Poleward expansion of North Pacific gyres circulation during the warm early Eocene inferred from inter-model comparisons

The larger-scale oceanic gyre circulation regulates temperature, salinity and nutrient flow throughout the ocean, profoundly influencing the biological environment and climate. Here, we investigate the response of the Pacific gyre circulation during the warm climate of the early Eocene in eight models from the Deep-Time model intercomparsion project (DeepMIP). Our DeepMIP results suggest a northward expansion of the North Pacific subtropical gyre by up to 10 degrees latitude in the Eocene, maintaining a similar strength to the present day. This simulated poleward expansion of the North Pacific gyre circulation is corroborated by proxy evidence, including poleward shifts in low sedimentation rate and high clay concentration during the Eocene. In the southern Pacific, the super subtropical gyre is much stronger during the Eocene due to the southward position of Australia that leads to a wide-open Indonesian gateway. The poleward shifted boundary between the subtropical and subpolar gyre in North Pacific occurs as a result of the northward shifted westerly winds maxima, as also corroborated by an analysis of the Sverdrup transport. The Sverdrup transports describes the upper circulation during the Eocene further poleward than modern day mainly due to their continental differences. The upper circulation corresponds to Sverdrup transport up to ~53°N for the North Pacific, slightly further north than modern day of 50°N, and up to ~55°S for the South Pacific that is much further south than in the modern ocean and continents (~45°S).