The North Pacific in Warm(ing) Climates: effects on ocean circulation and biogeochemical cycles

The North Pacific hosts the both one of the largest oceanic reservoirs of sequestered carbon and extensive oxygen minimum zones in the world ocean, which will likely intensify and expand under future climate warming scenarios, yielding significant consequences for ecosystems, biogeochemical cycles, and living resources. At present, relatively better-oxygenated subsurface North Pacific Intermediate Water (NPIW) mitigates OMZ development, but on instrumental time scales, data the past decades indicate decreasing NPIW ventilation, induced by a freshening and increased stratification of surface and thermocline waters. Longer variations in these oceanographic boundary conditions were, however, large and are thus able to hinder assessment of anthropogenic influences against natural background shifts. We previously provided evidence modern well-ventilated waters underwent significant millennial-scale variations over the last ca. 12,000 years (Lembke-Jene et al., 2018), with a prominent “tipping point” around 4,500 years before present.Crossing such mid-Holocene threshold led to the Okhotsk Sea becoming the modern ventilation source it is today, although the underlying forcing and physical boundary conditions characteristics remain largely enigmatic. A combination of sea ice loss, higher water temperatures, and remineralization rates may be able to induce a nonlinear change into a different mean state in this region. To constrain these factors we present combined surface, mesopelagic and bathyal ocean proxy records from key study sites in the Western Subarctic Pacific, the Okhotsk Sea and Bering Sea, and the Gulf of Alalska, with submillennial-scale resolution to assess changes in upper ocean stratification, nutrient characteristics and resulting changes on mid-depth water ventilation. Our results imply that under assumed past hemispheric warmer- than-present conditions, regional surface temperatures and upper ocean stratification were increased and changed in a nonlinear mode during the last 4-5,000 years, associated with changing primary productivity patterns and biogeochemical feedback mechanisms. Results from complementary Earth System Model simulations provide evidence for the interaction between the high-latitude North Pacific marginal seas and thePacific Western Subarctic Gyre circulation, with effects on mesopelagic ventilation dynamics and its consequences for large oceanic regions.