The Southern Ocean (SO) surface westerly wind stress plays a fundamental role in driving the Antarctic Circumpolar Current and the global meridional overturning circulation. Here we investigate the contributions of atmospheric wind fluctuations to the mean, variability and trend of SO wind stress over the last four decades using NCEP and ERA-Interim reanalysis products. Including wind variability at synoptic frequencies (2-8 days) and higher in the stress calculation is found to increase the strength of the mean SO wind stress by almost 40% in both reanalysis products. The Southern Annular Mode index is found to be a good indicator for the strength of the mean wind and mean wind stress, but not as good an indicator for wind fluctuations, at least for the chosen study period. Large discrepancies between reanalysis products emerge regarding the contributions of wind fluctuations to the strengthening trend of SO wind stress. Between one-third and half of the stress trend in NCEP can be explained by the increase in the intensity of wind fluctuations, while the stress trend in ERA-Interim is due entirely to the increasing strength of the mean westerly wind. This trend discrepancy may have important climatic implications since the sensitivity of SO circulation to wind stress changes depends strongly on how these stress changes are brought about. Given the important role of wind fluctuations in shaping the SO wind stress, studies of the SO response to wind stress changes need to account for changes of wind fluctuations in the past and future.