Spin-up; monthly forcing; daily forcing; relaxation T and/or S Topics;

1. 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. 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 (5th or 6th 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)

5. AMOC, last 4 cycles

6. NA Sub-Polar Gyre SSH, last 4 cycles

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

10. F I R F R I MICOM Dynamical SPG influence SSH EOF (Häkkinen and Rhines, Science, 2004) Altimetry

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

12. NB: Relationship also valid for temperature

13. Forcing mechanisms (I) Hatun et al., in prep.

14. Forcing mechanisms (II) Hatun et al., in prep

15. Wind stress and NAO consistent with the Gyre Index before - but not after - 1995

16. Conclusions (1) – 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)

17. 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

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

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

20. SST variability (Atl inflow), Faroe Islands Record high temperature (and salinity) Simulated temperature (Hátún, AGU Monogr 158, 2005)

21. 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 1 °C increase in T of Atlantic Water entering the Nordic Seas since 1997 (ca 0-600 m)

22. Temperature anomaly (ºC), 50-200 m Barents Sea Ingvaldsen, Loeng and Ådlandsvik, 2007

23. Observed vertically avg temperature, Fram Strait Ursula Schauer, AWI

24. Observed air temperature, Nuuk (W Greenland) Stein (2007)

25. Jakobshavn glacier, West Greenland http://svs.gsfc.nasa.gov

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

28. The model system used

29. MICOM The NERSC version 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 Simulated salinity Nordic Seas Global model:(40 km in the Nordic Seas) run for the 1948-2003 period; forced with daily atmospheric NCAR/NCEP re-analyses fields

30. Subpolar gyre source water Western North Atlantic Water (WNAW) Western Subtropical gyre Source water Eastern North Atlantic Water (ENAW) Simulated salinity Source Waters for the Atlantic Inflows

31. Hatun et al., Science (2005) Irminger (I) Faroe (F) Rockall (R) Observedand simulated salinity anomalies at three locations in the northern North Atlantic

32. Gyre index I R Rockall Trough (obs) Gyre index Irminger Current (obs)

33. Atmospheric forcing Hatun et al., 2007

34. Atmospheric forcing Hatun et al., 2007

35. Q1How does the subpolar gyre (SPG) respond to a persistent, decadal time scale positive – or negative – NAO forcing? Q2How linear is the response of the SPG forced with positive – or negative – phases of the NAO? Q3How important was the ocean initial state in 1995 for the strong and rapid weakening of the SPG?

36. Q1How does the subpolar gyre (SPG) respond to a persistent, decadal time scale positive – or negative – NAO forcing? Q2How linear is the response of the SPG forced with positive – or negative – phases of the NAO? Q3How important was the ocean initial state in 1995 for the strong and rapid weakening of the SPG?

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

38. Sea surface height, NAO+minus NAOn

39. Steric height, NAO+minus NAOn T S

40. SPG index, NAO+ minus NAOn Weak Strong

41. Sea surface height, NAO-minus NAOn

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

43. 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 • NAOnforcing can reveal temporal / spatial nonlinearities in ocean’s response to NAO (for analysis, use e.g. NAO+ minus NAOn and NAO- minus NAOn)

44. Q3How important was the ocean initial state in 1995 for the strong and rapid weakening of the SPG? NAO index

45. Simulated strength of the SPG 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

46. Post 1995 forcing (I) Simulated strength of the SPG Control integration (“reality”)

47. Post 1995 forcing (II) Simulated strength of the SPG Control integration (“reality”)

48. 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

49. Post 1982 forcing Simulated strength of the SPG Control integration 1995

50. 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