An estimate of Southern Ocean volume in temperature-salinity (T-S) classes is made with a focus on cold water masses south of the Polar Front, at depths shallower than 2,000 m. The volumetric diagram for waters below 5° C shows a deficit at temperatures near 0° C and 34.2 psu, surrounded by a ring of larger volumes, related to water mass formation near the Polar Front and near the continental margins as well as mixing effects. The cold, fresh extreme of the Antarctic Intermediate Water T-S relation shows a volumetric maximum, apparently distinct from the interior T-S relationship. We identify this mode as Antarctic Mode Water with a volumetric maximum centered near 2.0° C and 33.9 psu. This maximum lies on a large-scale ridge of high volume representing the Antarctic Surface Water mixed layers south of the Polar Front, at the northern edge of the seasonal sea-ice cover. Additional maxima on the diagram near freezing temperatures seem to be related to processes operating on the slope and shelf, though the data coverage in these regions is much reduced. The origin of the Antarctic Mode Water is ultimately due to sea-ice melt, which systematically shifts the T-S relation of surface water to lower salinities, whereas its thickness and distribution is linked to circumpolar northward Ekman transport and the eddy fluxes of the Polar Front.