[1] Variability of Antarctic Intermediate Water salinity in the South Atlantic is investigated on interannual and intradecadal timescales. Novel observations of slow, westward propagating salinity anomalies in Argo data are presented. The features have no corresponding signal in temperature and are anomalous in density. Analysis of 40 years of model output supports the existence of these westward propagating salinity anomalies and indicates that they are typical occurrences in time. The features are intensified in a latitude band around 30°S associated with the propagation of Agulhas rings. However, the features are much larger than Agulhas rings and occur on decadal timescales in the model. They propagate westward with speeds of 2.3 cm/s in observations, and 1.7 cm/s in model data. They are more consistent with planetary waves than with the advection of large‐scale salinity anomalies. The observation of these features has implications for the interpretation of salinity anomalies, such as the linking of hydrological cycle changes to salinity changes.