Interannual changes of the ocean wave variability in the North Atlantic and in the North Sea

A high-resolution ocean wave hindcast for 1981 to 1993 with the WAM wave model is used to study the mean sea state and its dominant modes of variability in the North Atlantic and in the North Sea, The wind and wave data share high correlation coefficients with the North Atlantic Oscillation (NAO) index of the winter season and rather low correlation coefficients with the monthly NAO index, The low correlations are induced by the different modes of variability, The NAO index varies at semiannual and shorter periods and the wind and wave data vary with the seasonal cycle, In addition, considerably unsteady variability is apparent at synoptic scales. The dominant modes of that variability, retrieved by a wavelet analysis, are similar for wind and wave data of the same year but may change considerably from year to year. From 1987 to 1989, when the NAO index increases, the dominant period of variability also increases. The same trend is inferred from wind and wave measurements at Ocean Weather Station M. In contrast, the dominant periods inferred from data of the southern North Sea show a reverse trend, This indicates that the wave variability in the North Sea is reduced when the westerlies are more zonal, and is intensified when the westerlies are less zonal through more meridional contributions initiated by the Northeast Atlantic wind and wave variability.