Interdecadal water mass changes in the Indian - Western Pacific sectors of the Southern Ocean were investigated using the Japanese Antarctic Research Expeditions and historical hydrographic observations from the 1950s to 1990s. Freshening and cooling occurred on the neutral density surfaces of 27.0 kg·m⁻³ equatorward of Sub-Antarctic Front. Results for the area south of the Polar Front show warm and saline anomalies and oxygen decreases on the surfaces around 27.9 kg·m⁻³, which correspond to the Upper Circumpolar Deep Water. These latter anomalies are most simply explained by the mixing of these shallow waters with warmer and fresher surface waters. Steric sea level has also increased with an average change of 1mm·yr⁻¹ from the 1970s to 1990s. The changes are larger north of the Sub-Antarctic Front, implying a strengthening of the Antarctic Circumpolar Current. It appears that the observed changes are consistent with the results from coupled climate model results for a similar period.