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Deep Ocean Circulation: Present, Past, and Future
- 1. Deep Ocean Circulation Present,Past, and Future Andreas Schmittner College of Earth, Ocean, and Atmospheric Sciences Oregon State University Portland State University Nov. 6, 2017
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- 6. Antarctic Bottom Water Antarctic Intermediate Water NorthAtlantic Deep Water Meridional Overturning Circulation (aka thermohaline circ. / great conveyor belt)
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- 10. Pacific Deep Water Circumpolar Deep Water NorthPacific Intermediate Water Pacific (150°W)
- 11. Global Deep WaterCirculation a.k.a.Thermohaline Circulation: driven by temperature and salinity (density) differences or Meridional (North-South) Overturning Circulation (MOC)
- 12. Why is thereno deep water formation in North Pacific?
- 13. Sea Surface SalinityObservations Rockies Andes Water vapor transport from Pacific to Atlantic blocked by mountains Water vapor transport from Atlantic to Pacific across Central America Atlantic = salty Pacific = fresh (g salt per kg water) E.Africa
- 14. Climate Models Equations basedon conservation of • mass • momentum (Navier-Stokes) • energy • water, salt • carbon • ... Interactive Components: • Atmosphere • Ocean • Sea Ice • Land Surface (vegetation, snow, soil moisture, runoff) Prescribed (fixed): • Land Ice Sheets
- 15. Flat World Real World Schmittneret al. (2011) J. Climate Model streamfunction 2 Sv isolines (1 Sv = 106 m3 s-1) (Amazon = 0.2 Sv) Climate Model Simulations
- 16. Conclusions Present • Deepocean circulation transports heat from southern to northern hemisphere • North Atlantic is saltier than North Pacific because of topographic effects of mountains on atmospheric water vapor transport • This causes surface waters to sink in the North Atlantic, setting up the global deep water circulation
- 17. Past Last Glacial Maximum ~20,000years ago
- 18. Method: Produce different AMOCmodel states Compare with sediment reconstructions Reduced southward moisture flux in SH causes saltier and stronger AABW and weaker NADW and AMOC We don’t claim that this is the correct mechanism for changing the circulation. Test method first with modern data. Muglia et al. (in review) Sci. Adv.Funded by NSF
- 19. Can we reconstructthe modern Atlantic Meridional Overturning Circulation (AMOC) using carbon isotopes (14C and 13C) only at sparse locations of LGM sediment cores? Muglia et al. (in review) Sci. Adv. Yes, we can!
- 20. Pre-Industrial Control LGM_13 LGM_8 LGM_0 Muglia etal. (in review) Sci. Adv.Funded by NSF
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- 22. Muglia et al.(in review) Pre-Industrial Control LGM_13 LGM_8 LGM_0 Atlantic Pacific
- 23. LGM_13 LGM_8 LGM_13 SOFe LGM_8 SOFe RMSE 1.2 1.2 0.7 1.8 Both weak AMOC+ SO Fe fertilization needed to fit sediment data!
- 24. Weak AMOC +SO Fe maximize carbon storage!
- 25. Conclusions Past • Duringthe Last Glacial Maximum the Atlantic overturning circulation was much weaker (by about 50%) and shallower than today • Increased iron fertilization from dust enhanced phytoplankton productivity and deep ocean carbon storage • Together, these two effects explain much of the increased carbon storage in the ice age oceanduring the Last Glacial Maximum
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- 27. Bakker et al.(2016) Geophys. Res. Let.Funded by NOAA Greenland Ice Sheet Melting only Probabilistic Projections with Simple Box Model Comprehensive Models
- 29. Conclusions Future • Climatemodels project a decrease of the Atlantic overturning circulation due to warming, intensification of the atmospheric hydrological cycle, and (to a lesser degree) melting of the Greenland ice sheet. • Higher carbon emissions will lead to a stronger decline. • A collapse of the circulation can be avoided by reducing carbon emissions.
- 30. Back to thePresent
- 31. Frajka-Williams (2015) Geophys.Res. Let. In Situ Observations Linear Trend -0.2 Sv/yr Reconstruction Based on Satellite Data -0.13 Sv/yr 1 Sv = 106 m3 s-1 For Comparison: Model Trend post 2006 -0.04 Sv/yr Model Natural Decadal Variability +-0.08 Sv/yr Observations
- 32. Final Thought • Currentlyobserved decrease is likely mostly due to natural variability but anthropogenic could also already play a role
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- 37. Buizert et al.(2015) Nature Rapid Climate Change During Last Ice Age