Previous studies have focused exclusively on the northern center of the North Atlantic Oscillation (NAO) over two twenty-year periods (i.e. 1958-1977 and 1978-1997) and found that during the half-century reanalysis period it shifted eastward during recent couple of decades. This study focuses on the displacements of both NAO centers of action in the 20th Century Reanalysis data for December through March, 1871 through 2008. The slow movement of the NAO centers of action is identified after applying EOF analysis to sea level pressure over 20-year running windows that are moved by one year for each new analysis period.

Results from our analysis indicate that variability in the position of the southern center of action is more pronounced than in the northern center over the longer analysis period. We find that the southern center moved along a northeast-southwest axis, which is different from the east-west movement of the northern center. Over all the 20-year periods, the movement of two nodes followed a general pattern, that is, when one center moved, another center stayed around. The displacement of both NAO centers of action was prevalent with a cycle of 60-70 years.

However, the definition of the conventional NAO index assumes implicitly that the NAO is stationary over a long period of time. In order to better interpret the variability of the relative location between the northern and southern center, we define a new index to describe the tilting degree of the two centers on the decadal time scale. The performance of displaying the corresponding pattern in geophysical fields (e.g. wind and temperature) to the shifting NAO by the new index is investigated through composite analysis. We find that the new index can capture the influence of the displacement of the NAO on geophysical fields especially when two centers of action are more tilted, which is not well represented by the conventional NAO index. The future work will explore and illustrate the relationship between the conventional NAO index and the new index in the more general instance.