Atlantic Meridional OverturningSST Feedback

Some modeling studies have noted that over the course of the next few centuries, the Atlantic MOC may move to an off state in response to increasing greenhouse gases (Cubasch et al., 2001). Some, on the other hand, find no such reduction (Gent, 2001; Latif et al., 2000) and others find very little reduction (Cubasch et al., 2001). The reduction in Atlantic MOC strength associated with increasing greenhouse gases leads to a negative feedback to warming in and around the North Atlantic. That is, through reducing the transport of heat from low to high latitudes, SSTs are cooler than they would otherwise be if the Atlantic MOC were left unchanged. As such, warming is reduced over and downstream of the North Atlantic. It is important to note that in all models where the Atlantic MOC weakens, warming still occurs downstream over Europe due to the radiative forcing of increasing greenhouse gases. In different models the competing effects of differential heat and freshwater flux forcing between low and high latitudes fundamentally determine the MOC-SST feedback.

There is some suggestion (Stocker, 1998; Broecker; 1998) that a reduced cross-equatorial heat transport to the North Atlantic with a reduced Atlantic MOC would at quasi-equilibrium lead to enhanced SSTs in the South Atlantic. Thus, a reduction in Atlantic MOC and the stabilization of South Atlantic surface water would suggest a positive feedback to anthropogenic warming in and around the South Atlantic.

Many future projections show that once the radiative forcing is held fixed, reestablishment of the Atlantic MOC occurs at a state similar to that of the present day. During this reestablishment phase the Atlantic MOC acts as a positive feedback to warming in and around the North Atlantic and at equilibrium there is close to zero net feedback. Whether reestablishment of the MOC occurs depends on the parameterization of ocean mixing (Manabe and Stouffer, 1999), as well as the emission rate and eventual stabilization scenario for atmospheric greenhouse gases (Stocker and Schmittner, 1997).

The fundamental MOC-SST feedback is well understood although different models yield different projections in the strength of the MOC over the twenty-first century. This is not because the underlying feedback is unknown but because the feedback is ultimately linked to the air-sea exchanges of heat and freshwater. The basic physics of the latter is still a matter of investigation, and the present observational network needed to constrain the physics and its parameterization in models is far from ideal. The recommendation regarding air-sea exchanges of fresh water in the preceding section is thus critical to better understanding the local feedbacks that might be associated with changes in the Atlantic meridional overturning.

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