Southern Ocean dominance and basin asymmetry in centennial-scale bottom-water return

Abstract. Return of abyssal bottom waters toward the upper ocean is a key component of overturning closure because it reconnects the deep ocean with surface-influenced layers and helps regulate the redistribution of heat, carbon, and nutrients. Yet the large-scale organization and timing of this return remain incompletely quantified. Here we combine the 1/12° GLORYS12 global ocean reanalysis with Constituent-oriented Age and Residence time Theory to diagnose bottom-water concentration, ventilation thickness, concentration-weighted diapycnal transport, and water age within a single framework. The results reveal a globally connected but strongly basin-asymmetric return structure. The Southern Ocean forms the dominant return hub, where strong cross-isopycnal exchange and broad outcropping pathways provide the principal route through which abyssal waters reach lighter, upper-ocean-connected density classes within the centennial diagnostic window. Beyond this circumpolar branch, the Atlantic exhibits broad and comparatively efficient upward penetration, the Pacific retains older bottom waters and weaker ascent through much of the basin, and the Indian Ocean displays more spatially focused subtropical and ridge-associated return corridors. These results show that abyssal return is not a single diffuse background upwelling branch, but a basin-dependent structure shaped by Southern Ocean upwelling, topographic mixing, equatorial exchange, and basin-scale overturning geometry. Together, the diagnostics provide a global view of where bottom-water influence rises, how strongly it fills the water column, and how renewal timescales differ among basins.