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Modeling the MOC

Modeling the MOC. http://www.andrill.org/iceberg/blogs/julian/images/greatoceanconveyor.jpg. Ronald J Stouffer Geophysical Fluid Dynamics Laboratory NOAA. The views described here are solely those of the presenter and not of GFDL/NOAA/DOC or any other agency or institution.

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Modeling the MOC

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  1. Modeling the MOC http://www.andrill.org/iceberg/blogs/julian/images/greatoceanconveyor.jpg Ronald J Stouffer Geophysical Fluid Dynamics Laboratory NOAA The views described here are solely those of the presenter and not of GFDL/NOAA/DOC or any other agency or institution.

  2. Modeling the MOCOutline • Role of MOC in maintaining mean climate • Northward Heat Transport • Northward Salt transport • Role of MOC in Abrupt Climate Change • Unforced • Forced • Predictability

  3. How well do models simulate the T, S structure in the ocean?AR4 ensemble mean error Temperature -2.5 +2.5 IPCC WGI Chapter 8

  4. AR4 ensemble mean errorSalinity EQ 90N 90S +1.0 -1.0 -0.4 -0.2 0.2 0.4 PSU IPCC WGI Chapter 8 Suppl. Material

  5. Role of MOC in Heat Transport

  6. Impact of MOC on Climate(SAT) AOGCMs EMICs MOC “on” minus “off” oC Conclusion – MOC warms NH, locally large values, MOC cools SH

  7. Impact of MOC on Climate Zonally averaged Precipitation differences (mm/day) Red lines control Blue lines difference Dashed – EMICs Solid - AOGCMs Conclusion – MOC on => ITCZ toward north

  8. Impact of MOC on ClimateSalinity MOC “on” minus “off” (PSU)

  9. Role of MOC in maintaining mean climate • North Atlantic saltier than without MOC • Rest of world ocean surface more fresh • Northern Hemisphere warmer than without • Particularly the N Atlantic • Role of atmosphere mixing heat

  10. Ocean MOC Role in Abrupt Climate Changes • Unforced • Hall and Stouffer Nature • Forced • Idealized (Hosing ) - Stouffer et al. JoC 2006

  11. TransientAn Anomalous Event (Unforced)

  12. Maximum Negative Anomaly

  13. Maximum Positive Anomaly

  14. Surface Air TemperatureDecadal Mean Difference

  15. Surface variables/THC

  16. Summary of Physical Mechanism

  17. Unforced MOC variability • If model if “realistic” • Can we predict this event? • Complicating GHG changes • Possible explanation of some parts of paleo-record

  18. Experimental DesignManabe Climate Model “MCM” • R30 AOGCM coupled model • Idealized Water hosing • 1 Sv for 100 years • After 100 years, stop hosing - allow recovery • Case 1: Hosing 50N to 70N in Atlantic • Case 2: Hosing south of 60S in Southern Ocean • Compare to long control integration

  19. Simulated Global MOC SH Index box NH Index box

  20. Atlantic THC Response Atlantic THC does not respond in a seesaw-like manner NH Hosing – NH THC shuts down SV Years

  21. SH THC Response SH Hosing – SH THC weakens. SH THC does not shut down SV Years

  22. SAT Difference mapSH hosing K Years 51-100 hosing minus 1-200 control

  23. Surface Salinity Response NH P S U SH 0 100 200 Years 0 100 200 Years NH SSS anomaly – Intense and confined SH SSS anomaly – Weaker and spreads

  24. Differences in Sea Surface Salinity (PSU)Southern Freshwater Escape 25 years 100 years Hosing minus Control

  25. Sea Surface Temperature Response NH K SH 0 100 200 Years 0 100 200 Years Response more symmetrical than SSS Magnitude also becoming more similar

  26. Surface Air Temperature Response NH K SH 0 100 200 Years 0 100 200 Years Response remarkably symmetrical (first 100 yrs) Magnitude very similar

  27. Precipitation Response NH Cm/ day SH 0 100 200 Years 0 100 200 Years Response very symmetric Magnitude very similar ITCZ shifts toward warmer hemisphere

  28. Hosing Experiment Summary • Symmetrical Atmospheric Response • Much less symmetry in ocean • Why? • Strong Circum-Antarctica winds • Northward flowing surface waters • Freshwater “escapes” into other basins • Far a field impacts • Less local impacts

  29. MOC Predictability • Unforced Changes • Are MOC predictable? • How long into future • Manifest in surface changes? • Forced - GHG increase • Does MOC weaken? • How much? • Likelihood of complete shutdown?

  30. Predictability Investigations just starting • Perfect model experiments • Use ICs from long control • Ocean ICs unchanged from control • Atmosphere ICs shifted in time by day or 2 • Probably “best case” for predictability • No model errors • Ocean ICs perfectly known

  31. Predictability of Atlantic Meridional Overturning Circulation (AMOC) in GFDL CM2.1 Climate Model

  32. Are past ocean observations good enough to constrain MOC? • Past ocean observations mainly XBTs • Temperature only • Upper 700 m or so • Since 2003 or so – ARGO • T, S • Upper 2 km

  33. Can Observations constrain the MOC? Need ARGO and atmospheric data to constrain MOC Other research suggests this may be too pessimistic.

  34. MOC and Forced Climate Change A AR4 WG1 Assessment: MOC very likely to weaken MOC shutdown very unlikely

  35. Why does MOC slow down as GHG increase? • Role of surface fluxes • Heat fluxes • Water fluxes

  36. Design of Partially Coupled ExperimentsGregory et al. 2006 GRL • Run control and 1% per year CO2 increase experiment • Save out surface fluxes • Use water fluxes from control in 1% run • TRAD_CH2O • Use water fluxes from 1% in control • CRAD_TH2O • Isolates role of heat and water fluxes

  37. Summary of Partially Coupled Experiment Conclusions: 1. Heat fluxes changes always weaken MOC 2. Water fluxes changes mixed, but usually weaken MOC 3. Response to heat flux changes fairly uniform

  38. Warming greatest over land and at most high northern latitudes and least over Southern Ocean and parts of the North Atlantic Ocean Weakening of MOC contributes to minimum in cooling in N Atlantic => smaller climate change => a positive impact? Surface Warming Pattern A1B, 2090-2099 relative to 1980-1999 IPCC WGI SPM

  39. Summary:Ocean’s Role in Abrupt Climate Change • Unforced • Possible to have large abrupt climate changes in AOGCMs • Forced • Idealized • Allows easy study of climate response • Application to paleo-data and future climate changes • GHG increase • Predictability of MOC changes

  40. Questions • Basic Issues • Why does the MOC exist? • How much mixing is there in the ocean? • Do models have too much/too little? • Impacts of mixing on the MOC • What are the physical processes? • What is the role of ocean eddies? • How does MOC changes impact biology and associated changes in atmospheric pCO2?

  41. Questions • Variability • Are observations good enough to constrain MOC? • Are the MOC changes predictable? • Time scale? • Does it matter for where people live? • Paleo-data tests? • What is role of MOC variability/changes in tropical Atlantic SST/hurricane changes? • Future • Is future weakening of the MOC “bad”? • Interactions with Greenland/Antarctic ice melting

  42. Thank you

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