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Arnaud Czaja

Rotating Fluid -Part II A “GFD view” of the Ocean and the Atmosphere (a follow up Raymond’s Lectures). Arnaud Czaja. Source / sink flows –see Raymond’s lectures. “Basin”. “Channel”. Source / sink flows –see Raymond’s lectures. “Basin”. “Channel”. No distinction between

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Arnaud Czaja

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  1. Rotating Fluid -Part IIA “GFD view” of the Ocean and the Atmosphere (a follow up Raymond’s Lectures) Arnaud Czaja

  2. Source / sink flows –see Raymond’s lectures “Basin” “Channel”

  3. Source / sink flows –see Raymond’s lectures “Basin” “Channel” No distinction between Ocean & Atmosphere…

  4. Central idea • Constraint 1: Ocean & Atmosphere are rapidly rotating fluids: geostrophy is the leading order dynamics. • Constraint 2: The two fluids must transport energy poleward (cold parcels move equatorward and warm parcels poleward)

  5. Central idea • This brings a key distinction between basins (~ocean) and channel (~atmosphere)’s geometry: Basins: walls provide dP/dx and a large scale (eddy free) geostrophic heat transport is possible. Channels: no zonally integrated dP/dx and the heat transport must involve eddies and / or ageostrophic effects (e.g., Hadley cell).

  6. Outline • The energy constraint • Basin dynamics • Channel dynamics

  7. The energy constraint

  8. The energy constraint Geometry: more energy impinging at low than high latitudes

  9. ASR IR Assume infra-red radiation and albedo is uniform Observations Stone, 1978.

  10. The energy constraint

  11. The energy constraint Poleward motion in ocean & atmosphere

  12. Basin: Northern Oceans, Atmosphere • Background • Geostrophic mass transport calculation • Heat transport • Complications…

  13. A classic: oxygen distribution at 2500m (from Wüst, 1935).

  14. A classic: oxygen distribution at 2500m (from Wüst, 1935). -Spreading from high latitude North Atlantic source region -Large spatial scale of `tongue’ considering the narrowness of ocean currents

  15. More recent section along the `great tongue’

  16. The “great oceanic conveyor belt”

  17. The “great oceanic conveyor belt”

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

  19. Atlantic ocean’s meridional overturning streamfunction NB: From an OGCM constrained by data (Wunsch, 2000)

  20. Can we measure the ocean circulation in basins using the Geostrophic calculation? • All you need is the thermal wind: Coriolis parameter East-west density gradient North-South velocity Gradient with height

  21. Global “inverse” ocean circulatioin and heat transport Ganachaud and Wunsch, 2003

  22. RAPID – WATCH array at 26N

  23. RAPID array calculation

  24. RAPID array calculation

  25. Blackboard calculations…

  26. Heat Transport Up Warm water North Cold water 26N East

  27. Heat Transport Up Mo≈ 20 Sv & Δθ≈10K yields Ho≈1PW as required Warm water North Cold water 26N East

  28. Are there basins in the atmosphere? Z Density profile H~7km X OCEAN ATMOSPHERE

  29. Different situation in the Tropics Trade wind inversion 2-3km … “isolated” low level layer

  30. East-African Highlands & the Indian Monsoon Orography Northward flow across the equator

  31. Low level winds climatology (June-August) ERA40 Atlas

  32. Channel: Atmosphere, Southern Ocean • Hadley cell • Oceanic & atmospheric eddies How to satisfy the energy constraint In a geometry in which <dP/dx> = 0?

  33. Zonally averaged atmospheric circulation (annual mean) ~100Sv NB: Ocean: ~10-20Sv

  34. Zonally symmetric motions are the key energy carriers in the Tropics Total Transient eddies Stationnary eddies Axisymmetric motions

  35. Zonally averaged atmospheric circulation (annual mean) Ω Eq df/dy max at equator Frictional effects dominate

  36. Zonally averaged atmospheric circulation (annual mean) Inertial effects dominate

  37. Critical (moist) temperature distributions leading to the onset of Hadley cell Emanuel (1995)

  38. Poleward heat transport in Hadley cell –see Q3 High gz Low gz

  39. Eumetsat/MetOffice infrared picture (daily composite)

  40. Eddy motions are the key energy carriers in midlatitudes Total Transient eddies Stationnary eddies Axisymmetric motions

  41. Ocean eddies: the Movie

  42. Ocean eddy heat transport from a ¼ º ocean GCM Total heat transport Eddy heat transport From Jayne & Marotzke (2002)

  43. “Shallow” Ocean (heat trspt ≠0) “Deep” Ocean (heat trspt=0) P T Height V Longitude

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