An essential element of modern ocean circulation and climate is the Atlantic meridional overturning circulation(AMOC), which includes deep-water formation in the subarctic North Atlantic. However, a comparable overturningcirculation is absent in the Pacific, the world’s largest ocean, where relatively fresh surface waters inhibit North Pacificdeep convection. We present complementary measurement and modeling evidence that the warm, ~400–ppmv(parts per million by volume) CO2world of the Pliocene supported subarctic North Pacific deep-water formationand a Pacific meridional overturning circulation (PMOC) cell. In Pliocene subarctic North Pacific sediments, we reportorbitally paced maxima in calcium carbonate accumulation rate, with accompanying pigment and total organiccarbon measurements supporting deep-ocean ventilation-driven preservation as their cause. Together with highaccumulation rates of biogenic opal, these findings require vigorous bidirectional communication between surfacewaters and interior waters down to ~3 km in the western subarctic North Pacific, implying deep convection. Redox-sensitive trace metal data provide further evidence of higher Pliocene deep-ocean ventilation before the 2.73-Ma(million years) transition. This observational analysis is supported by climate modeling results, demonstratingthat atmospheric moisture transport changes, in response to the reduced meridional sea surface temperaturegradients of the Pliocene, were capable of eroding the halocline, leading to deep-water formation in the westernsubarctic Pacific and a strong PMOC. This second Northern Hemisphere overturning cell has important implica-tions for heat transport, the ocean/atmosphere cycle of carbon, and potentially the equilibrium response of thePacific to global warming.