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Geothermal heating : the unsung diva of abyssal dynamics

Geothermal heating : the unsung diva of abyssal dynamics. Julien Emile-Geay Lamont-Doherty Earth Observatory, Palisades, NY, USA Gurvan Madec LODYC, Paris, France. Solid Earth cooling in the abyss. The spatial structure. Introduction. “Q geo ~ 100 mW.m -2 / Solar is ~100 W.m -2 ”.

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Geothermal heating : the unsung diva of abyssal dynamics

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  1. Geothermal heating : theunsung diva of abyssal dynamics Julien Emile-Geay Lamont-Doherty Earth Observatory, Palisades, NY, USA Gurvan Madec LODYC, Paris, France

  2. Solid Earth cooling in the abyss

  3. The spatial structure

  4. Introduction “Qgeo ~ 100 mW.m-2 / Solar is ~100 W.m-2” Why is geothermal heating generally neglected in dynamical oceanography ? (except by Scott, Adcroft and Marotzke, JGR, 2001) AABW

  5. Outline Analytical balance Density-binning Numerical approach Geothermal Heating is a Driving force of the MOC

  6. 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”

  7. Geothermal Heating vs Diapycnal Mixing (2) (z=-3500m)

  8. A simple scaling law

  9. Results Geothermal circulation is commensurable to the Stommel-Arons circulation

  10. Density-binning the abyssal ocean Geothermal Circulation Transformation equation : Formation equation : (Steady-state)

  11. 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

  12. 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*y=2 * [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

  13. Control runs Kz=0.1cm2.s-1 Cold bottom water Kz=0.1 Kz=1 Hadley center

  14. Effect of a uniform heatflow(CBW)

  15. Effect of a uniform heatflow (STD) Transformation (Sv)

  16. Effect of vertical physics

  17. 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

  18. 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%)

  19. 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

  20. Geothermal Heating vs Diapycnal mixing (1) Downward Heat Flux =

  21. What happens to the Sverdrup balance ? • If , then : (Sverdrup balance) • Now , then : • Integrating : (Joyce et al. [1986])

  22. Life cycle of AABW Formation Deep convection, cabelling Transformation Entrainment, Downhill mixing, Consumption Diapycnal mixing Upwelling (NADW) Getohermal Heating

  23. Density-binning the abyssal ocean Transformation equation : (Steady-state)

  24. Effect of a spatially variable heatflow

  25. Impact on the circulation

  26. Impact on the thermal structure

  27. 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

  28. Three sets of experiments

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