by Rahmstorf (1996), the presence of this slow salt-advection feedback is critical to the existence of stable multiple equilibria.

Climate and Earth system models have been used to investigate the stability of the AMOC, in particular the number of stable states that the system can exist in, which is an important characteristic to know for fully understanding the climate system. Carefully designed non-linear modeling experiments using Earth system Models of Intermediate Complexity (EMICs; and also the FAMOUS AOGCM; Hawkins et al., 2011) have revealed a model-dependent threshold beyond which an active AMOC cannot be sustained (Rahmstorf et al., 2005; see Figure 2.2). However, analysis of the AMOC in the models that submitted simulations in support of the third phase of the Community Model Intercomparison Project1 (CMIP3; Meehl et al., 2007a) suggested that the CMIP3 models were overly stable (Drijfhout et al., 2011; Hofmann and Rahmstorf, 2009), i.e., that an abrupt change in the AMOC was not likely to be simulated in the models even if it were to be likely in reality.

Several studies (de Vries and Weber, 2005; Dijkstra, 2007; Weber et al., 2007; Huisman et al., 2010; Drijfhout et al., 2011; and Hawkins et al., 2011) have suggested that the sign of the net freshwater flux into the Atlantic across its southern boundary via the overturning circulation determines whether or not the AMOC is in a monostable or bistable regime. Observations suggest that the present day ocean resides in a bistable regime, thereby allowing for multiple equilibria and a stable “off” state of the AMOC (Hawkins et al., 2011). By examining the preindustrial control climate of the CMIP3 models, Drijfhout et al. (2011) found that the salt flux was mostly negative (implying a positive freshwater flux), indicating that these models were mostly in a monostable regime. This was not the case in the CMIP5 models where Weaver et al. (2012) found that 40 percent of the models were in a bistable regime throughout their integrations. Although this question of the number of stable states of the system is important for a complete understanding of the climate system, it is important to emphasize that regardless of this stability question, the CMIP5 models also show no evidence of an abrupt collapse for the 21st century.

In addition to the main threshold for a complete breakdown of the circulation, other thresholds may exist that involve more-limited changes, such as a cessation or diminishment of Labrador Sea deep water formation (Wood et al., 1999). Rapid melting of the Greenland ice sheet causes increases in freshwater runoff, potentially weakening the AMOC. None of the CMIP5 simulations include an interactive ice sheet component. However, Jungclaus et al. (2006), with parameterized freshwater melt as high

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1http://www-pcmdi.llnl.gov/ipcc/about_ipcc.php.



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