Resolved tropical cyclones trigger CO2 uptake and phytoplankton bloom in an Earth system model simulation

David Marcolino Nielsen , Fatemeh Chegini, Nuno Serra, Arjun Kumar, Nils Brüggemann, Cathy Hohenegger, Tatiana Ilyina

The ocean carbon cycle is directly impacted by storms in the atmosphere. Tropical cyclones (TCs), particularly, are known to drive intense air-sea CO2 fluxes and to trigger phytoplankton blooms. However, the latest generation of Earth system models (ESM) cannot realistically represent TCs due to their coarse spatial resolution (typically 100-200 km grid spacing). Here, we present the first km-scale coupled, global, storm- and eddy-resolving (5 km ocean, 5 km atmosphere) ESM simulation including ocean biogeochemistry that is able to resolve TCs, and the cascade of physical-biogeochemical mechanisms that unfold in their response. Our simulated TCs enhance CO2 fluxes by 20-40 times and cool the surface ocean by 2-3°C, thus contributing to inverting the CO2 flux direction from ocean outgassing to uptake. Our TCs furthermore trigger a phytoplankton bloom in autumn in the western North Atlantic, which is missed by coarser ESMs. While our TCs boost primary production, they also warm the ocean subsurface, which enhances organic matter remineralization and attenuates their impact on carbon export to depth. Our novel model configuration reproduces mechanisms underlying the ocean carbon cycle variability that remained so far unresolved in ESMs. By representing fine-scale atmosphere-ocean biogeochemistry interactions in our ESM, we pave the way for future work to constrain uncertainties in the role of km-scale events in the ocean carbon cycle at global and climatic scales.