Transient overturning changes cause an upper-ocean nutrient decline in a warming climate.

Models and proxy data suggest multi-centennial nutrient reorganization and biological productivity changes under sustained climate warming. These changes have traditionally been attributed to processes in the Southern Ocean. Here we instead show that transient overturning circulation adjustments, associated with changes in the Atlantic Meridional Overturning Circulation (AMOC), dominate the global nutrient reorganization on centennial timescales. Following an AMOC weakening, a typical feature of a warming climate, a transient overturning circulation develops in the Indo-Pacific basins, characterized by enhanced southward transport in the deep ocean. Coupled with the vertical nutrient structure, these transient overturning changes produce a net transport of nutrients from the Indo-Pacific into the Southern Ocean. Meanwhile, isopycnal surfaces deepen and bring nutrient-depleted waters to greater depths, causing nutrient concentrations to decline in much of the global upper ocean. Given the close link between nutrients and carbon, our findings suggest that transient overturning circulation changes across different basins can critically affect the marine carbon cycle.