The relative abundance of diatom taxa in 216 core tops has been quantified and compared, using cluster analysis, with assemblages in six cores located beneath different water masses of the North Pacific Ocean. All but one sequence span at least 1.5 m.y., (early Pleistocene to Holocene), and one extends to 2.6 Ma. The quantitative abundance of each taxon and the space-time occurrence of each cluster are interpreted as tracers of oceanographic conditions. In addition to high-frequency (glacial-interglacial) oscillations, the records display rapid steplike changes at 2.4 Ma, 1.6–1.5 Ma, 1.0 Ma, 0.8–0.7 Ma, and 0.35 Ma. Two secondary transitions, recorded only in the northwestern cores, occur at 2.0 Ma and 1.1 Ma. We conclude that both the high-frequency oscillations and the stepwise transitions reflect variations in extent of subarctic water masses, under the influence of the Siberian high-pressure and Aleutian low-pressure atmospheric cells. We further conclude that (1) the subarctic water mass has developed progressively and attained its modern structure only in the late Pleistocene; (2) some stages of this evolution may have occurred earlier in the eastern subarctic; (3) the Holocene is an atypical “warm” extreme, so that the present is not a very good key to the past; (4) there were several glaciations between 2.4 and 2.0 Ma; and (5) the Subtropical Gyre is more conservative than the Subarctic Gyre, and major changes in the region have been due to forcing from the north. We propose a conceptual model of oscillating metastable climatic modes, maintained by internal feedback; and suggest that steplike changes may occur when one set of boundary conditions is temporarily exceeded.