Abstract Southern Ocean mode waters act as a major sink for both anthropogenic heat and carbon, yet the mechanistic understanding of their variability remains incomplete. We use observational-based data to examine an oceanic pathway that begins at the Indonesian Throughflow (ITF), extends through the South Equatorial Current and Agulhas Current, and ultimately encircles the Southern Ocean via the Antarctic Circumpolar Current (ACC). Our analysis reveals that interannual temperature-salinity variations observed in the ITF region propagate into the Southern Ocean along this pathway, taking approximately three years to propagate from the Indonesian Archipelago to the Agulhas Retroflection region. Lagrangian particle tracking reveals that both Agulhas Current waters and waters from the Atlantic and further poleward shape the temperature-salinity characteristics of ACC waters in the western Indian Ocean, although the relative contributions of these oceanic pathways and atmospheric forcings have yet to be quantified. In the western Indian Ocean, mixed-layer depth (MLD) and Subantarctic Mode Water (SAMW) volume and temperature-salinity properties all show strong quasi-biennial variability. These anomalies are advected eastward with the ACC as vertically coherent structures extending over the top 1000 m, reemerging in the eastern Indian Ocean approximately one year later, reaching the southeast Pacific in 4–5 years and further to the east of Drake Passage a year afterward. The propagation speed of these anomalies is around 40–55° longitude per year along the ACC, or equivalently 0.15–0.2 m s −1 , matching the advective speed of water particles driven by mean geostrophic currents in the ACC. In specific years, the eastward advection of anomalies can complete a full circumpolar circuit around the Southern Ocean.
Li et al. (Wed,) studied this question.