The annual and interannual variations of sea level pressure (SLP) over the Southern Ocean are investigated using the NCEP/NCAR reanalysis data from 1968 to 2005. Results show that the weakening of the Semiannual Oscillation (SAO) has continued to present day but does not occur homogeneously across all longitudes. While the second harmonic of SLP has decreased over the South Pacific Ocean, its signal is still strong over the Atlantic sector up to 2005. This suggests that the mechanism influencing the amplitude of the SAO over South Atlantic Ocean is different to that in the Pacific and Indian oceans. Spectral analysis of zonally averaged SLP at 50¿S and 65¿S shows that significant energy is associated with the period of 4 years. The same period is found for the time series of the zonally averaged position of the circumpolar trough and for the meridional temperature gradient at 500 hPa. This interannual signal is even stronger in the time series of the SLP difference between those two latitudes: the SAO index. Wavelet analysis of the SLP time series at mid and high southern latitudes showed significant interannual energy, also with a 4 year period, at 50¿S, 65¿S, and in the time series of their difference. When analyzed by ocean basins, spectral analysis reveal that there is no interannual variability at the 4 year period for the Atlantic, indicating that the known weakening of the SAO is basin-dependent.