THE ATMOSPHERIC SIGNATURE OF THE NORTH ATLANTIC OSCILLATION

The North Atlantic Oscillation (NAO) is thought to be the major mode of variability in the North Atlantic storm track. It is associated with a dipolar anomaly pattern in the sea level pressure over the Atlantic with opposing centers of action over Iceland and over the Azores. The NAO also has a large impact on the Northern Hemisphere circulation, though the dominant mode of variability for the whole hemisphere has been identified recently as the so-called the Arctic Oscillation (AO). The sea level pressure pattern associated with the AO is more zonally symmetric and has a pronounced center of action over the Pacific. In the presentation, the relationship between the two modes of variability will be put forward. At the same time some comments on the physical interpretation of the two will be made. Evidence is shown that indicates that the NAO, as opposed to the AO, is the dominant physical mode of variability in the Northern Hemisphere midlatitude circulation.