The Pacific Center of Action of the Northern Hemisphere Annular Mode: Real or Artifact?

The leading empirical orthogonal function (EOF) of the sea level pressure (SLP) field, referred to as the Arctic Oscillation (AO) or Northern Hemisphere annular mode (NAM), consists of a dipole between the polar cap region and the surrounding zonal ring centered along 458N. Embedded within the outer ring are centers of action over the Euro-Atlantic and Pacific sectors in which SLP fluctuates in phase. That the observed SLP fluctuations at these two centers of action are virtually uncorrelated raises the question of whether the Pacific center in the annular mode could be an artifact of EOF analysis. It is argued that sea level pressure fluctuations at the Pacific and Euro-Atlantic centers of action of the AO/ NAM would be more strongly correlated were it not for the fact that SLP variability over the North Pacific is dominated by a pattern in which fluctuations over the North Atlantic and North Pacific are inversely related. Evidence of the coexistence of such a pattern, which resembles an augmented version of the Pacific–North American pattern, is presented.