1. Department of Geophysics University of Bergen Spin-up; monthly forcing; daily forcing; relaxation T and/or S Topics; Variability guided by observed changes (mainly) in the Atlantic; particularly the gyre variability Inflow to the Nordic Seas/Arctic Water mass transformation in the Nordic Seas AMOC variability Physical forcing of the marine biota Idealised experiments; role of the ocean preconditioning and atmospheric forcing Have used NCAR/NCEP and ERA40, error in implementation of CORE; will be rerun

2. Department of Geophysics University of Bergen Spin-up … min full 4 cycles with daily forcing; usually 6 full cycles (ca. 300 yr) Always start with (strong) SSS-relaxation; 30 days for 50 m thick ML; scales with ML depth; limited to |ΔSSS<0.5| everywhere; no relaxation under max sea ice extent Diagnose SSS-nudging when model is steady (5 th or 6 th cycle); applying diagnosed SSS-fluxes for the production runs with very weak Newtonian relaxation (360 days time scale for 50 m ML and |ΔSSS<0.5| ) Temperature relaxation is not critical (in our system)

3. Department of Geophysics University of Bergen

4. Department of Geophysics University of Bergen

5. Department of Geophysics University of Bergen AMOC, last 4 cycles

6. Department of Geophysics University of Bergen NA Sub-Polar Gyre SSH, last 4 cycles

7. Department of Geophysics University of Bergen

8. Department of Geophysics University of Bergen

9. Department of Geophysics University of Bergen eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee 3) Increased salinity in the Subpolar gyre (SPG) 2) Increased salinity in the subtropical gyre (STG) 1) Relative contribution from the two gyres (Dynamics) 4) Enhanced Evaporation minus Precipitation (E-P) Possible mechanisms causing the rapid increase in salinity (and temperature) Hatun et al., Science (2005)

10. Department of Geophysics University of Bergen Altimetry MICOM SSH EOF (Häkkinen and Rhines, Science, 2004) I I F FR R Dynamical SPG influence

11. Department of Geophysics University of Bergen R I A longer term perspective, using the simulated Gyre Index as a proxy for the circulation Irminger Current (obs) Rockall Through (obs) Gyre index (model)

12. Department of Geophysics University of Bergen NB: Relationship also valid for temperature

13. Department of Geophysics University of Bergen Forcing mechanisms (I) Hatun et al., in prep.

14. Department of Geophysics University of Bergen Hatun et al., in prep Forcing mechanisms (II)

15. Department of Geophysics University of Bergen Volume of LSW in layers 15 and 16 and the Gyre Index

16. Department of Geophysics University of Bergen Heat (buoyancy) flux coherent with the Gyre Index

17. Department of Geophysics University of Bergen Sea Level Pressure consistent with the Gyre Index before (but not after) 1995

18. Department of Geophysics University of Bergen Wind stress and NAO consistent with the Gyre Index before - but not after - 1995

19. Department of Geophysics University of Bergen – SPG is of key importance for the decadal-scale variations in the Atlantic inflow to the Arctic Mediterranean and along the cost off South Greenland – The strength of the SPG is governed by the Subpolar (winter) buoyancy forcing – The latter follows, in general, the North Atlantic wind stress (NAO) forcing, but not after 1995/96 – The Subpolar Gyre Index, rather than NAO, should be used as a proxy for the variability of the marine climate in the North Atlantic region (can be deduced from observations or hind-cast model simulations) Conclusions (1)

20. Department of Geophysics University of Bergen Recent North Atlantic Warming and Some Consequences Thereof Helge Drange, Katja Lohmann, Mats Bentsen, Hjalmar Hatun, Anne Britt Sandø and colleagues helge.drange@gfi.uib.no

21. Department of Geophysics University of Bergen 0 1 2 3 4 5 6 7 8 9 10°C Orvik and Niiler, GRL, 2002 Observed sea surface temperature Greenland Norway UK Iceland This presentation: Focus on the rapid (northern) North Atlantic warming after 1995/96 Not addressed here: The superimposed global warming signal

22. Department of Geophysics University of Bergen 0 1 2 3 4 5 6 7 8 9 10°C Barents Faroes Observed hydrography, N Atl/Nordic Seas (NB: post 95-changes!) Fram Strait Svinøy section

23. Department of Geophysics University of Bergen (Hátún, AGU Monogr 158, 2005) SST variability (Atl inflow), Faroe Islands Record high temperature (and salinity) Simulated temperature

24. Department of Geophysics University of Bergen 1 °C increase in T of Atlantic Water entering the Nordic Seas since 1997 (ca 0-600 m) Orvik, Kjell Arild; Skagseth, Øystein (2005), Heat flux variations in the eastern Norwegian Atlantic Current toward the Arctic from moored instruments, 1995–2005. Geoph. Res. Lett., 32, No. 14, L14610 10.1029/2005GL023487

25. Department of Geophysics University of Bergen Ingvaldsen, Loeng and Ådlandsvik, 2007 Temperature anomaly (ºC), 50-200 m Barents Sea

26. Department of Geophysics University of Bergen Ursula Schauer, AWI Observed vertically avg temperature, Fram Strait

27. Department of Geophysics University of Bergen Observed air temperature, Nuuk (W Greenland) Stein (2007)

28. Department of Geophysics University of Bergen http://svs.gsfc.nasa.gov Jakobshavn glacier, West Greenland

29. Department of Geophysics University of Bergen

30. Department of Geophysics University of Bergen Rapid warming is observed throughout the North Atlantic since 1995/96 Followed by rapid changes in the cryosphere and the marine ecosystems, possibly/likely linked to the North Atlantic warming North Atlantic salinity is increasing in concert with temperature (post 1995)

31. Department of Geophysics University of Bergen The model system used

32. Department of Geophysics University of Bergen MICOM The NERSC version Simulated salinity Global model:(40 km in the Nordic Seas) run for the 1948-2003 period; forced with daily atmospheric NCAR/NCEP re-analyses fields Nordic Seas Regional model: (20 km in the North Atlantic) run for the same period (1948-2003); forcing fields as for the global model; boundary fields from the global model

33. Department of Geophysics University of Bergen Subpolar gyre source water Western Subtropical gyre Source water Western North Atlantic Water (WNAW) Eastern North Atlantic Water (ENAW) Source Waters for the Atlantic Inflows Simulated salinity

34. Department of Geophysics University of Bergen Irminger (I) Faroe (F) Rockall (R) Observed and simulated salinity anomalies at three locations in the northern North Atlantic Hatun et al., Science (2005)

35. Department of Geophysics University of Bergen R I Gyre index Irminger Current (obs) Rockall Trough (obs) Gyre index

36. Department of Geophysics University of Bergen Atmospheric forcing Hatun et al., 2007

37. Department of Geophysics University of Bergen Hatun et al., 2007 Atmospheric forcing

38. Department of Geophysics University of Bergen Q1 How does the subpolar gyre (SPG) respond to a persistent, decadal time scale positive – or negative – NAO forcing? Q2 How linear is the response of the SPG forced with positive – or negative – phases of the NAO? Q3 How important was the ocean initial state in 1995 for the strong and rapid weakening of the SPG?

39. Department of Geophysics University of Bergen Q1 How does the subpolar gyre (SPG) respond to a persistent, decadal time scale positive – or negative – NAO forcing? Q2 How linear is the response of the SPG forced with positive – or negative – phases of the NAO? Q3 How important was the ocean initial state in 1995 for the strong and rapid weakening of the SPG?

40. Department of Geophysics University of Bergen Idealized experiments (Lohmann et al., Clim Dyn, 2008) 40 year sensitivity experiments, forced with persistent NAO+, NAO- and NAOn fields (cycling through the marked years) NAO index High NAO-years Low NAO-years

41. Department of Geophysics University of Bergen Sea surface height, NAO+ minus NAOn

42. Department of Geophysics University of Bergen Steric height, NAO+ minus NAOn T S

43. Department of Geophysics University of Bergen SPG index, NAO+ minus NAOn Strong Weak

44. Department of Geophysics University of Bergen Sea surface height, NAO- minus NAOn

45. Department of Geophysics University of Bergen Sea surface height, NAO+ minus NAO- Looks like a gradual (linear) change, but is a composite of two different temporal and spatial responses

46. Department of Geophysics University of Bergen Conclusions (2) NAO+ Initial strengthening of SPG After ~10 years replaced by weakening Advective warming overruns local buoyancy forcing NAO- Steady weakening of SPG No ocean advective feedback Nonlinearity: SPG response to NAO+ and NAO- forcing is nonlinear, so the difference NAO+ minus NAO- is misleading NAOn forcing can reveal temporal / spatial nonlinearities in ocean’s response to NAO (for analysis, use e.g. NAO+ minus NAOn and NAO- minus NAOn)

47. Department of Geophysics University of Bergen NAO index Q3 How important was the ocean initial state in 1995 for the strong and rapid weakening of the SPG?

48. Department of Geophysics University of Bergen Sensitivity experiments Post 1995 forcing (red arrow) applied to ocean initial state from ✓ 1975, 1980, 1985, 1990, 2000, 2005, and ✓ every year between 1991 and 1997 Simulated strength of the SPG

49. Department of Geophysics University of Bergen Post 1995 forcing (I) Control integration (“reality”) Simulated strength of the SPG

50. Department of Geophysics University of Bergen Control integration (“reality”) Post 1995 forcing (II) Simulated strength of the SPG

51. Department of Geophysics University of Bergen Additional sensitivity experiments Post 1982 forcing (red arrow) applied to ocean initial state from ✓ 1975, 1985, 1990, 1995, 2000, 2005 Simulated strength of the SPG

52. Department of Geophysics University of Bergen Post 1982 forcing Control integration 1995 Simulated strength of the SPG

53. Department of Geophysics University of Bergen Conclusions (3) What happened after 1995? ✓ SPG close to maximum strength and approaching weakening even with unchanged NAO+ forcing (preconditioning) ✓ NAO forcing dropped from high to low value the winter 1995/96 (right forcing) ✓ Important implications for decadal-scale predictability of the climate in the North Atlantic – the ocean initial state is of key imortance

54. Department of Geophysics University of Bergen Large increase in the blue whiting SSB Large changes in the distribution of redfish Changes since 1995 Cod is heading back to Greenlandic waters Warming Herring is heading back to Icelandic and Faroese waters

55. Department of Geophysics University of Bergen Hatun et al, Prog. Oceanogr. (2009)

56. Department of Geophysics University of Bergen Response of phytoplankton (PCI) and zooplankton (Cal. Fin.)