The future of the Atlantic Meridional Overturning Circulation

1. The future of the Atlantic Meridional Overturning Circulation Helge Drange Nansen-Zhu International Research Centre

2. Outline • What is the Atlantic Meridional Overurning circulation? • Climate implications induced by changes in the Atlantic MOC • Observed changes in the North Atlantic/Arctic climate system since the 1960’s • Recent observed and simulated changes in the northern North Atlantic • Conclusions

3. AMOC • Nomenclature • Meridional Overturning Circulation (MOC): Total northward/southward flow, over latitude and depth • Thermohaline Circulation (THC): Part of the MOC set up by gradients in T and S, and driven by heat & water exchange with atmosphere • MOC is an observable quantity; THC is an interpretation • Often used synonymously • No consistency in literature

4. Deep water formation Overflows Wind forcing Basin scale density difference Deep water formation Southern wind stress Mixing THE cartoon! Central processes governing the Atlantic MOC Broecker, 1987

5. AMOC • Important! • Variations in the Atlantic MOC are associated with variations in the pole-ward transport of heat • In general: Strong Atlantic MOC means strong pole-ward ocean transport by heat • Pole-ward transport of heat at 20°N in the Atlantic is about 1.2 TW • Details about the link between the Atlantic Ocean circulation and the pole-ward transport of heat is still not complete • Maximum strength of the Atlantic MOC is about 18 Sv

6. What are the climate implications of a weakened or collapsed Atlantic MOC?

7. Simulated temperature change after a collapse of the Atlantic MOC (Vellinga and Wood 2002) Global surface temperature response: 1. Strong cooling in the Northern Hemisphere 2. Weak warming in the Southern Hemisphere

8. Simulated temperature change after a collapse of the Atlantic MOC (Vellinga and Wood 2002) Global surface temperature response: 1. Strong cooling in the Northern Hemisphere 2. Weak warming in the Southern Hemisphere 3. Profound SST-dipole in the Atlantic Ocean (tele-connection!)

9. ! The Atlantic MOC in the 21st century according to the IPCC (2001) models 1) Large model spread – large uncertainty 2) From 0 to 30% reduction in Atlantic MOC 3) No models show collapse of Atlantic MOC 4) The Atlantic region warms in the 21st century despite reduced heat transport with the ocean

10. Recent observed changes in the North Atlantic-Arctic climate system

11. Increased Eurasian river discharge Peterson et al 2002

12. Reduced Arctic sea ice extent from satellite 1978-2002 trend: ~8.1x105 km2 (~–3-4% / decade) Johannessen et al. (2002)

13. Reduced Arctic sea ice extent Johannessen et al. (2002)

14. Reduced sub-surface salinity 1963 to 2002 Dickson et al. 2002

15. Fresher More saline Observed change in salinity 1985-99 minus 1955-69 Curry et al. 2003

16. Incr river inputs Thinning & retraction of Arctic ice Incr Greenland f’w production Incr f’w loading of Nordic Seas Incr P-E Freshening Lab Sea W’column Freshening overflows Fresher DWBC Courtesy R. Dickson (2004)

17. A 3-D view of the Greenland-Scotland Ridge system

18. NWAC NWASC NIIC Fresh water export EGC NAC Inflow CSC Irminger Current Overflow Dense overflows NAC Courtesy Hatun, 2004

19. Greenland-Scotland Ridge Fluxes (Sv) 16-18 Sv = 4-6 Sv + 6 Sv + 6 Sv 2/3 Hansen et al., Science, 2004

20. Here! Observed overflow from the Nordic Seas

21. Hansen, Turrell and Osterhus (2001), updated by Hansen et al. (2004) Observed overflow through the Faroe-Bank Channel No trend – but the time series is short

22. Fresher More saline Observed change in salinity 1985-99 minus 1955-69 Curry et al. 2003

23. Fresher More saline Observed change in salinity 1985-99 minus 1955-69 Reason: Enhanced evaporation at low latitudes followed by enhanced precipitation at high latitudes Curry et al. 2003

24. Fresher More saline Observed change in salinity 1985-99 minus 1955-69 Reason: Enhanced evaporation at low latitudes followed by enhanced precipitation at high latitudes  Slow-down of Atlantic THC? Over last decade:Increasing to record-high salinities and temperatures in the northward-flowing water masses in the North Atlantic WHY? Curry et al. 2003

25. Observed increase in salinity May 1995 ICES

26. Observed increase in salinity May 1996 ICES

27. Observed increase in salinity May 1997 ICES

28. Observed increase in salinity May 1998 ICES

29. Observed increase in salinity May 1999 ICES

30. Observed increase in salinity May 2000 ICES

31. Observed increase in salinity May 2001 ICES

32. Observed increase in salinity May 2002 ICES

33. Observed increase in salinity May 2003 Highest salinity ever observed! ICES

34. Simulated salinity (Micom) Rockall Trough Hátún et al (2005)

35. Hatun et al. (2005) Science Mean S and T in upper waters (300-500m), 1997-2002

36. Influence of the Atlantic subpolar gyre on the Thermohaline Circulation Hjálmar Hátún, Anne Britt Sandø, Helge Drange, Bogi Hansen, Heðinn Valdimarsson Science, 309 (16 Sep)2005 STG: Sub-Tropical GyreSPG: Sup-Polar Gyre

37. Influence of the Atlantic subpolar gyre on the Thermohaline Circulation Hjálmar Hátún, Anne Britt Sandø, Helge Drange, Bogi Hansen, Heðinn Valdimarsson Science, 309 (16 Sep)2005 The record-high salinity of the Atlantic Inflow can be caused by (i) changes in the (local) air-sea flux of freshwater (evaporation minus precipitation, E-P) (ii) increasing salinities of the STG water (iii) increasing salinities of the SPG water (iv) dynamic changes in the relative contributions from the two gyres STG: Sub-Tropical GyreSPG: Sup-Polar Gyre

38. Influence of the Atlantic subpolar gyre on the Thermohaline Circulation Hjálmar Hátún, Anne Britt Sandø, Helge Drange, Bogi Hansen, Heðinn Valdimarsson Science, 309 (16 Sep)2005 The record-high salinity of the Atlantic Inflow can be caused by (i) changes in the (local) air-sea flux of freshwater (evaporation minus precipitation, E-P) (ii) increasing salinities of the STG water (iii) increasing salinities of the SPG water (iv) dynamic changes in the relative contributions from the two gyres: YES! STG: Sub-Tropical GyreSPG: Sup-Polar Gyre

39. Hatun et al. (2005) Science …in conclusion Both observations and model show that the cold and fresh Subpolar Gyre has weakened over the last 10 years, opening for northward transport of warm and saline water with the Subtropical gyre. This change will tend to strengthen the Atlantic THC!

40. Configuration of regional OGCM • Miami Isopycnic Coordinate Ocean Model (MICOM; Bleck et al., JPO, 1992) - a mixture of versions 2.6 to 2.8 is used • Dynamic-thermodynamic sea ice modules included • Reference pressure at the surface • 24verticallayers with potential density ranging from 23.54 to 28.10 • Stretched grids with focus in the North Atlantic-Arctic region (Bentsen et al., MWR, 1999) • Dailyatmosphericforcing, using NCEP/NCAR reanalysis data (Kalnay et al., 1996) • Period 1948 to present • 80, 40 and20 km • grid spacing in the Nordic Seas

41. Hatun et al. (2005) Science Observed and simulated salinity anomalies from about 1970 to present Good fit! Observed Simulated Irminger (I) Faroe (F) Rockall (R)

42. F I R F R I MICOM The subpolar gyre influence SSH EOF (Häkkinen and Rhines, Science, 2004) Altimetry Hatun et al. (2005) Science

43. Hatun et al. (2005) Science A longer term perspective I R

44. EOF1 upper ocean temp. from model 1960-2003 EOF1 from sat. altimetry 1992-2002 EOF1 SSH from model 1960-2003 Hatun et al. (2005) Science

45. WNAW ENAW (inverted) The gyre shape and the Rockall Trough Fresh Saline NB: Holds also for temperature Hatun et al. (2005) Science

46. EOF1 Wind stress curl (26%) EOF1 Buoyancy flux (19%) Red: SSH PC1 Black: SLP PC1 Green: PC1 Wind stressBlue: PC1 Buoyancy flux

47. Summary • Variations in the strengh and propeties of the Atlanic MOC have local, as well as global, climate implications • It is likely that the large-scale, multidecadal di-pole pattern in Atlantic SST is associated with changes in the strength of the Atlantic MOC • Changes in the GSR overflow may significantly alter the North Atlantic deep water (12 Sv out of 18 Sv) • Almost all surface and sub-surface waters in the northern North Atlantic have freshened since the 1960’s – this tend to weaken the Atlantic MOC • A recent increase in surface salinity is observed in the northern North Atlantic over the last years – this may increase the Atlantic MOC • It is not likely that the Atlantic MOC will decrese by more than 30% in the 21st century • Beyond the 21st century, melting of the Greenland ice cap may significantly alter the global ocean cirulation

48. Summary • Still a long way to go before key processes are adequately represented in climate GCMs • Open question how realistic climate GCMs need to be to realistically represent the stability and variability properties of the Atlantic MOC • Need to fully integrate instrumental observations, paleo time series, theory and modelling! • The role of variations in the Atlantic SST associated with changes in the strength of the Atlantic MOC are still poorly known – challenge for students and scientists!