The intensification and shift of the annual North Atlantic Oscillation in a global warming scenario simulation

The impact of global warming on the annual North Atlantic Oscillation (NAO) is investigated with a global warming scenario simulation of the ECHAM4/OPYC3 coupled general circulation model. It is shown that the annual meridional pressure gradient over the North Atlantic is significantly strengthened, and the two centers of action of the NAO, the Icelandic low and the Azores high, are intensified and shifted northeastward by 10degrees to 20degrees in latitude and 30degrees to 40degrees in longitude in the global warming scenario. The shift of the centers of action leads to a failure in capturing the NAO change with the traditional definition of the NAO index. A modified index is introduced that allows for this shift, and exhibits a tendency toward the positive phase and an enhancement of its intensity. The intensification of Icelandic low is tied up with zonal mean state change, and the strengthened Azores high is related to the stationary wave change. The shift of the centers of action of the NAO is associated with the stationary wave change.