This paper has a nice discussion of the definition of the Arctic Oscillation as compared to the North Atlantic Oscillation and the Northern Annular Mode as well as nuances of those definitions.
The paper examines CMIP-2 models which is two generations before the CMIP-5 models used in the IPCC assessment report which should be released in 2013. Hence, update with current models is needed.
The paper substantiates that the already observed changes in the North Atlantic Oscillation has a discernable greenhouse gas signal and suggests that this trend will continue into the future with the positive mode becoming more common.
There are differences in the trend based on the definition of the index.
Physical mechanisms are linked, at this writing, to changes in tropical sea surface temperature and modulation due to stratospheric processes.