Oceanic Pathways of an Active Pacific Meridional Overturning Circulation (PMOC)

In contrast to the modern‐day climate, North Pacific deep water formation and a Pacific meridional overturning circulation (PMOC) may have been active during past climate conditions, in particular during the Pliocene epoch (some 3–5 million years ago). Here, we use a climate model simulation with a robust PMOC cell to investigate the pathways of the North Pacific deep water from subduction to upwelling, as revealed by Lagrangian particle trajectories. We find that similar to the present‐day Atlantic Meridional Overturning Circulation (AMOC), most subducted North Pacific deep water upwells in the Southern Ocean. However, roughly 15% upwells in the tropical Indo‐Pacific Oceans instead—a key feature distinguishing the PMOC from the AMOC. The connection to the Indian Ocean is relatively fast, at about 250 years. The connection to the tropical Pacific is slower (∼800 years) as water first travels to the subtropical South Pacific then gradually upwells through the thermocline. Plain Language Summary Deep water formation may have occurred in the North Pacific Ocean during the Pliocene, enabling a Pacific Meridional Overturning Circulation (PMOC) that is absent in today's ocean. Here we trace water particles in a Pliocene‐like climate model, in which a PMOC has developed, to determine the subsurface pathways and destinations of the PMOC water after sinking from its surface source regions. As with the present‐day Atlantic overturning, most water upwells in the Southern Ocean. However, roughly 15% of the PMOC water returns to the surface in the tropical Pacific and Indian oceans. Such pathways are weak in the Atlantic overturning system and represent a unique feature of the PMOC and its contribution to the ocean environment of the Pliocene and possibly other past climate states. Key Points Lagrangian particles are used to assess the global pathways and timescales of the Pacific meridional overturning circulation (PMOC) After subduction most of the PMOC water returns to the surface in the Southern Ocean on centennial to millennial timescales, similar to the Atlantic meridional overturning circulation Approximately 15% of the PMOC water upwells in the tropical Pacific and Indian oceans on centennial timescales—a feature unique to the PMOC