North Atlantic Sub-Polar Gyre Background

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”)

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”)

Contributing to the observed warming off the coast of Greenland (150-600 m) 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 Holland et al. (2008)

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

…and changes in East Atl temperature (°C, 0-500 m, 1950-2008) Holliday et al. GRL (2008)

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

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)

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

(Climatic Research Unit, University of East Anglia) Winter NAO-index, 1825-2010 (Climatic Research Unit, University of East Anglia) High NAO years High NAO years

Observed spreading of Atlantic cod along the coast of Greenland, 1900-1940 1900 1917-18 1919 1922 Similar mechanism(s) compared to the post 1995/96 change? Implications for past and future climate 1927-30 1931-36 Late 1930s Hansen, 1940

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)

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)

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)

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 1995-96 Lohmann et al. Clim Dyn (2008) Lohmann et al. GRL (2009)

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

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)

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+

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

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

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

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

Met Office Ocean analysis (upper 500 m) Jon Robson, U of Reading anomalies relative to 1941-1996 climatology

Sea surface height, NAO- minus NAOn Gradual weakening

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-)

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?

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

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

Simulated strength of the SPG 2005

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

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 1960-2005 with post-95 forcing Predictability Ocean initial state of crucial importance