85 / 2024-09-07 07:36:56

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

Overturning Circulation,Atlantic Meridional Overturning Circulation,nutrients,Carbon Cycle

Models and proxy data suggest multi-centennial nutrient and biological productivity changes under sustained climate warming, with nutrient concentrations declining in much of the global upper ocean. These changes have traditionally been attributed to enhanced deep water upwelling and increased biological productivity in the Southern Ocean. Here we instead show that transient adjustments of the global ocean overturning circulation, associated with changes in the Atlantic Meridional Overturning Circulation (AMOC), dominate the global nutrient reorganization on centennial timescales. Following a weakening of the AMOC, a typical feature of a warming climate, a transient overturning circulation develops in the Indo-Pacific basins, characterized by an enhanced southward transport in the deep ocean that is opposite to changes in the Atlantic basin. Coupled with the vertical nutrient structure, these transient overturning changes produce a net transport of nutrients from the Indo-Pacific basins into the Southern Ocean that is unrelated to changes in surface wind. Meanwhile, isopycnal surfaces deepen and bring nutrient-depleted waters to greater depths, causing nutrient concentrations to decrease in much of the global upper ocean and thereby reduce biological productivity. 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.