An Atlantic-Pacific Seesaw in Circulation and Biogeochemistry

By regulating the supply of carbon, nutrients, and heat, ocean circulation at high latitudes plays a critical role in global climate.  During the last ice age, the Atlantic’s overturning circulation was repeatedly perturbed, associated with major changes in climate, but little is known of the response of biogeochemistry and circulation in the Pacific.  Here we present new high-resolution data that illuminate the coupled changes in circulation, CO₂ and nutrient supply, and biological productivity associated with rapid climate change events at northern high latitudes.  We show that abrupt stadial cold events are consistently associated with pulses of enhanced nutrient supply and diatom productivity at mid latitudes in the North Atlantic.  Abrupt changes are also seen in the North Pacific, but are anti-phased, with peaks of productivity and nutrient supply occurring during abrupt interstadial warming.  Using model simulations, we show that these productivity changes can be explained by abrupt switches in the mode of overturning circulation, with weakened overturning associated with accumulation of nutrients in the subsurface waters that supply the surface via winter mixing and upwelling, alongside a southward shift of nutrient-rich subpolar waters.  Our results demonstrate the persistent operation of an Atlantic-Pacific seesaw in overturning circulation and biogeochemistry on centennial to millennial timescales and provide a valuable test for simulation of interlinked changes in circulation, biogeochemistry, and climate.