9.5.1.3 Changes in the Meridional Overturning Circulation

It is possible that anthropogenic and natural forcing may have influenced the MOC in the Atlantic (see also Box 5.1). One possible oceanic consequence of climate change is a slowing down or even halting of the MOC. An estimate of the overturning circulation and associated heat transport based on a trans-Atlantic section along latitude 25°N indicates that the Atlantic MOC has slowed by about 30% over five samples taken between 1957 and 2004 (Bryden et al., 2005), although given the infrequent sampling and considerable variability it is not clear whether the trend estimate is robust (Box 5.1). Freshening of North East Atlantic Deep Water has been observed (Dickson et al., 2002; Curry et al., 2003; Figure 5.6) and has been interpreted as being consistent with an enhanced difference between precipitation and evaporation at high latitudes and a possible slowing down of the MOC. Wu et al. (2004) show that the observed freshening trend is well reproduced by an ensemble of HadCM3 simulations that includes both anthropogenic and natural forcings, but this freshening coincides with a strengthening rather than a weakening trend in the MOC. Therefore, this analysis is not consistent with an interpretation of the observed freshening trends in the North Atlantic as an early signal of a slowdown of the thermohaline circulation. Dickson et al. (2002) propose a possible role for the Arctic in driving the observed freshening of the subpolar North Atlantic. Wu et al. (2005) show that observed increases in arctic river flow (Peterson et al., 2002) are well simulated by HadCM3 including anthropogenic and natural forcings and propose that this increase is anthropogenic, since it is not seen in HadCM3 simulations including just natural forcing factors. However, the relationship between this increased source of freshwater and freshening in the Labrador Sea is not clear in the HadCM3 simulations, since Wu et al. (2007) find that recent freshening in the Labrador Sea is simulated by the model when it is driven by natural rather than anthropogenic forcings. Importantly, freshening is also associated with decadal and multi-decadal variability, with links to the NAO (Box 5.1) and the AMO (Box 5.1; Vellinga and Wu, 2004; Knight et al., 2005).