Influences of different definitions of the winter NAO index on NAO action centers and its relationship with SST

Winter North Atlantic Oscillation (NAO) indexes from observations based on various winter durations are compared. Results show that there are significant differences in the interannual and decadal variations of these NAO indexes. For the same data source, a different definition of winter duration can lead to different signs of NAO index in some years, which mainly appear to be in the period of decadal phase transition. The different winter durations induce different cycles of the observation-based NAO. The longer the winter duration, the stronger the decadal variation. The NAO defined by different winter durations also can generate different descriptions of the NAO action centers, including the position and movement. The longer the winter duration, the more southerly action centers appear to be. The movement of the action centers affects not only site-based NAO indexes but also those defined by other methods, such as empirical orthogonal function (EOF) analysis. The length of time used in EOF analysis has a great influence on the spatial pattern of the NAO mode, which results in a considerable difference between the corresponding NAO indexes. Regardless of which NAO index is used, the NAO-related SST anomalies show the same tripole pattern. The longer the winter duration, the more significant the relationship between the NAO and SST affected by the timescale of sea-air interaction.