Book 2.indb - US Climate Change Science Program

Abrupt Climate Changeinduced reduction in the AMOC (Alley andAgustdottir, 2005). There is now evidence ofa weakening of the overflows in the NorthAtlantic from sediment grain size and magneticproperties (Ellison et al., 2006; Kleiven et al.,2008), and also a replacement of NADW (withhigh δ 13 C ratios) by AABW (with low δ 13 Cratios) in the deep North Atlantic (Kleiven etal., 2008).While many of the deep-sea sediment recordsare only able to resolve changes on millennial tocentennial time scales, a recent study (Boessenkoolet al., 2007) reconstructs the strength of theIceland-Scotland overflow on sub-decadal timescales over the last 230 years. This grain-sizebasedstudy suggests that the recent weakeningover the last decades falls mostly within therange of its variability over the period of study.This work shows that paleoceanographic datamay, in some locations, be used to extend theinstrumental record of decadal- and centennialscalevariability.4.7 SummaryWe now have compelling evidence from avariety of paleoclimate proxies that the AMOCexisted in a different state during the LGM,providing concrete evidence that the AMOCchanged in association with the lower CO 2and presence of the continental ice sheets. TheLGM can be used to test the response of AMOCin coupled ocean atmosphere models to thesechanges (Sec. 5). We also have strong evidencefor abrupt changes in the AMOC during the lastdeglaciation and during the Heinrich events,although the relation between these changesand known freshwater forcings is not alwaysclear (Box 4.3). Better constraining both themagnitude and location of the freshwater perturbationsthat may have caused these changesin the AMOC will help to further refine themodels, enabling better predictions of futureabrupt changes in the AMOC. The relativelymodest AMOC variability during the Holocenepresents a challenge for the paleoclimate proxiesand archives, but further progress in this area isimportant, as it will help establish the range ofnatural variability from which to compare anyongoing changes in the AMOC.5. How Well Do theCurrent Coupled Ocean-Atmosphere ModelsSimulate the OverturningCirculation?Coupled ocean-atmosphere models are commonlyused to make projections of how theAMOC might change in future decades.Confidence in these models can be improvedby making comparisons of the AMOC bothbetween models and between models andobservational data. Even though the scarcityof observations presents a major challenge,it is apparent that significant mismatches arepresent and that continued efforts are neededto improve the skill of coupled models. Thissection reviews simulations of the present-day(Sec. 5.1), Last Glacial Maximum (Sec. 5.2), andtransient events of the past (Sec. 5.3). Modelprojections of future changes in the AMOC arepresented in Section 7.5.1 Present-Day SimulationsA common model-model and model-data comparisonuses the mean strength of the AMOC.Observational estimates are derived fromeither hydrographic data (Sec. 3.3; Ganachaud,2003a; Talley et al., 2003; Lumpkin and Speer,2007) or inventories of chlorofluorocarbontracers in the ocean (Smethie and Fine, 2001).The estimates are consistent with each other andsuggest a mean overturning of about 15–18 Svwith errors of about 2–5 Sv.Coupled atmosphere-ocean models usingmodern boundary conditions yield a wide rangeof values for overturning strength, which isusually defined as the maximum meridionaloverturning streamfunction value in the NorthAtlantic excluding the surface circulation.While the maximum overturning streamfunctionis not directly observable, it is a very usefulmetric for model intercomparisons. Present-daycontrol (i.e., fixed forcing) simulations yieldaverage AMOC intensities from model to modelbetween 12 and 26 Sv (Fig. 4.11; Stouffer et al.,2006), while simulations of the 20th centurythat include historical variations in forcinghave a range from 10 to 30 Sv (Randall et al.,145