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153.6.4 The Antarctic Meridional Overturning Circulation

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Sinking of AABW near Antarctica supplies about half of the deep and abyssal waters in the global ocean (Orsi et al., 1999). AABW spreads northward as part of the global overturning circulation and ventilates the bottom-most portions of much of the ocean. Observed widespread warming of AABW in recent decades (Section 3.5.4) implies a con- comitant reduction in its northward spread. Reductions of 1 to 4 Sv in northward transports of AABW across 24°N have been estimated by geostrophic calculations using repeat oceanographic section data between 1981 and 2010 in the North Atlantic Ocean (Johnson et al., 2008b; Frajka-Williams et al., 2011) and between 1985 and 2005 in the North Pacific (Kouketsu et al., 2009). A global full-depth ocean data assimilation study shows a reduction of northward AABW flow across 35°S of >2 Sv in the South Pacific starting around 1985 and >1 Sv in the western South Atlantic since around 1975 (Kouketsu et al., 2011). This reduction is consistent with the contraction in volume of AABW (Purkey and Johnson, 2012) discussed in Section 3.5.4.

Several model studies have suggested that changes in wind stress over the Southern Ocean (Section 3.4) may drive a change in the Southern Ocean overturning circulation (e.g., Le Quéré et al., 2007). A recent analysis of changes in chlorofluorocarbon (CFC) concentrations in the Southern Ocean supports the idea that the overturning cell formed by upwelling of deep water and sinking of intermediate waters has slowed, but does not quantify the change in transport (Waugh et al., 2013).

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