Bathymetry Imposes a Global Pattern of Cross-Front Transport in the Southern Ocean

The Southern Ocean plays an integral role in the global climate system, exchanging heat, salt, and carbon throughout the major ocean basins via the deep, fast-flowing Antarctic Circumpolar Current. The Antarctic Circumpolar Current is bounded by spatiotemporally varying fronts that partition distinct water masses. Locating and quantifying cross- front transport is crucial for understanding global patterns of interbasin exchange; however, this is challenging because fronts are typically defined by hydrographic properties, such as temperature or salinity, which are subject to external sources and sinks, rather than by properties of the flow itself. Here, we show that Southern Ocean fronts can be characterized by material contours that minimize deformation and cross-contour mixing over a prescribed time window. By tracking material contours in a sequence of such windows, we quantify cross-front transport and show that the fronts exhibit a global pattern of alternating poleward and equatorward transport caused by frontal meandering in regions downstream of prominent seafloor obstacles. These results highlight the importance of bathymetric features in controlling Southern Ocean dynamics and interbasin exchange.