Geothermal heating : the unsung diva of abyssal dynamics

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Geothermal heating theunsung diva of abyssal
dynamics

• Julien Emile-Geay
• Lamont-Doherty Earth Observatory, Palisades, NY,
  USA
• Gurvan Madec
• LODYC, Paris, France

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Solid Earth cooling in the abyss
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The spatial structure
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Introduction
Why is geothermal heating generally neglected in
dynamical oceanography ? (except by Scott,
Adcroft and Marotzke, JGR, 2001)
AABW
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Outline

• Analytical balance
• Density-binning
• Numerical approach

Geothermal Heating is a Driving force of the MOC
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Heat Equation
Bryan, 1987 MOC is controlled by the heat
supplied to the abyss
How big is geothermal heating in the heat budget ?
Diffusion
Geothermal Heatflow
Measured Kz 0.1 cm2.s-1 Implied Kz 1
cm2.s-1 (advection-diffusion balance) Munk,
1966

• 2 ways of comparing
• Plot downward heat flux
• Equivalent Kz

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Geothermal Heating vs Diapycnal Mixing (2)
(z-3500m)
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A simple scaling law
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Results
Geothermal circulation is commensurable to the
Stommel-Arons circulation
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Density-binning the abyssal ocean
Geothermal Circulation
Transformation equation
Formation equation
(Steady-state)
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Results
F
Uniform Heatflow
A

• Transformation of 6.5 Sv
• Centered on ? 45.90

Realistic Heatflow
Q

• Transformation of 6 Sv
• Shifted towards ? 45.85

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A numerical approach

• OPA 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)
• ORCA2 configuration
• ?x?y2 0.5(Tropics) 2 - 31 vertical
  levels ( 15 in upper 200m)
• Coupled to LIM (LLN sea-ice model)
• Equilibrium runs from Levitus (1998) forced by
  climatological fluxes
• Geothermal Heat flux passed like a surface flux

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Control runs
Kz0.1cm2.s-1 Cold bottom water
Kz0.1
Kz1 Hadley center
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Effect of a uniform heatflow(CBW)
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Effect of a uniform heatflow (STD)
Transformation (Sv)
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Effect of vertical physics
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Conclusions

• 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

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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)

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Conclusion
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 ?)

Viewed as a heat engine, the ocean circulation
is extraordinarily inefficient. Viewed as a
mechanically-driven system, it is a remarkably
effective transporter of the energy Walter Munk
and Carl Wunsch, 1998
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Geothermal Heating vs Diapycnal mixing (1)
Downward Heat Flux
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What happens to the Sverdrup balance ?

• If , then
  (Sverdrup balance)
• Now , then
• Integrating

(Joyce et al. 1986)
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Life cycle of AABW
Formation
Deep convection, cabelling
Transformation
Entrainment, Downhill mixing,
Consumption
Diapycnal mixing Upwelling (NADW) Getohermal
Heating
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Density-binning the abyssal ocean
Transformation equation
(Steady-state)
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Effect of a spatially variable heatflow
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Impact on the circulation
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Impact on the thermal structure
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Three views of the problem

• Geothermal Heating as a source of mixing
• Gordon and Gerard (1970)
• Huang (1999)
• Localized hydrothermal venting
• Stommel (1983)
• Helfrich and Speer (1995)
• The new wave
• Adcroft et al (2001), Scott et al (2001)
• This study

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Three sets of experiments