Impact of Geothermal Heating on the General Circulation

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Impact of Geothermal Heating on the General
Circulation
Alistair Adcroft
Jeffery Scott
MIT
MIT
Jochem Marotzke
SOC
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Geothermal heating in context

• Solar radiation 175,000 TW
• Eq. to Pole 3,000 TW
• Geothermal heat flux 32 TW
• Tidal dissipation 3.7 TW

Sandströms Theorem A closed steady circulation
can be maintained in the ocean only if the
heating source is at a lower level than the
cooling source.
Geothermal heating is at depth
Munk Wunsch 98, Huang 99
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Geothermal / Hydrothermal
Stommel 82, Joyce Speer 89, Helfrich Speer
95, . . .
Hydrothermal
34 Low temp 10-20 ºC 1 High temp 300-400 ºC
Geothermal 35-50 mW m-2
10TW
22TW
Sclater et al., 1980 Stein et al., 1995
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Vorticity balance
Joyce et al., 86
? ? v dz f w(z1) - Fb(zb)/?)
which is negligible
Also argue a heating rate
0.04 ºC / 100 yr for a 1000 m column
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Diffusive limit
Vertical diffusion of ? anomaly
cp?? ?z ? Qgeo
using Qgeo 50 mW m-2 ? 5 x
10-5 m2 s-1
Levitus
?? 0-H H ?z ? 1.25 ºC
diffusive anomaly
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Sector model

• Simple forcing
• No wind
• SST restoring

Qsurf - ? ( ? - a cos(y) )
z
4500 m
64 ºN
y
x
Eq
60 º
Qgeo 50 mW m-2
Scott et al. 00
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Global model

• MIT OGCM
• Realistic topography (no Artic!)
• Seasonal forcing
• Obs. Heat, Freshwater Fluxes, Wind stress
• Surface restoring to Levitus
• Gent/McWilliams eddy parameterization
• Convective adjustment
• Uniform goethermal heat flux of 50 mW m-2

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Summary

• 50 mW m-2 geothermal heat flux
• ? 2 Sv perturbation OC in Pacific
• Advective rather than diffusive response
• Response intimately linked with existing
  circulation and stratification
• Uniform geothermal heating with no other
• forcing does not drive a large scale flow
• . . . probably smaller than model errors!