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 overturning circulation is absent in the Pacific, the world’s largest ocean, where relatively fresh surface waters inhibit North Pacific deep convection. We present complementary measurement and modeling evidence that the warm, ~400–ppmv (parts per million by volume) CO2 world of the Pliocene supported subarctic North Pacific deep-water formation and a Pacific meridional overturning circulation (PMOC) cell. In Pliocene subarctic North Pacific sediments, we report orbitally paced maxima in calcium carbonate accumulation rate, with accompanying pigment and total organic carbon measurements supporting deep-ocean ventilation-driven preservation as their cause. Together with high accumulation rates of biogenic opal, these findings require vigorous bidirectional communication between surface waters and interior waters down to ~3 km in the western subarctic North Pacific, implying deep convection. Redoxsensitive 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, demonstrating that atmospheric moisture transport changes, in response to the reduced meridional sea surface temperature gradients of the Pliocene, were capable of eroding the halocline, leading to deep-water formation in the western subarctic Pacific and a strong PMOC. This second Northern Hemisphere overturning cell has important implications for heat transport, the ocean/atmosphere cycle of carbon, and potentially the equilibrium response of the Pacific to global warming.
Active Pacific meridional overturning circulation (PMOC) during the warm Pliocene
Ocean to Climate 
This interesting paper suggests that a Pacific Meridional Overturning Circulation (PMOC) existed during the Pliocene, which is a warm period of the Earth that lasted from 5.3 million to 2.6 million years ago. During the mid-Pliocene, the CO2 level was about 400 ppm, very close to today’s CO2 level (405ppm). So, some scientists believe that the Pliocene can be a reference for the current and future warming.
The main argument here is that the reduced meridional SST gradient in the North Pacific during the Pliocene decreased the poleward moisture transport, which in turn reduced rainfall and increased surface salinity in the subarctic North Pacific, ultimately producing deep water formation in the subarctic North Pacific. Interestingly, the AMOC was much reduced in their model simulation for the Pliocene compared to the preindustrial period.
One caveat is that the models used in the Pliocene Model Intercomparison Project (PMOC) do not show any evidence of the PMOC, as also pointed out in this paper.
Sang-Ki