Projected AMOC Weakening Controls Future Diatom Decline in the Subpolar North Atlantic Ocean

Climate models consistently project a decline in diatom net primary production (NPP) under high‐emission scenarios in the highly productive subpolar North Atlantic. However, the magnitude of this decline remains highly uncertain and varies substantially across models, limiting our ability to anticipate its ecological and biogeochemical consequences. Several recent studies using individual Earth system models have identified a mechanistic relationship between the projected weakening of the Atlantic Meridional Overturning Circulation (AMOC) and the availability of for phytoplankton growth in the subpolar North Atlantic. These findings suggest that uncertainty in the future evolution of the AMOC may be a major driver of the spread in diatom NPP projections. Here, we analyze an ensemble of Earth system models to test this hypothesis and to better constrain projections of diatom NPP. We first show that a robust inter‐model relationship exists between projected changes in AMOC intensity and availability in the subpolar North Atlantic. We then apply an emergent‐constraint framework to estimate the future sensitivity of diatom NPP to nitrate changes, using satellite‐based reconstructions of historical NPP and nitrate variability. Combining these two approaches yields a constrained relationship between projected AMOC weakening and the associated decline in diatom NPP. Our results confirm that uncertainty in the magnitude of future AMOC weakening constitutes the dominant source of uncertainty in diatom NPP projections. Nevertheless, they suggest a smaller decrease in diatom NPP than previously anticipated for a plausible range of AMOC decline.