Detection of anthropogenic weakening of the Atlantic meridional overturning circulation ('MOC') may be improved by monitoring a limited number of key oceanic variables. We have identified a set of potentially useful ocean hydrographic observations in the climate general circulation model HadCM3. When the model is forced by an anthropogenic climate forcing scenario for the 21st century, a pattern is found that captures co-variability of these variables and strength of the MOC. Fingerprints of this pattern emerge clearly on time scales of 10 years from background noise caused by internal variability. If similar results are found in other climate models this will provide useful information for the design of an MOC observing system.