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Eddies and Ocean Biogeochemistry

Eddies and Ocean Biogeochemistry. Andreas Oschlies IFM-GEOMAR. The Biological Pump: Traditional 1D View. CO 2 , O 2. z. Sea surface. z(mix)~z(euph). inorganic nutrients. organic matter. nutrients,  CO 2. ``relatively constant´´ C:N:P:-O 2. (2). particulate and dissolved

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Eddies and Ocean Biogeochemistry

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  1. Eddies and Ocean Biogeochemistry Andreas Oschlies IFM-GEOMAR

  2. The Biological Pump: Traditional 1D View CO2, O2 z Sea surface z(mix)~z(euph) inorganic nutrients organic matter nutrients, CO2 ``relatively constant´´ C:N:P:-O2

  3. (2) particulate and dissolved organic matter Biological Pump in 3D CO2, O2 low lats high lats Z(euphot. zone) (1) Z(winter mixed layer)

  4. (3a) newly-remineralised dissolved inorganic matter (2) particulate and dissolved organic matter Biological Pump in 3D CO2, O2 low lats high lats Z(euphot. zone) (1) Z(winter mixed layer)

  5. (3b) newly-generated inorganic matter deficit (3a) newly-remineralised dissolved inorganic matter (2) particulate and dissolved organic matter Biological Pump in 3D CO2, O2 low lats high lats Z(euphot. zone) (1) Z(winter mixed layer) (Oschlies & Kähler, 2004)

  6. Potential of the biological pump Mean profile Present-day sea-surface nitrate concentrations mmol/m3 (Conkright et al., 1994) Controls are not fully understood

  7. Potential of the biological pump Mean profile Present-day sea-surface nitrate concentrations mmol/m3 Subtropical deserts (Conkright et al., 1994) Controls are not fully understood

  8. Surface Chlorophyll Biogeographical Provinces

  9. Nitrate distribution • Subtropical nitrate “bowl” • Observed biological production requires net supply across NO3=const surfaces. • Gauss’s theorem: mean advection cannot contribute to transport across mean iso-surface. • diapycnal mixing • Ekman transport anomalies • eddy stirring (isopycnal & dia-nutrial)

  10. Where have eddies come into play? Apparent observational discrepancy in oligotrophic subtropical gyres: • Large-scale biogeochemical estimates of export production >> local direct measurements • Still not fully resolved despite several decades of research

  11. Subtropical desert conundrums Simulated NO3 supply to euphotic zone Eastern basin: ~factor 12 • high rates of O2 consumption, (Jenkins, 1982: ~0.6 mol N m-2 yr-1)

  12. Subtropical desert conundrums Simulated NO3 supply to euphotic zone Eastern basin: ~factor 12 • high rates of O2 consumption, (Jenkins, 1982: ~0.6 mol N m-2 yr-1) • low15NO3 uptake, diff. NO3 supply (Lewis et al., 1986: ~0.05 mol N m-2 yr-1)

  13. Subtropical desert conundrums Simulated NO3 supply to euphotic zone Eastern basin: ~factor 12 • high rates of O2 consumption, (Jenkins, 1982: ~0.6 mol N m-2 yr-1) • low15NO3 uptake, diff. NO3 supply (Lewis et al., 1986: ~0.05 mol N m-2 yr-1) • low simulated NO3 supply (Oschlies, 2002: ~0.02 mol N m-2 yr-1)

  14. Subtropical desert conundrums Simulated NO3 supply to euphotic zone Eastern basin: ~factor 12 • high rates of O2 consumption, (Jenkins, 1982: ~0.6 mol N m-2 yr-1) • low15NO3 uptake, diff. NO3 supply (Lewis et al., 1986: ~0.05 mol N m-2 yr-1) • low simulated NO3 supply (Oschlies, 2002: ~0.02 mol N m-2 yr-1) Bermuda: ~ factor 4 • high rates of 3He supply (Jenkins, 1988 : ~0.6 mol N m-2 yr-1) • lower sedimentation rates (Michaels et al.,1994:~0.15 mol N m-2 yr-1)

  15. Subtropical desert conundrums Simulated NO3 supply to euphotic zone Eastern basin: ~factor 12 • high rates of O2 consumption, (Jenkins, 1982: ~0.6 mol N m-2 yr-1) • low15NO3 uptake, diff. NO3 supply (Lewis et al., 1986: ~0.05 mol N m-2 yr-1) • low simulated NO3 supply (Oschlies, 2002: ~0.02 mol N m-2 yr-1) Bermuda: ~ factor 4 • high rates of 3He supply (Jenkins, 1988 : ~0.6 mol N m-2 yr-1) • lower sedimentation rates (Michaels et al.,1994:~0.15 mol N m-2 yr-1) • Substantial interannual variability (Lipschultz, 2001; Oschlies, 2001)

  16. Subtropical desert conundrums Simulated NO3 supply to euphotic zone Eastern basin: ~factor 12 • high rates of O2 consumption, (Jenkins, 1982: ~0.6 mol N m-2 yr-1) • low15NO3 uptake, diff. NO3 supply (Lewis et al., 1986: ~0.05 mol N m-2 yr-1) • low simulated NO3 supply (Oschlies, 2002: ~0.02 mol N m-2 yr-1) Bermuda: ~ factor 4 • high rates of 3He supply (Jenkins, 1988 : ~0.6 mol N m-2 yr-1) • lower sedimentation rates (Michaels et al.,1994:~0.15 mol N m-2 yr-1) • Substantial interannual variability (Lipschultz, 2001; Oschlies, 2001) Discrepancy based on different tracers! Role of conversion factors?

  17. Where have eddies come into play? • Large-scale biogeochemical estimates of export production >> local direct measurements • Trace metal contamination, sediment trap problems,… => underestimated local production rates?

  18. Where have eddies come into play? • Large-scale biogeochemical estimates of export production >> local direct measurements • Trace metal contamination, sediment trap problems,… => underestimated local production rates? • Unintended tradition of undersampling!  under-representation of episodic eddy events?

  19. Evidence for episodic nutrient supply Section Azores – Cape Farewell (Strass, 1992)

  20. Evidence for episodic nutrient supply by eddies (McNeil et al., 1999) Bermuda Testbed Mooring Time series 4 months, eddy time scale 15 days.

  21. Eddy pumping concept (vertical one) (McGillicuddy et al., 1998)

  22. On the relevance of eddy pumping • “vertical flux of nutrients induced by the dynamics of mesoscale eddies is sufficient to balance the nutrient budget” • “Eddy pumping and wintertime convection are the two dominant mechanisms transporting new nutrients into the euphotic zone” • Nutrient flux by eddy pumping “is more than an order of magnitude higher than the diapycnal diffusive flux as well as … vertical transport due to isopycnal mixing”.

  23. First counterargument: Statistics • Should we have missed the important events? Undersampling of episodic events  under-representation of episodic events? • chance of under-representation = change of over-representation

  24. Undersampling = under-representation? T, z = 0m Hawaii Ocean Timeseries Site (Karl et al., 2003) T, z = 200m

  25. Undersampling = under-representation? SLA >3 years BATS vs Topex-Poseidon (Siegel et al., 1999)

  26. Counter argument 2: high estimates based on models • Falkowski et al. (1991), eddy off Hawaii: infer ~< 20% enhancement of large-scale primary production by eddies. • based on direct fluorescence measurements of primary production

  27. Counter argument 2: high estimates based on models • Falkowski et al. (1991), eddy off Hawaii: infer ~< 20% enhancement of large-scale primary production by eddies. • based on direct fluorescence measurements of primary production • McGillicuddy et al. (1998), eddy off Bermuda: infer ~100% enhancement of large-scale nutrient supply. • based on nitrate-density relationship and inconsistent model assumptions

  28. Altimetry-based eddy-pumping estimate SLA Estimated nitrate supply zeuph (Siegel et al., 1999)

  29. Altimetry-based eddy-pumping estimate Based on mutually exclusive assumptions: • All eddy events contribute to local nitrate flux (wave-like eddies) • 100% of upwelled nutrient is taken up locally (slow growth at base of euphotic zone  water must be trapped in moving eddy) • Plausible efficiency more likely 20-25% (Martin & Pondaven, 2003)

  30. Model-based assessment Spring bloom in eddy-resolving model (1/9x2/15 degrees) ecosystem model, (Oschlies & Garcon, 1999) (Oschlies, 2002)

  31. Model statistics at BATS rms vert. displ. of iso-NO3 surfaces Corr2(r,NO3)) mean NO3 Corr2(SSH,Z(NO3)) BATS (1/9)o model ~ correct amplitude for lifting of iso-NO3 surfaces

  32. Model-based assessment at BATS SSH mg Chl/m3 Chlorophyll Siegel et al. (1999) method would predict 6 times too much NO3 supply associated with eddy event (1). mmol NO3/m3 Nitrate 0.05mol/m2 after spring (Lipschultz (2001): 0.07 in 1992, 0.04 in 1993) cumulative NO3 supply (Oschlies, 2002)

  33. Counter argument 3: Recharging issues Eddy-pumping process . recharging time

  34. Counter argument 3: Recharging issues Eddy-pumping process • Sinking is diapycnal transport . recharging time

  35. Counter argument 3: Recharging issues Eddy-pumping process • Sinking is diapycnal transport • Recharging of nutrients on shallow isopycnals matters. . recharging time

  36. Counter argument 3: Recharging issues Eddy-pumping process • Sinking is diapycnal transport • Recharging of nutrients on shallow isopycnals matters. . • Recharging requires diapycnal nutrient transport (local or remote). recharging time

  37. Counter argument 3: Recharging issues Eddy-pumping process • Sinking is diapycnal transport • Recharging of nutrients on shallow isopycnals matters. . • Recharging requires diapycnal nutrient transport (local or remote). • Bottleneck is diapycnal transport rather than isopycnal uplift! recharging time (Oschlies, 2002)

  38. What about eddy-resolving models? • Idealised models (frontal dynamics)

  39. Idealised models Large local impacts > 100% increase in regional production. Often run in spin-up or spin-down mode. Representative of steady-state large-scale mean? (Levy et al., 2001)

  40. “We can never do merely one thing”(Hardin, 1985) New production increases with finer and finer resolution. No convergence seen, yet. SST New Production (Mahadevan & Archer, 2000)

  41. “We can never do merely one thing”(Hardin, 1985) 120 day mean higher NO3 supply lower SST 0.36oC cooling over 120/2=60 days BATS: Hawaii for MLD = 50m: 14 W/m2 18 W/m2 for MLD = 100m: 28 W/m2 36 W/m2 Heat transport constraint on nutrient transport? (Mahadevan & Archer, 2000)

  42. Basin-scale models (i) permitting Spring bloom in eddy-resolving model (1/9x2/15 degrees) ecosystem model, (Oschlies & Garcon, 1999) (Oschlies, 2002)

  43. Surface heat “flux correction” 1/9o model, forced by ECMWF 1989-93 ERA About 25W/m2 additional heating required to reproduce observed SSTs, (little less at higher resolution: ML restratification by eddies) 1/3o model, forced by ECMWF 1989-93 ERA (Oschlies, 2002)

  44. z light warm dense cold y (Nurser & Zhang, 2000) Eddy-induced stratification Baroclinic instabilities in ML generate stratification Heat flux required to balance eddy-induced stratification of ML. Eddies stratify and heat the surface ML in most areas. (Oschlies, 2002)

  45. Sub-mesoscale heatflux NH winter Inferred from altimetry: Positive everywhere! SH winter (Fox-Kemper & Ferrari, 2008)

  46. Simulated North Atlantic spring bloom Surface chlorophyll (mg/m3) 1/3 x 2/5 degrees 1/9 x 2/15 degrees Eddy resolving looks “better”. Is there a significant net impact? (Oschlies, 2002)

  47. Simulated annual NO3 supply into upper 126m (mol m-2 yr-1) Small difference in oligotrophic subtropical gyre. Some difference at gyre’s margins. eddy permitting viscous eddy resolving (Oschlies, 2002)

  48. Eddy impacts on mean nutrient supply total supply by eddies Eddy supply by vertical excursions (includes “eddy pumping”) Eddy supply by lateral stirring (exceeds vertical eddy contribution over large parts of subtropical gyre!) (Oschlies, 2002)

  49. Role of lateral stirring • Supply by lateral stirring might reach larger distances for organic nutrients with longer lifetimes/slower utilisation rates (Lee & Williams, 2000) t=3 months t=1 year

  50. Basin-scale models (ii) McGillicuddy et al. (2003): • Nutrient transport model • z = 0-104m: N uptake rate = m min(QN,L) • z > 104m: Remineralisation: 1/t [ NO3obs(x,y,s0) – NO3] • Sensitivity experiments: t = 10, 30, 60 days (results only shown for t = 10 days, though) NO3 s0 (McGillicuddy et al., 2003)

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