Poleward Migration of the Subtropical Front and Its Implications for the Tasman Outflow and Antarctic Circumpolar Current in the Southern Ocean

The Southern Ocean plays a critical role in global climate regulation, acting as a key pathway for heat, momentum, and biochemical transport. Using expanded hydrographic observations from the WOCE/CLIVAR SR03 section (1991–2018), a choke point between Tasmania and Antarctica, along with high-resolution GLORYS12 reanalysis data (1993–2018), we investigate long-term changes in the Antarctic Circumpolar Current (ACC) and Tasman Outflow (TO) and their relationship with the southward migration of the Subtropical Front (STF). Our results reveal a significant southward migration of the STF, while the Subantarctic Front remains meridionally stable. This STF migration is associated with an intensified TO and strengthened Ekman convergence, leading to enhanced subduction of cold, low-salinity intermediate water in the Subantarctic Zone and increased subsurface warming (0.4–0.8°C per decade) north of the ACC. The TO has strengthened significantly (+3.1 Sv per decade), while the ACC’s geostrophic transport shows an increasing trend (+3.2 Sv per decade), indicating the poleward expansion of subtropical circulation towards Antarctica. Additionally, the correlation between TO transport and the Southern Annular Mode (SAM) index (r = 0.48, p < 0.05) suggests that westerly wind variability plays a key role in modulating ocean transport in this region. Our findings emphasize the importance of STF migration and TO expansion in shaping Southern Ocean circulation under climate change. The poleward shift of westerlies and subtropical systems underscores the ongoing sensitivity of transport processes to global warming. Further research with extended observations and high-resolution climate models is needed to refine projections of circulation changes and their impact on oceanic heat transport in the Southern Ocean.