1. North Atlantic Sub-Polar Gyre
Background

2. North Atlantic Ocean Major water masses
Simulated
salinity
Cold and fresh
polar waters
Low salinity Subpolar water meets salty water in the NAC. These water masses mix along the front producing the relatively fresh WNAW as the NAC enters the Rockall region. The more saline ENAW enters the Rockall region the south.
Polar + subtropical
mixed water
Warm and saline
eastern waters
Warm and saline
subtropical waters
(“Golf Stream”)

3. Record-high temperatures
Heat enters
the Arctic
Heat enters
the coasts of Greenland
Large changes to fish stocks
New (warm water) fish species
Happened winter 1995/96 (“perfect storm”)

4. Contributing to the observed warming
off the coast of Greenland ( m)
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
Holland et al. (2008)

5. …to the (dynamic) thinning of the outlet glaciers on Greenland
(m per year)
20 m per yr
Pritchard et al., Nature (2009)

6. …and changes in East Atl temperature
(°C, m, )
Holliday et al. GRL (2008)

7. Defines the Gyre Index Strong gyre: Low SSH Weak gyre: High SSH
Given by altimeter obs form 1992,
from vertical temperature prior to 1992

8. Observed and simulated gyre index
[SSH (cm) averaged for 60°W–15°W, 48°N–65°N]
3.0
1.5
-1.5
-3.0
Weak
Strong
Lohman et al. (2009)

9. Proposed mechanisms for SPG-variability
Atm2Ocean: Wind stress forcing
Atm2Ocean: Thermal forcing
Arctic: Fresh water input
Ocean: Preconditioning
Winter 1995/96:
Likely combined effect of ocean preconditioning and abrupt drop in NAO

10. (Climatic Research Unit, University of East Anglia)
Winter NAO-index,
(Climatic Research Unit, University of East Anglia)
High NAO years
High NAO years

11. Observed spreading of Atlantic cod
along the coast of Greenland,
1900
1919
1922
Similar mechanism(s) compared to the post 1995/96 change?
Implications for past and future climate
Late 1930s
Hansen, 1940

12. Hatun et al., Science (2005); Prog. Ocenogr. (2009)
Observed and simulated salinity anomalies at three locations in the northern North Atlantic
Irminger (I) Faroe (F) Rockall (R)

13. Hatun et al., Science (2005); Prog. Ocenogr. (2009)
Observed and simulated salinity anomalies at three locations in the northern North Atlantic
Irminger (I) Faroe (F) Rockall (R)

14. Hatun et al., Science (2005); Prog. Ocenogr. (2009)
Observed and simulated salinity anomalies at three locations in the northern North Atlantic
Why post-95 change?
(T, S, marine biota)
Irminger (I) Faroe (F) Rockall (R)

15. Why post-95 change?
(T, S, marine biota)
Ocean preconditioning of the SPG due to several years with positive NAO-like forcing
Abrupt drop in NAO-forcing during winter
Lohmann et al. Clim Dyn (2008)
Lohmann et al. GRL (2009)

16. Dynamics discussed and partly understood; SPG involved
Less understanding; role of SPG unclear
Hatun et al, Prog. Oceanogr. (2009)

17. Q1: How does the 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?
Lohmann et al. Clim Dyn (2008)
Lohmann et al. GRL (2009)

18. NAO index based on ERA40 Force Bergen isopycnic OGCM with repeating
(i) NAO+, (ii) NAO- or (iii) NAOn fields for 40 years
NAOn
NAO-
NAO+

19. Sea surface height, NAO+ minus NAOn (starting from 1961)
meter
meter
Yr 4-14: Intensified gyre

20. Sea surface height, NAO+ minus NAOn (starting from 1961)
meter
meter
Yr 4-14: Intensified gyre
Yr 15-25: Weakened gyre

21. Mixed layer T and S, NAO+ minus NAOn (starting from 1961)oC oC
psu
psu

22. Advection of warm and saline waters
Mixed layer T and S, NAO+ minus NAOn (starting from 1961) oC oC
Advection of warm and saline waters
psu
psu

23. Met Office Ocean analysis (upper 500 m)
Jon Robson,
U of Reading
anomalies relative to climatology

24. Sea surface height, NAO- minus NAOn
Gradual weakening

25. Conclusions (1) NAO+ forcing  NAO- forcing  Asymmetry 
- Initial strengthening of SPG
- After ~10 years replaced by weakening
- Advection of warm water counteracts local cooling
NAO- forcing 
- Gradual weakening of SPG, approaching a minimum value
Asymmetry 
- Potentially misleading to look at/analyse (NAO+ minus NAO-)

26. Q3: What caused the abrupt drop in the SPG after 1995
Q3: What caused the abrupt drop in the SPG after 1995? Q4: What is the role of the actual ocean initial state?

27. Sensitivity experiments
same model as before (Bergen isopycnic OGCM)
post 1995 forcing applied to initial conditions from 1975, 1980, 1985, 1990, 2000, 2005, and every year between 1991 and 1997

28. Control integration (1995; “real model world”)
Post 1995 forcing (I)
Control integration (1995; “real model world”)

29. Simulated strength of the SPG
2005

30. Control integration (1995; “real model world”)
Post 1995 forcing (II)
Control integration (1995; “real model world”)

31. Conclusions (2) SPG drop in 1995  Note Predictability
SPG at maximum strength and approaching break-down after a long period with NAO+ forcing
NAO forcing changed from high to low value the winter 1995/96
The combined effect lead to an unprecedented collapse of SPG
Note
SPG would also have collapsed in 1994 with post-1995 forcing
Otherwise no collapse for the period with post-95 forcing
Predictability
Ocean initial state of crucial importance