A three dimensional descriptaion of the two-degree discontinuity (TDD) in the North Atlantic western basin is made from the TTO and1981 North Atlantic section data. In 1981 this break in the &thgr;-S slope of the Atlantic deep waters is found at &thgr;=1.90¿ C, S=34.895¿/00, near the western flank of the basin, and it seems to follow an isopycnal at &sgr;4 =45.89¿/00. It encounters the western boundary at the level of the Western Boundary Undercurrent (WBUC). Based on nutrient data, the TDD is more sharply defined along the western flank of the basin than along the Mid-Atlantic Ridge, where the TDD water is fresher and colder and where one finds more Antarctic Bottom Water (AABW) above the discontinuity. The TDD is consistent with vertical mixing being dominantly a boundary process; along the western flank of the basin, upward mixing of AABW is stopped at the WBUC level by the strong southward flow of North Atlantic Deep Water (NADW). Along the ridge, boundary mixing allows AABW to mix upward with waters from shallower levels.