A NORTHERN HEMISPHERE ANNULAR MODE AS THE COMBINATION OF THE NAO AND THE PNA

This paper demonstrates that an annular mode can be constructed from the combination of the North Atlantic Oscillation (NAO) and the Pacific/North American (PNA) patterns. The quasi-annularity, meridional dipole and vertical barotropy of the constructed annular mode resemble those of the Atlantic Oscillation (AO) pattern. It is also a dominant mode in terms of the variance contribution. Moreover, its temporal correlation with the AO is quite strong. This new annular mode has the advantage over the AO in that it incorporates a large portion of the PNA and makes the center of action in the Pacific stronger and more physically relevant than that of the AO. Or, more generally, it may be regarded as a physical mode unlike the AO. The results of this study also indicate the NAO-PNA perspective contains most of the information of the AO, whereas the AO perspective only contains about half of the information of the NAO-PNA. Consequently, the NAO-PNA perspective is regarded by the authors to be more comprehensive than that of the AO.