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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.

Abstract. This study evaluates the response of winter-average sea surface temperature (SST) to the winter North Atlantic Oscillation (NAO) simulated by 13 Coupled Model Intercomparison Project Phase 5 (CMIP5) Earth system models in the North Atlantic (NA) (0–65∘ N) on an interannual scale. Most of the models can reproduce an observed tripolar pattern of the response of the SST anomalies to the NAO on an interannual scale. The model bias is mainly reflected in the locations of the negative-response centers in the subpolar NA (45–65∘ N), which is mainly caused by the bias of the response of the SST anomalies to the NAO-driven turbulent heat flux (THF) anomalies. Although the influence of the sensible heat flux (SHF) on the SST is similar to that of the latent heat flux (LHF), it seems that the SHF may play a larger role in the response of the SST to the NAO, and the weak negative response of the SST anomalies to the NAO-driven LHF anomalies is mainly caused by the overestimated oceanic role in the interaction of the LHF and SST. Besides the THF, some other factors which may impact the relationship of the NAO and SST are discussed. The relationship of the NAO and SST is basically not affected by the heat meridional advection transports on an interannual timescale, but it may be influenced by the cutoffs of data filtering, the initial fields, and external-forcing data in some individual models, and in the tropical NA it can also be affected by the different definitions of the NAO indices.