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The Atlantic Meridional Overturning Circulation
Reference
Baehr, J., Haak, H., Alderson, S., Cunningham, S.A., Jungclaus, J.H. and Marotzke, J. 2007. Timely detection of changes in the meridional overturning circulation at 26°N in the Atlantic. Journal of Climate 20: 5827-5841.

Background
The authors note that several modeling studies suggest that the Meridional Overturning Circulation (MOC) of the North Atlantic Ocean "is potentially sensitive to anthropogenic climate change," and that "a weakening or collapse of the Atlantic MOC is expected to entail a reduction in the North Atlantic heat transport, leading to a significant cooling over the North Atlantic and its adjacent regions," a virtual-world finding that has led to all sorts of rabid climate-alarmist speculation.

What was done
Baehr et al. investigated how quickly changes in the North Atlantic MOC could be detected by projecting simulated observations onto a time-independent spatial pattern of natural variability, which was derived by regressing the zonal density gradient along 26°N against the strength of the MOC at 26°N within a model-based control climate simulation, which pattern was compared against observed anomalies found between the 1957 and 2004 hydrographic occupations of this latitudinal section.

What was learned
Looking to the future, this exercise revealed that Atlantic MOC changes could likely be detected with 95% reliability after about 30 years, using continuous observations of zonal density gradients that can be obtained from a recently-deployed monitoring array, while looking to the past, they report that "for the five hydrographic occupations of the 26°N transect, none of the analyzed depth ranges shows a significant trend between 1957 and 2004, implying that there was no MOC trend over the past 50 years."

What it means
In spite of climate model projections of a weakening or total collapse of the Atlantic MOC in response to global warming, which climate alarmists claim to have produced late-20th-century temperatures that were unprecedented over the last two millennia or more, there is still no compelling or even suggestive evidence for such a response in the real world's real Atlantic Ocean.

Reviewed 2 April 2008