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Geothermal heating : theunsung diva of abyssal dynamics
Geothermal circulation is commensurable to the Stommel-Arons circulation
Density-binning the abyssal ocean
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F(σ ) = A(σ ) +∂D
∂σ
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M(σ ) =∂ 2D
∂σ 2−
∂F
∂σ= Ψc (Steady-state)
Transformation equation :
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B=-α 4
Cp
Qgeo(x, y)
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A(σ ) = u • ndSSσ
∫€
F(σ ) = B(x, y) δ ′ σ (x, y) −σ( )∫∫ dxdy
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D(σ ) = κ∂σ
∂nSσ∫ dS
Formation equation :
Geothermal Circulation
Results :Results :
A
Q
F
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F(σ)≈A(σ)Q(σ)
•Transformation of ~6.5 Sv•Centered on = 45.90
•Transformation of ~6 Sv•Shifted towards = 45.85
Uniform Heatflow
Realistic Heatflow
A numerical approachA numerical approachOPA model v8.1 (Madec et al, 1998):•Primitive equation model, non-linear equation of state •Horizontal physics : Isopycnal mixing with Gent & McWilliams•Conservation of haline content (Roullet and Madec 2000)
Equilibrium runs from Levitus (1998) forced by climatological fluxes•Geothermal Heat flux passed like a surface flux
Control runsControl runs
Kz=0.1cm2.s-1
Cold bottomwater
Kz=0.1
Kz=1Hadley center
Effect of a uniform heatflow(CBW)Effect of a uniform heatflow(CBW)
Effect of a uniform heatflow (STD)Effect of a uniform heatflow (STD)
Transformation (Sv)
Effect of vertical physicsEffect of vertical physics
ConclusionsConclusions
• Qgeo ~ Kz = 1.2 cm2.s-1 (at 3500m)
•Three independent approaches predict a circulation of 5-6 Sv, inversely proportional to deep temperature gradients(modulated by mixing)
•Changes the thermal structure to first order (cf Scott et al.), in particular the meridional temperature gradient
Geothermal Heating is a major AABW consumer
Major forcing of the abyssal circulation
Summary (continued)Summary (continued)
•Details of the spatial structure are secondary :
Circulation is weakened by ~ 20% (STD)
Warming enhanced in the NADW depth range weakened on abyssal plains
(by ~10-20%)
ConclusionConclusion
“Viewed as a heat engine, the ocean circulation is extraordinarily inefficient. Viewed as a mechanically-drivensystem, it is a remarkably effective transporter of the energy”
Walter Munk and Carl Wunsch, 1998
Geothermal Heating is a major actor of abyssal dynamics
• Influences mostly PE, not KE • Provides 1/3 of APE for deep mixing• May help resolve the “diffusivity dilemna”• Does it have a role in climate change ? (Little Ice Age ? Glacial THC ?)
Geothermal Heating vs Diapycnal mixing (1)
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Kz∂zTLevitusDownward Heat Flux =
What happens to the What happens to the Sverdrup balance ?Sverdrup balance ?