9 knot

  32. Surface currents measured from Space “Geostrophic balance” Standard deviation of sea surface height Time mean sea surface height

  33. Momentum balance Rotation rate f/2 East to west acceleration f V East to west deceleration up North NB: f = 2 Ω sinθ East

  34. Geostrophic balance! Rotation rate f/2 High Pressure Low Pressure East to west acceleration f V East to west deceleration up North East

  35. 10-yr average sea surface height deviation from geoid Subtropical gyres

  36. 10-yr average sea surface height deviation from geoid Subpolar gyres Antarctic Circumpolar Current

  37. ARGO floats (since yr 2000) T/S/P profiles every 10 days Coverage by lifetime Coverage by depths

  38. All in-situ observations can be interpolated dynamically using numerical ocean models Overturning Streamfunction (Atlantic only) From Wunsch (2000)

  39. RAPID – WATCH array at 26N Q2

  40. RAPID – WATCH array at 26N 14 millions £

  41. The movie…

  42. Part IIIKey physics

  43. Because T is conserved by fluid motion the temperature structure simply reflects transport by waves and mean currents Downward heat transport Upward heat transport = Sea surface Zo No internal heat source/sink Z Ocean bottom X, Y

  44. This simply happens when warm water goes up or cold water goes down Downward heat transport Upward heat transport = Sea surface Zo No internal heat source/sink Z Ocean bottom X, Y

  45. This happens when warm water goes down or cold water goes up… Downward heat transport Upward heat transport = Sea surface Zo No internal heat source/sink Z Ocean bottom X, Y

  46. Requires mechanical forcing (winds/tides)! Downward heat transport Upward heat transport = Sea surface Zo No internal heat source/sink Z Ocean bottom X, Y

  47. “Historical” view Sea surface Zo Z Ocean bottom X, Y

  48. “Historical” view “Conveyor-belt” upwelling/downwelling Sea surface Zo Z Ocean bottom X, Y

  49. Q6 Broecker, 2005 NB: 1 Amazon River ≈ 0.2 Million m3/s

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