H. Jesse Smith It has been suggested that Atlantic meridional overturning circulation (MOC; the northward flow of water in the upper kilometer of the north Atlantic Ocean and southward flow below) could be affected by global warming, in turn substantially affecting the climate of the Northern Hemisphere, particularly in Europe. Observations of the strength of the circulation, taken over ~1-month periods in different seasons of each of the years 1957, 1981, 1992, 1998, and 2004, have been used to argue that the MOC has slowed by about 30% over that interval. Searl et al. have now analyzed simulations of the MOC from the HadCM3 climate model and found no significant trend in the strength of the MOC over the period in question. Furthermore, variations in the MOC as large as 30% were very unusual.
The discrepancy between the model and measurements may be due not to inherent limitations of the model, but rather to the small number of observations, short-term variability of the MOC, and measurement errors, which together inhibit accurate estimates of multidecadal trends from the data. Indeed, after taking all such factors into account, the HadCM3 results are fully consistent with the observations. To determine with confidence how the MOC may be changing on interannual and seasonal time scales in the warming world, continuous monitoring of water transport appears to be needed. -- HJS
Geophys. Res. Lett. 34, L03610 (2007).