1 / 29

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 ”.

cassie
Download Presentation

Geothermal heating : the unsung diva of abyssal dynamics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related