Collapse of the North Atlantic Phytoplankton Bloom and its link to the Subpolar Gyre Tipping Point

The North Atlantic bloom (NAB) is the massive phytoplankton biomass increase that occurs each spring in the North Atlantic Ocean and which provides the base of the North Atlantic food web. Secondary producers (e.g. calanoid copepods) and the species that feed upon them have, over evolutionary timescales, developed life cycle synchronicities to coincide with the timing of the bloom. This makes understanding the future of the NAB – both in terms of its magnitude and timing – crucial for understanding the impacts of climate change on the North Atlantic ecosystem. The NAB is driven by nutrients supplied to the surface ocean by deep winter mixing, but the North Atlantic Subpolar Gyre (SPG) has been identified as a tipping element in the climate system (Armstrong McKay et al., 2022). If a tipping element crosses a critical threshold – a tipping point – associated with a given level of climate change, non-linear, abrupt and potentially irreversible (over human timescales) changes are projected to occur, altering the tipping element into a qualitatively different state. The SPG may be on the verge of a tipping point characterised by a deep convection collapse. This could significantly impact the availability of surface nutrients in the region, and may cause an abrupt decline in the NAB. We use a coupled climate model (UKESM1) forced under different emissions scenarios, as well as a high resolution ocean-only model forced by UKESM1’s model atmosphere, to investigate whether the SPG undergoes a convection collapse and how this is linked to future changes in the NAB.