Why should the AMOC Change as Part of Ongoing Climate Change

In their fourth comprehensive climate assessment the Intergovernmental Panel of Climate Change (IPCC) considers it 'very likely' that the AMOC will have gradually slowed down by the end of the twenty-first century as a consequence of the Greenhouse climate [38]. Climate model projections all of which are based on the greenhouse gas emission scenario A1B [38] predict a reduction between 0 and 50% by the year 2100 [39], such that a complete (and possibly irreversible) AMOC shutdown is considered 'unlikely'. The future evolution of the AMOC in several selected climate model projections is shown in Fig. 4.

Future greenhouse gas emission scenarios carry a high level of uncertainty as they depend on parameters such as economic and population growth, technology development and basic political and social conditions, all of which are difficult to predict. Also, none of the present-day climate models have a sufficiently fine spatial resolution to resolve the processes that govern either the

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FIGURE 4 Evolution of the AMOC as defined by the maximum overturning at 24°N for the period 1900 2100 in nine different climate models forced with the greenhouse gas emission sce nario A1B. The AMOC evolutions of integrations with a skill score larger than one are shown as solid lines, those from models with a smaller skill score as dashed lines [39]. The weighted ensemble mean is shown by the thick black curve together with the weighted standard deviations (thin black lines). This figure was published by Schmittner et al. [39].

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FIGURE 4 Evolution of the AMOC as defined by the maximum overturning at 24°N for the period 1900 2100 in nine different climate models forced with the greenhouse gas emission sce nario A1B. The AMOC evolutions of integrations with a skill score larger than one are shown as solid lines, those from models with a smaller skill score as dashed lines [39]. The weighted ensemble mean is shown by the thick black curve together with the weighted standard deviations (thin black lines). This figure was published by Schmittner et al. [39].

sinking or the rising, and have to rely on parameterisations instead. Both aspects may significantly add to the uncertainty in the prediction of the long-term AMOC evolution.

Reasons for a long-term greenhouse gas induced reduction of the strength of the AMOC include straightforward effects, such as warming of surface waters, melting of continental ice sheets acting to reduce high latitude salinity (a mechanism not included in many climate models), and intensification of the hydrological cycle [40]. All of these act to impede deep-water formation.

More complex feedbacks (that either stabilise or de-stabilise the AMOC) also involve wind field changes in the deep-water formation regions leading to buoyancy flux anomalies [41] and oceanic teleconnections driven by changes in the freshwater budget of the tropical Atlantic and South Atlantic [36,42 45].

Huang et al. [46] found a significant increase in wind stress and its energy input into the Southern Ocean between 1950 and 2000, which may have been caused by decreasing stratospheric ozone concentrations [47]. Using a climate model, Shindall and Schmidt [48] predicted the positive wind stress trend over the Southern Ocean, which will prevail until 2100 as a consequence of anthropogenic Greenhouse gas induced global warming. While oceanographers have not yet been able to establish a relationship between the multidecadal wind stress increase and changes in the ocean circulation which may partly be a consequence of unavailability of a sufficient number of suitable observations over the last 50 years one might speculate that in the future, beyond the end of this century, in a then different climate 'pulling', the AMOC in the Southern Ocean might gain in importance relative to 'pushing' it in the North Atlantic [49].

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