Historical hydrographic data from the Southern Ocean are projected into a baroclinic stream function space, generating a three-dimensional gravest empirical mode (GEM) parameterized by pressure, geopotential height, and longitude. The low-dimensional GEM projection is intrinsically steady, vertically coherent, and equivalent barotropic. The GEM fields capture more than 97% of the total density and temperature variance in the Antarctic Circumpolar Current region. ¿ 2001 American Geophysical Union