Tuesday, 11 February 2003: 3:45 PM
Arctic Oscillation versus North Atlantic Oscillation: Arguments based on the principal component analysis methodology
The North Atlantic Oscillation (NAO) has been recognized for a long time as the most important mode of circulation variability in the Northern Hemisphere (NH). Recently, an alternative concept of the Arctic Oscillation (AO) has been introduced. This stimulated a discussion on which of the two concepts is more suitable to describing the NH circulation variability and should be preferred. In the contribution, we examine the AO / NAO patterns obtained by principal component analysis (PCA) of NH wintertime monthly sea level pressure (SLP) and 500 hPa height anomalies. It is shown that the AO pattern appears only for unrotated PCA of the covariance matrix. Unrotated PCA of the correlation matrix results in a pattern resembling the AO, but with the Aleutian center considerably weakened, spatially more extensive and displaced southwards. In rotated PCA (regardless of the number of rotated PCs within reasonable limits), the NAO is detected. Based on the concept of a simple structure, which is crucial in the interpretation of principal components, we argue that only the rotated, not the unrotated components should be interpreted. It is also found that the Aleutian center is included in the AO pattern because of high SLP variance in its region, not because of joint variability with the other two centers of action. It can therefore be concluded that the AO does not appear to be an intrinsic variability mode and that in interpreting the NH SLP variability, the NAO should be given preference to the AO.