A possible mechanism for the strong weakening of the North Atlantic subpolar gyre in the mid‐1990s

[1] The extent and strength of the North Atlantic subpolar gyre (SPG) changed rapidly in the mid-1990s, going from large and strong in 1995 to substantially weakened in the following years. The abrupt change in the intensity of the SPG is commonly linked to the reversal of the North Atlantic Oscillation (NAO) index, changing from strong positive to negative values, in the winter 1995/96. In this study we investigate the impact of the initial SPG state on the subsequent behavior of the SPG by means of an ocean general circulation model driven by NCEP-NCAR reanalysis fields. Our sensitivity integrations suggest that the weakening of the SPG cannot be explained by the change in the atmospheric forcing alone. Rather, for the time period around 1995, the SPG was about to weaken, irrespective of the actual atmospheric forcing, due to the ocean state governed by the persistently strong positive NAO during the preceding seven years (1989–1995). Our analysis indicates that it was this preconditioning of the ocean, in combination with the sudden drop in the NAO in 1995/96, that lead to the strong and rapid weakening of the SPG in the second half of the 1990s. This hypothesis explains the diverging evolution of the strength of the SPG and the atmospheric forcing (winter NAO) after 1995, as has been suggested recently.

[1]  S. Häkkinen.  Variability of the simulated meridional heat transport in the North Atlantic for the period 1951–1993 , 1999 .

[2]  David M. Holland,et al.  Acceleration of Jakobshavn Isbræ triggered by warm subsurface ocean waters , 2008 .

[3]  Rong‐Hua Zhang.  Coherent surface‐subsurface fingerprint of the Atlantic meridional overturning circulation , 2008 .

[4]  Carsten Eden,et al.  Mechanism of Interannual to Decadal Variability of the North Atlantic Circulation , 2001 .

[5]  Rainer Bleck,et al.  Salinity-driven Thermocline Transients in a Wind- and Thermohaline-forced Isopycnic Coordinate Model of the North Atlantic , 1992 .

[6]  R. Gerdes,et al.  Response of the northern North Atlantic and Arctic oceans to a sudden change of the North Atlantic Oscillation , 2005 .

[7]  C. Eden,et al.  North Atlantic Interdecadal Variability: Oceanic response to the North Atlantic Oscillation (1865-1997) , 2001 .

[8]  Gurvan Madec,et al.  Is the thermohaline circulation changing , 2006 .

[9]  H. Drange,et al.  Response of the North Atlantic subpolar gyre to persistent North Atlantic oscillation like forcing , 2009 .

[10]  A. Biastoch,et al.  Decadal variability of subpolar gyre transport and its reverberation in the North Atlantic overturning , 2006 .

[11]  S. Häkkinen,et al.  Decline of Subpolar North Atlantic Circulation During the 1990s , 2004, Science.

[12]  H. Drange,et al.  Influence of the Atlantic Subpolar Gyre on the Thermohaline Circulation , 2005, Science.

[13]  Mark R. Payne,et al.  Large bio-geographical shifts in the north-eastern Atlantic Ocean: From the subpolar gyre, via plankton, to blue whiting and pilot whales , 2009 .

[14]  Rainer Bleck,et al.  A Wind-Driven Isopycnic Coordinate Model of the North and Equatorial Atlantic Ocean 1 , 1990 .

[15]  R. Reynolds,et al.  The NCEP/NCAR 40-Year Reanalysis Project , 1996, Renewable Energy.

[16]  Mark R. Payne,et al.  The North Atlantic subpolar gyre regulates the spawning distribution of blue whiting (Micromesistius poutassou) , 2009 .

[17]  Syukuro Manabe,et al.  Interdecadal Variations of the Thermohaline Circulation in a Coupled Ocean-Atmosphere Model , 1993 .