We investigated the distribution of the deep mixed layer in the Southern Ocean, where Subantarctic Mode Water (SAMW) forms, by using Argo profiling floats and the ocean general circulation model for the Earth simulator (OFES). The mixed-layer depth (MLD) generally increased eastward from 50°–180°E and from 180°–80°W, with distinct local maxima near major bathymetric features. A sudden transition from deep to shallow mixed layer was found along the equatorward limit of the deep mixed layer. This diagnosed MLD front coincides with the line of u _g · σ _s = 0, indicating the distributions of the general deepening and local maxima of MLD were affected by near-surface geostrophic flow. The flow is steered by the major bottom topographies, and the local MLD maxima occurred where the flow direction was deflected southward. This implies a link between the bathymetric features and formation of thick SAMW.