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Quantifying eddy transport and mixing in atmospheric and oceanic flows

Quantifying eddy transport and mixing in atmospheric and oceanic flows. Emily Shuckburgh DAMTP, Cambridge. Eddies in the Southern Ocean. Southern Ocean & Antarctic Circumpolar Current (ACC). Do eddies play order-one role in dynamics of ACC? Does the ACC act as a barrier to transport?

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Quantifying eddy transport and mixing in atmospheric and oceanic flows

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  1. Quantifying eddy transport and mixing in atmospheric and oceanic flows Emily Shuckburgh DAMTP, Cambridge

  2. Eddies in the Southern Ocean Southern Ocean & Antarctic Circumpolar Current (ACC) • Do eddies play order-one role in dynamics of ACC? • Does the ACC act as a barrier to transport? • Are there similarities with the eddy transport structure of the atmosphere? Start by quantifying eddy diffusivities…

  3. Complex tracer geometry: strong eddy diffusivity Simple tracer geometry: weak eddy diffusivity Coordinate transformation Eddy diffusivity quantification Nakamura 1996 Rhines & Young 1983

  4. Eddy diffusivity Strong eddy mixing, large K Diffusion enhanced by differential advection • stretching and folding produces small-scale structure in the tracer • consequence of eddies Weak eddy mixing, small K

  5. Passive tracer • Non-div surface velocity field derived from altimetry • Initialize tracer with streamlines (cross-stream diffusivities) • Calculate keff from tracer as it evolves in the flow 1/20th deg resolution

  6. Keff independent of numerical diffusivity k Small k Nu=Keff/k Large k Pe

  7. Cross-stream particle dispersion Pdf of particles Tracer with keff Shuckburgh & Haynes 2003

  8. k=50, 10 m2s-1

  9. Eddy diffusivity 2000 1500 Near surface keff 1000 500 Caution: average along streamlines

  10. Pacific patches -55 -50 -45 -40 Equivalent latitude

  11. Overturning circulation 2000 Eulerian-mean 1500 1000 Residual 500 Eddy Marshall et al 2005

  12. Tropopause Polar vortex ACC Jets and barriers to transport Gulf Stream

  13. Potential vorticity

  14. Jets and barriers - atmosphere Zonal-mean winds Haynes & Shuckburgh 2000 Shuckburgh et al 2001

  15. Jets and barriers - ocean Zonal-mean velocities Cerovecki and Plumb 2006

  16. Jets and barriers - kinematic flows

  17. No mean flow k=50, 10 m2s-1

  18. No mean flow -55 -50 -45 -40 Equivalent latitude

  19. Eddy activity in the Pacific Eddy kinetic energy

  20. Eddy activity in the Pacific Eddy kinetic energy

  21. 60 50 40 30 20 10 0 ocean Leq -60 -50 -40 -30 120 100 80 60 40 20 0 Leq stratosphere -60 -50 -40 -30 Differences: continents; scales; mixing & barrier strengths

  22. 60 50 40 30 20 10 0 -60 -50 -40 -30 k=5x10m2s-1 k=3x105m2s-1

  23. Summary • Effective diffusivity diagnostic • useful for quantifying eddy diffusivities in atmosphere and ocean • can be calculated from a range of data • robust diagnostic (i.e. largely independent of the calculation method) • Meridional overturning circulation in Southern Ocean • eddy-driven component nearly balances wind • Reduced diffusivities at core of ACC • jets often associated with barriers in atmosphere and ocean • much greater range diffusivities in atmosphere

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