Lateral mixing and subduction

1. Lateral mixing and subduction Jean-Baptiste Sallée Clivar meeting, Sydney, February 2009 K.Speer, R.Morrow, S.Rintoul, R.Lumpkin, S.Wijffels The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

2. Introduction 1- How much and where surface water is exported into Subantarctic Mode Water (AAIW) and Antarctic Intermediate Water (AAIW) layers? 2- What are the mechanisms? The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

3. Introduction Relevant to climate signal Wong et al., 1999 Strongest climate signal in the mode and intermediate waters The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

4. Outlines • Subduction calculation • Lateralmixing • Circumpolar average • Regional Variability The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

5. Subduction calculation (ii) (iii) (iv) (i) Volume budget • Subduction = rate by which ventilated water enters the ocean interior • Subduction= Divergence of horizontal transport • = Divergence ( Ekman flow, Mean geostrophic flow, Eddy flow) Geostrophic flow and eddy induced flow Are very different within or below the ML H(t) Need to resolve the ML seasonal cycle! ARGO Hwinter ? interior The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

6. Subduction calculation Mixed layer • Argo => seasonal cycle + good coverage 1) Annual mean 2) Mode 1 (90%): seasonal cycle The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

7. Subduction calculation (ii) (iii) (iv) (i) Volume budget • Subduction= Divergence of horizontal transport • = Divergence ( Ekman flow, Mean geostrophic flow, Eddy flow) ARGO Geostrophic flow and eddy induced flow Are very different within or below the ML Quickscat H(t) Need to resolve the ML seasonal cycle! ARGO Hwinter ? interior The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

8. Subduction calculation Eddy Flow : 2-D residual framework Karsten, Marshall et al. Ek= /f eddy = K by/bz = K.s res = mean+ eddy B = B/byB = ML diabatics PF SAF 20 Sv AAIW layer transport Salinity The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

9. Subduction calculation Argo Eddy Flow • Subduction= Divergence ( Ekman flow, Mean geostrophic flow, Eddy flow) Gent&McWilliams, 1990 Need to know about the lateral mixing ! H(t) Hwinter interior The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

10. Outlines • Subductioncalculation • Lateral mixing • Circumpolar average • Regional Variability The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

11. II. Lateral mixing Effective diffusivity background Average over particles A lot of theoritical studies. Applications require a lot of particles. The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

12. II. Lateral mixing Map Computations with 10 years of surface drifter data. Diffusion • High diffusion in the Western boundary currents and when ACC interacts with the bathymetry. • High values compared to previous studies. The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

13. GDP - flotteurs réels II. Lateral mixing Average Altimeter drifters: ACC Same grid as real Real drifters Average along streamlines Real drifters without WBC PF SAF SAF-N Ekman South North The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

14. GDP - flotteurs réels II. Lateral mixing Issues (Marshall et al., 2006) Nakamura (96) approach (Sallée et al., 2008) ACC Real drifters ACC PF SAF SAF-N What are the differences between the Taylor and Nakamura estimates? What does experience a tracer in the ocean? Which estimate (if any) is more suitable to climate models? South North The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

15. II. Lateral mixing Application to mass transport • Subduction= Divergence ( Ekman flow, Mean geostrophic flow, Eddy flow) H(t) Seddy Hwinter interior Balance Ekman pumping: first order term! The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

16. Outlines • Subductioncalculation • Lateral mixing • Circumpolar average • Regional Variability The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

17. II. Circumpolar average Across isopycnal transport in the surface layer ACC Residual ACC South North AAIW SAMW The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

18. II. Circumpolar average Sensivity to diffusion choice? (i) Subduction ACC Residual SAMW AAIW Subduction of dense SAMW Upwelling of AAIW The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

19. II. Circumpolar average 3) Kappa sensitivity test Residuals High sensitivity on the transport Eddy flow Low sensitivity on the subduction The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

20. II. Circumpolar average (ii) Representative of regional regimes? Subduction ACC Residual SAMW AAIW No! Sensivity to diffusion choice? (i) Yes! Consistency with thermodynamics? Subduction of dense SAMW Upwelling of AAIW The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

21. Outlines • Subductioncalculation • Lateral mixing • Circumpolaraverage • Regional variability: The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

22. III. Regional Variability 1) Bathymetry constraint Campbell Plateau Fracture Zone Drake Kerguelen SAMW Subducted water exported away? Or: Stay below the ML and reabsorbed downstream? Dense SAMW AAIW The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

23. III. Regional Variability 1) SAMW layers: interior structure and circulation Low PV High PV Red spots = subduction hotspots Lines = circulation The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

24. Conclusions • Large local peaks of surface diffusion in WBC and bathymetry Eddy-induced contributionis a first order term • Large diabatic and adiabatic local impacts There is a strong regional variability of the subduction process. The circumpolar average picture hides strong spots of subduction Bathymetry is a constraint on the position of these hotspots. The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

25. Conclusions 1. Lateral mixing: What are the differences between the Taylor and Nakamura estimates? What does experience a tracer in the ocean? Which estimate (if any) is more suitable to climate models? 2. Horizontal transport and subduction: Does the GM90 represents the full complexity of mesoscale eddies? How smaller scale eddies contribute to subduction/ventilation? What is the time variability of this subduction structure; how eddies react to a wind intensification (SAM)? The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

26. Conclusions 3. Transition layer ?? Few 1000s m2.s-1 ??? Few 100s m2.s-1 The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

27. Outlines • Subductioncalculation • Lateral mixing • Circumpolaraverage • Regional variability: • Modewaterlayers: interior structure and circulation • 2) AAIW layers: interior structure and circulation The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

28. III. Regional Variability 2) AAIW layers: interior structure and circulation Low PV High PV Red spots = subduction hotspots Lines = circulation The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

29. Diffusion 4) Consistency with thermodynamics Winter climatological heat diffusion Important adiabatic impact (GM) but also important local diabatic impact! The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

30. II. Circumpolar average 4) Consistency with thermodynamics Buoyancy transport Eddy mixing JRA-25 NCEP-2 NOC-adjusted Buoyancy forcings Buoyancy transport Buoyancy forcings Comparison The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

31. Introduction The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

32. The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology