rotating fluid -part ii a “gfd view” of the ocean and the atmosphere (a follow up raymond’s...
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Rotating Fluid -Part II
A “GFD view” of the Ocean and the Atmosphere
(a follow up Raymond’s Lectures)
Arnaud Czaja
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Source / sink flows –see Raymond’s lectures
“Basin”
“Channel”
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Source / sink flows –see Raymond’s lectures
“Basin”
“Channel”No distinction betweenOcean & Atmosphere…
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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)
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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).
x
Pfv
o
1
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Outline
• The energy constraint
• Basin dynamics
• Channel dynamics
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The energy constraint
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The energy constraint
Geometry: more energyimpinging at low than high
latitudes
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Stone, 1978.
Assume infra-red radiation and albedois uniform
Observations
ASR IR
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The energy constraint
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The energy constraint
Poleward motionin ocean & atmosphere
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Basin: Northern Oceans, Atmosphere
• Background
• Geostrophic mass transport calculation
• Heat transport
• Complications…
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A classic:
oxygen distribution at 2500m
(from Wüst, 1935).
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A classic:
oxygen distribution at 2500m
(from Wüst, 1935).
-Spreading from high latitude North Atlantic source region
-Large spatial scale of `tongue’ consideringthe narrowness of ocean currents
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More recent sectionalong the `great tongue’
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The “great oceanic conveyor belt”
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The “great oceanic conveyor belt”
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Broecker, 2005NB: 1 Amazon River ≈ 0.2 Million m3/s
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Sv2010max
Atlantic ocean’s meridional overturning streamfunction
NB: From an OGCMconstrained by data(Wunsch, 2000)
136101 smSv
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Can we measure the ocean circulation in basins using the
Geostrophic calculation?
• All you need is the thermal wind:
x
g
z
vf
o
Coriolis parameter
North-South velocityGradient with height
East-westdensity gradient
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Global “inverse” ocean circulatioin and heat transport
Ganachaud and Wunsch, 2003
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RAPID – WATCH array at 26N
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RAPID array calculation
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RAPID array calculation
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Blackboard calculations…
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Heat Transport
26N
Warm water
Cold water
East
North
Up opopoo McdxdzvcH
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Heat Transport
26N
Warm water
Cold water
East
North
Up opopoo McdxdzvcH
Mo ≈ 20 Sv & Δθ≈10Kyields Ho≈1PW as required
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Are there basins in the atmosphere?
Z
Density profileH~7km
OCEAN ATMOSPHEREX
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Trade wind inversion
Different situation in the Tropics
2-3km
… “isolated” low level layer
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Orography
Northward flow across the equator
East-African Highlands & the Indian Monsoon
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Low level winds climatology (June-August)
ERA40 Atlas
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Channel: Atmosphere, Southern Ocean
• Hadley cell
• Oceanic & atmospheric eddies
How to satisfy the energy constraintIn a geometry in which <dP/dx> = 0?
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Zonally averaged atmospheric circulation (annual mean)
~100Sv
NB: Ocean: ~10-20Sv
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Zonally symmetric
motions are the key energy
carriers in the Tropics
Total
Transient eddies
Stationnary eddies
Axisymmetricmotions
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Zonally averaged atmospheric circulation (annual mean)
Frictionaleffects dominate
Ω
Eq
df/dy max at equator
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Zonally averaged atmospheric circulation (annual mean)
Inertialeffects dominate
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Critical (moist)temperaturedistributions leading to the onset of Hadley cell
Emanuel (1995)
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Poleward heat transport in Hadley cell –see Q3
High gz
Low gz
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Eumetsat/MetOffice infrared picture (daily composite)
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Eddy motions are
the key energy
carriers in midlatitudes
Total
Transient eddies
Stationnary eddies
Axisymmetricmotions
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Ocean eddies: the Movie
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Ocean eddy heat transport from a ¼ º ocean GCM
From Jayne & Marotzke (2002)
Eddyheat transport
Total heat transport
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“Shallow” Ocean (heat trspt ≠0)
“Deep” Ocean (heat trspt=0)
P
T
VLongitude
Height
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