1. Mixing & Turbulence Mixing leads to a homogenization of water mass properties Mixing occurs on all scales in ocean –molecular scales (10’s of  m) –basin scales (1000’s of km) Turbulence interactions cascade energy from big to small scales

2. Mixing & Turbulence 10 cm eddies Small-scale turbulence Shear-driven

3. Mixing & Turbulence 200 km eddies Mesoscale Geostrophic

4. Mixing & Turbulence 4 km eddies Submesoscale ???

5. Mixing & Turbulence Stirring efficiently mixes property fields All scales of motion contribute Nearly all scales interact Think of a kid’s playroom full of colored balls in neat piles (before the kids come) Works if no restoring forces are present

6. Stirring vs. Mixing A single patch will be chaotically strained by flow Reversible

7. A Stirring Example? Advection of a blob of passive tracer Seeded eddy model [Dyke & Robertson, 1985] Collection of Gaussian vortices that advect themselves & tracer distribution Used in Abraham [1998] Nature, 391, 577

9. Different Initial Seeds Different Squiggly Patterns

10. Repeating many times …

11. Averaging over space … … it looks like “diffusion”

12. Stirring vs. Eddy Diffusion Snapshot Ensemble of Many Snapshots

13. Stirring vs. Eddy Diffusion Snapshots & synoptic surveys –Evidence of eddying motions, straining, etc. Averaging smoothes the edges –The average result of stirring can be modeled as a diffusive process –Diffusion is now acting on larger (or eddy) scales

14. Stirring vs. Mixing If this experiment were repeated many times, the average patch will appear to be “diffused” Concept of eddy diffusion Eddy diffusion is >> molecular diffusion

15. Buoyancy Dense water sinks - light water floats –Density profile will increase with depth –Upward force due to  ’s in  is called the buoyancy force Buoyancy restricts vertical mixing of water masses

16. Buoyancy & Mixing Buoyancy is important to vertical mixing for two major processes –Asymmetric mixing in ocean interior –Convection

17. Example Water mass 1 T=20 & S=36 Water mass 2 T=22 & S=35  1 >>  2 Water mass 2 is buoyant relative to 1 1 2

18. Example Water mass 1 T=20 & S=36 Water mass 2 T=17 & S=35  1 ~  2 No net buoyancy 1 2

19. Buoyancy Waters of same  mix easily, waters of different don’t (oil & vinegar) Potential energy differences must be overcome by mechanical energy inputs Mixing along isopycnal surfaces will be >>> than mixing across them

20. Purposeful Tracer Release SF 6 release in Brazil Basin Column inventory (  mol m -2 ) Top 14 mo Bottom26 mo

21. Purposeful Tracer Release SF 6 release in Brazil Basin Spatial average Dashed - initial Solid - 1 yr later In one year, the SF 6 plume has spread 400 m in vertical & 400 km in horizontal

22. Purposeful Tracer Release SF 6 in color Isopycnal surfaces in white lines Top 14 mo Bottom26 mo

23. Stratified Mixing Waters mix rapidly along isopycnals & slowly across them –Vertical eddy diffusivity ~10 -5 - 10 -3 m 2 /s –Horizontal eddy diffusivity ~10 2 - 10 4 m 2 /s Gives rise to constancy in properties following isopycnals

24. Atlantic Temperature eWOCE gallery – www.ewoce.org

25. Atlantic Salinity

26. Atlantic Oxygen

27. Atlantic Phosphate

28. Convection Air-sea cooling & evaporation creates cool & saline surface waters These waters are then denser than those just beneath them and they sink Process is called convection Annual & diurnal time scales

29. Seasonal Temperature Change OWS Papa 50N 145W

30. Seasonal Temperature Change OWS Papa 50N 145W

31. Convection & the Conveyor Belt NADW production drives the conveyor

32. Convection & the Conveyor Belt AABW NADW AAIW

33. Role of Sea Ice Formation of seasonal sea ice also is important Sea ice salinities are ~2 to 5 psu Reject brine water when formed –which is salty, cold and dense!! Source of AABW

34. Seasonal Sea Ice

35. Mixing, Buoyancy, etc. Turbulence drives mixing in the sea –Flow variations on many scales Buoyancy is important –Drives convection –Asymmetric mixing within the interior Active area of research