The Semi-Annual Oscillation (SAO), first detected by Van Loon (1967), is a marked half-yearly variations of the circumpolar trough of low pressure over the Antartctic Ocean. The mechanisms that explain the SAO are related to the temperature gradient in the middle troposphere, between 50S and 65S latitudes, aand to the heat budget of the oceanic upper layers. The semiannual temperature gradient modulates the annual cycle of zonal winds and surface pressure in middle and high southern latitudes. The magnitude of the SAO as seen from sea level pressure has been changing since 1980s, reducing the amplitude of the second harmonic in annual cycle. This work aims to analyze the changes occurred in SAO using the NCEP/NCAR reanalyses, from the period 1948 to 2002. The Southern Ocean is characterized by its seasonal and annual climatologies. The annual cycle of zonally averaged sea level pressure for 50S latitude presents a maximum in march and august, while for 65S it exhibits a minimum in march and october. The harmonic analyses reveals a significant second harmonic in sea level pressure over some regions of Southern Ocean. The anomalies time seriesrelative to the mean annual cycle show accentuated negative trends in sea level pressure at 50S and 65S latitudes. Spectral analyses of this time series shows predominant interannual variability with a period of approximately 4 years at 50S and 65S. The difference between these two latitudes reveals a strenghtening of this signal. This variability could be associated with the Antartic Circumpolar Wave that has a similar period. Significant peaks at lower frequencies are also evident in the spectral analyses with a period of oscillation of 17 years for both latitudes and their difference. The decadal climatology of sea level pressure shows a weakening of SAO phenomenon that indicates an interference of interannual signals. It can be showed that this change is associated with a warming tendency at 65S latitude.