Abrupt Changes in the Timing and Magnitude of the North Atlantic Bloom Over the 21st Century

The North Atlantic subpolar gyre (SPG) is a key region for the North Atlantic bloom (NAB), the phytoplankton foundation of the regional food web. The NAB depends on nutrients seasonally introduced into the surface ocean by deep winter convection. Under climate change, this pattern is threatened by increasing water column stratification, representing a potential "tipping point" in the Earth system, and may "collapse" as a result. We investigate changes in winter mixing and the impacts on the SPG and the northern North Atlantic using a spread of future projections from a low-resolution Earth system model (UKESM) and a high-warming projection of a high-resolution ocean-only configuration of the same model (NEMO-MEDUSA). Both models project significant declines in the strength of the NAB during the 21st century. In UKESM, this occurs across all projections, but with low spatiotemporal coherence. In NEMO-MEDUSA, changes in mixed layer depth, nutrients, and chlorophyll concentrations are abrupt and more highly spatiotemporally correlated. We find a >30-day phenological shift in the peak of the bloom aligned with the timing of this change, which may affect food web dynamics. Defining "collapse" as halving of surface chlorophyll, we find that the NAB collapses this century regardless of future projection. However, the spatial-temporal coherence of the timing and abruptness of this collapse is greater in our high-resolution model. Because key physical processes driving biogeochemical responses are poorly represented in low-resolution models, especially at high latitudes, this suggests that higher resolution may be essential for predicting abrupt and irreversible changes, particularly those involving ecosystem dynamics.