1. Geothermal heating : the unsung 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 Solid Earth cooling in the abyss

3. The spatial structure

4. Introduction “Q geo ~ 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 1.Analytical balance 2.Density-binning 3.Numerical approach Geothermal Heating is a Driving force of the MOC

6. Heat Equation 2 ways of comparing : 1.Plot downward heat flux 2.“Equivalent K z ” Bryan, 1987 : MOC is controlled by the heat supplied to the abyss How big is geothermal heating in the heat budget ? DiffusionGeothermal Heatflow Measured K z : ~0.1 cm 2.s -1 Implied K z : ~1 cm 2.s -1 (advection-diffusion balance) Munk, 1966

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

8. A simple scaling law

9. Results BasinAtlanticIndianPacificGlobal Area (10 14 m 2 ) 0.341.900.50.450.960.80.990.425.21.78-6.5  T ( C) Scaling(Sv) Geothermal circulation is commensurable to the Stommel-Arons circulation

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

11. Results : A Q F Transformation of ~6.5 Sv Centered on  = 45.90 Transformation of ~6 Sv Shifted towards  = 45.85 Uniform Heatflow Realistic Heatflow

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 K z =0.1cm 2.s -1 Cold bottom water K z =0.1 K z =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 Q geo ~ K z = 1.2 cm 2.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

19. 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%)

20. Conclusion “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 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 ?)

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

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

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

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

25. Effect of a spatially variable heatflow

26. Impact on the circulation

27. Impact on the thermal structure

28. Three views of the problem 1.Geothermal Heating as a source of mixing Gordon and Gerard (1970) Huang (1999) 2.Localized hydrothermal venting Stommel (1983) Helfrich and Speer (1995) 3.The new wave Adcroft et al (2001), Scott et al (2001) This study

29. Three sets of experiments Set Experiments Q geo (mW.m -2 ) K z (cm 2.s -1 ) CBW CBWCBW_Q_uni0 86.40.10.1 STD STDSTD_Q_uniSTD_Q_var086.4 Q geo (x,y) 0.10.10.1 MIX MIXMIX_Q_var0 1 (Hadley)