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The Circulation of the Deep Oceans

The Circulation of the Deep Oceans. a.k.a. abyssal circulation a.k.a. thermohaline circulation a.k.a. meridional overturning circulation a.k.a. global conveyor belt. Josh Willis. Joshua.k.willis@jpl.nasa.gov.

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The Circulation of the Deep Oceans

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  1. The Circulation of the Deep Oceans a.k.a. abyssal circulation a.k.a. thermohaline circulation a.k.a. meridional overturning circulation a.k.a. global conveyor belt Josh Willis Joshua.k.willis@jpl.nasa.gov

  2. In 1798, Englishman Count Rumford postulated that currents bring cold water from the polar regions to fill the abyss In 1845, Emil von Lenz, a Russian-German physicist, noticed the shoaling of the thermocline near the equator and proposed two hemispheric cells.

  3. In 1935, Georg Wüst, a German oceanographer, considering salinity contours suggested a more complicated picture.

  4. Prior to the late 1950s, estimates of overturning in the Atlantic based on hydrographic data suggested only 6-8 Sv of overturning, or inter-hemispheric exchange. Stommel, Arons and Faller – series of papers on theory of deep circulation suggest 15 – 25 Sv!!! References: Stommel H. 1958. The abyssal circulation. Deep-Sea Research 5 (1): 80–82. Stommel H., A.B. Arons, and A.J. Faller. 1958. Some examples of stationary flow patterns in bounded basins. Tellus 10 (2): 179–187. Stommel H., and A.B. Arons. 1960. On the abyssal circulation of the world ocean—II. An idealized model of the circulation pattern and amplitude in oceanic basins. Deep-Sea Research 6: 217–233.

  5. Stommel, Arons and Faller – series of papers on theory of deep circulation suggest 15 – 25 Sv!!! • Three fundamental assumptions: • Deep water supplied by convection in Greenland & Irminger Seas in the North & Weddell Sea in the South. • Uniform mixing brings cold water back toward surface • Deep circulation is geostrophic in the interior.

  6. Theory of the Deep Circulation DeepOcean Upper Ocean Each Contour is 10 Sv Stommel, H.M., 1957. A survey of ocean current theory. Deep-Sea Research 4, 149–184.

  7. Theory of the Deep Circulation Circulation is poleward in interior with narrow deep boundary current Stommel H. 1958. The abyssal circulation. Deep-Sea Research 5 (1): 80–82.

  8. Theory of the Deep Circulation Stommel H., A.B. Arons, and A.J. Faller. 1958. Some examples of stationary flow patterns in bounded basins. Tellus 10 (2): 179–187.

  9. Theory of the Deep Circulation Stommel H., A.B. Arons, and A.J. Faller. 1958. Some examples of stationary flow patterns in bounded basins. Tellus 10 (2): 179–187.

  10. The Deep Western Boundary Current in the Southern Hemisphere Potential Temperature at 30S Salinity at 30S Deep Western Boundary Current Tomczak, Matthias & J Stuart Godfrey: Regional Oceanography: an Introduction 2nd edn (2003), Chapter 13.

  11. The Global Overturning Circulation Reviewsof GeophysicsVolume 45, Issue 2, pages n/a-n/a, 24 APR 2007 DOI: 10.1029/2004RG000166http://onlinelibrary.wiley.com/doi/10.1029/2004RG000166/full#rog1618-fig-0001 From Kuhlbrodt et al., Rev. Geophys., 2007

  12. The Global Overturning Circulation The polar view of the Global Overturning reminds us that the ACC acts as a huge mix-master, mixing deep water masses together and redistributing them to every ocean basin.

  13. Overturning in the North Atlantic The surface (red, orange, yellow) and deep (violet, blue, green) currents in the North Atlantic. The North Atlantic Current brings warm water northward where it cools. Some sinks and returns southward as a cold, deep, western-boundary current. Some returns southward at the surface. From Woods Hole Oceanographic Institution. http://oceanworld.tamu.edu/resources/ocng_textbook/chapter13/chapter13_01.htm

  14. Processes that set abyssal water properties deep convection: 1000 to 1500 dbar (or more) overturn due to buoyancy loss (mostly cooling that causes densification) brine rejection: salt rejected from sea ice during formation, most effective when mixed into a shallow layer, say, on a continental shelf. B.R. in some special sites makes the densest ocean waters. upwelling and surface transformation: SouthernOcean diffusion: mixing of heat and salt. Diapycnal diffusion is essential for deep waters to warm and upwell diapycnally (balances the other two densification processes). Includes local vigorous mixing e.g. strait overflows, and broad-scale

  15. X X B B B X B B Brine rejection in all sea ice areas X X B B B B Ross Sea Weddell Sea Deep convection and brine rejection sites Labrador Sea Greenland Sea Mediterranean Red X From Descriptive Physical Oceanography: An Introduction, 6th edition, by Talley, Pickard, Emery, and Swift

  16. Abyssal circulation: diapycnal diffusion due to vigorous mixing at strait overflows Example: Mediterranean Sea, also the Nordic Seas Inflow of surface water, densification within sea, outflow of denser water through strait, descent with vigorous mixing and entrainment From Descriptive Physical Oceanography: An Introduction, 6th edition, by Talley, Pickard, Emery, and Swift

  17. Intermediate water production sites: major impacts on salinity Labrador Sea Water (fresh) Mediterranean Water (salty) Red Sea Water (salty) Antarctic Intermediate Water (fresh) North Pacific Intermediate Water (fresh) Source Waters for Abyssal Circulation Deep and bottom water production sites: North Atlantic Deep Water (densest portion of it) Antarctic Bottom Water From Descriptive Physical Oceanography: An Introduction, 6th edition, by Talley, Pickard, Emery, and Swift

  18. North Atlantic Deep Water Saline, high oxygen, low nutrient, water mass around 2000 m depth as it exits the N. Atlantic to the south. Signature found throughout world ocean. Sources: Upper layer water of N. Atlantic, from Gulf Stream through subpolar gyre, including Antarctic Intermediate Water and surface water from the Indian Ocean Nordic Seas Overflow Water: Dense, cold overflows from intermediate-deep convection in Greenland Sea Labrador Sea Water: Intermediate depth convection in (fresher) Labrador Sea Mediterranean Water: Evaporated, saline waters from Mediterranean Sea Antarctic Bottom Water: Very dense, cold water from Antarctic

  19. AAIW MOW LSW NADW NSOW AABW Atlantic 25W salinity and water mass names

  20. T-S Plots from various Basins Abyssal waters are a complex mixture of water from various sources Other important tracers: Oxygen, Silicates, Phosphates, 3He, 3H http://oceanworld.tamu.edu/resources/ocng_textbook/chapter13/chapter13_03.htm

  21. Why do we care about the overturning? • At 24N: • Gulf Stream Carries 40 Sv at ~ 18C • DWBC Returns 14 Sv. At ~ 2C (14 x 106 m3/s * 16 C) x 1030 kg/m3 x 4000 J/(kg C) = 0.9 petawatts 1.2 petawatts is the accepted value today! ~ 25% of net northward heat transport Moderates winters in Europe

  22. Why do we care about the overturning?

  23. What’s up with the ice sheet?

  24. Evacuate the Country!!!

  25. How many grad. students will it take to couple my paleoclimate model to an AOGCM?

  26. The Great Ocean Conveyor Belt

  27. 20,000 years ago in North America

  28. Ice Discharge from Glaciers

  29. Could this Really Happen?

  30. Both Arctic Sea Ice and Greenland Ice Sheet are shrinking

  31. The Hosing

  32. Cooling: Hurricane Connection? US Rainfall African Drought Brazilian Rainfall SW Australian Drought ∆SST Coupled Model: Shutting Down the AMOC ∆Precip Vellinga & Wood, Climatic Change, 2002

  33. BUT! Shutdown unlikely with realistic melt 0.29 1.65 0.19 0.54 0.03 0.15 Hu et al., GRL, 2009

  34. Probably safe for today….

  35. Still, AMOC could be important for regional climate HadCM3 1400-year unforced, coupled model run Knight et al., GRL, 2005

  36. Atlantic Hurricanes and the AMO # of Big Hurricanes AMO Goldenberg et al., Science, 2001

  37. How to Measure the Overturning?

  38. RAPID Array Temperature and salinity profiles measured near the boundaries using moorings

  39. RAPID Array Transport through the Florida Straights is measured using the voltage across a cable Transport in the Ekman layer is estimated using wind observations

  40. RAPID Array Overturning Streamfunction Adding these all together makes it possible to estimate zonally averaged overturning

  41. RAPID Array Adding these all together makes it possible to estimate zonally averaged overturning

  42. Subsurface Floats Float Deployments have suggested a more complicated picture than Stommel’s Deep Western Boundary Current DWBC is highly variable and interior pathways are also important From Lozier, Science, 2010 (http://www.sciencemag.org/content/328/5985/1507.full.html)

  43. Satellite Observations of SSH Floats

  44. Subsurface velocity level of known motion Profile data provide geostrophic shear Argo Floats Limited by 2000 m isobath

  45. Computing subsurface displacements A few hours A few hours x x x x ~7 days A few hours Park et al., JTECH, 2005

  46. Altimeter Data

  47. ‘04-’06 mean – 1000 db Velocity SSH Can we integrate, west to east? GeostrophicVelocity

  48. Northward flow of surface water Difference dynamic height at 2000 m isobath NADW Return flow Boundary Current Separated ‘04-’06 mean – 1000 db Velocity Steep Topography

  49. Time Series at 41°N

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