Sensitivity of North Atlantic subpolar gyre and overturning to stratification-dependent mixing: response to global warming

Abstract

We use a reduced complexity climate model with a three-dimensional ocean component and realistic topography to investigate the effect of stratificationdependent mixing on the sensitivity of the North Atlantic subpolar gyre (SPG), and the Atlantic meridional overturning circulation (AMOC), to idealized CO 2 increase and peaking scenarios. The vertical diffusivity of the ocean interior is parameterized as j * N -a , where N is the local buoyancy frequency. For all parameter values 0 B a B 3, we find the SPG, and subsequently the AMOC, to weaken in response to increasing CO 2 concentrations. The weakening is significantly stronger for a C a cr & 1.5. Depending on the value of a, two separate model states develop. These states remain different after the CO 2 concentration is stabilized, and in some cases even after the CO 2 concentration has been decreased again to the pre-industrial level. This behaviour is explained by a positive feedback between stratification and mixing anomalies in the Nordic Seas, causing a persistent weakening of the SPG.

Stratification‐dependent mixing may increase sensitivity of a wind‐driven Atlantic overturning to surface freshwater flux

“…The mechanism explained in this study does however not depend on a hysteresis behavior of the model, and not even on the details of the North Atlantic circulation. While the results of Marzeion et al [2009], using the same model set up forced with idealized CO 2 forcing, depend on the North Atlantic sub‐polar gyre circulation changes and therefore potentially may be sensitive to changes in the salinity distribution, we do not expect the results presented here to be sensitive to the specific model. Nonetheless, a detailed comparison remains as an important next step for future work.…”

“…The reason is the weakening of the diffusive, and strengthening of the advective pathway in the propagation of the fresh anomaly, which leads to a reduced meridional density gradient in the northern deep return flow of the AMOC. Similarly, the experiments of Marzeion et al [2007Marzeion et al [ , 2009 have shown a greatly enhanced weakening of the AMOC in response to freshwater pulses and idealized CO 2 forcing when stratification dependent mixing was employed. Therefore, the sensitivity of the AMOC to changes in the forcing found in models employing constant vertical mixing may have to be understood as a lower limit when considering the complexity of the processes linking vertical mixing in the ocean interior to the AMOC.…”

“…In the experiments presented here, the parameter space between a = 0 (constant diffusivity) and a = 2 (stronger sensitivity to stratification, constant rate of energy conversion by mixing, see Nilsson et al [2003] and Marzeion et al [2007] for a more detailed discussion of significance of the parameter values) is explored. This range of a includes the value of a cr as given by Marzeion et al [2007Marzeion et al [ , 2009, and implies that the vertical diffusivity for a = 2 varies between 0.1 Á 10 À4 k 0.3 Á 10 À4 m 2 s À1 in the upwelling branch of the AMOC (i.e., between 450 and 800 m depth).…”

Hysteresis Behavior of the Atlantic Ocean Circulation Identified in Observational Data

“…This relatively short MOC adjustment time to convective changes is determined by the waves' propagation time (Dö scher et al 1994). Two separate states of the MOC, associated with two distinct configurations of the North Atlantic subpolar gyre, were also simulated in response to idealized carbon dioxide increase (Marzeion et al 2010). Unlike the case of an advectively generated hysteresis, which includes an off state of the overturning, the convectively generated hysteresis includes states of relatively strong and weak MOC.…”

Societal need for improved understanding of climate change, anthropogenic impacts, and geo-hazard warning drive development of ocean observatories in European Seas

“…Mediterranean Intermediate Water (MIW) entering the North Atlantic is warming and becoming more saline . Continued warming is anticipated to increase stratification in large areas of the ocean including the North Atlantic , Marzeion et al 2010. Related changes to heat content, wind stress, and circulation across European seas are also likely with cascading impacts into biogeochemistry and ecology.…”