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For the first time, scientists have resolved extremely intense tropical cyclones and their effect on the ocean carbon cycle in a global Earth system model. Using two category-4 hurricanes in the North Atlantic as examples, the study reveals a cascade of physical-biogeochemical effects including uptake of carbon dioxide and regional-scale phytoplankton bloom.
Tropical cyclones are enormously powerful: The destructive wind speeds, fierce gusts, and intense rainfall leave obvious marks wherever they pass. They also affect the ocean. Because the storms stir up the water surface, they trigger mixing of water masses as well as exchange of heat and carbon with the atmosphere. For the first time, scientists from the Max Planck Institute for Meteorology and the University of Hamburg have represented these interactions in a global storm- and eddy-resolving Earth system model, revealing the cascade of physical-biogeochemical mechanisms that unfold in response to tropical cyclones.
“Traditional Earth system models have a coarse grid spacing of 100 to 200 kilometers, which does not allow them to resolve very intense tropical cyclones realistically, especially category 4 and 5 cyclones,” explained David Nielsen, first author of the study. “Using the ICON model with a horizontal resolution of five kilometers and including the ocean biogeochemistry component HAMOCC, we could see category-4 tropical cyclones in the simulation and study their impact on the carbon cycle.” Specifically, the team investigated two hurricanes in the North Atlantic with wind speeds exceeding 200 kilometers per hour, which appeared in the one-year simulation in September 2020, about a week apart.
Effects on carbon and phytoplankton
The scientists showed that the hurricanes triggered an immediate release of carbon dioxide from the ocean into the atmosphere that was 20 to 40 times stronger than under normal weather conditions. However, the hurricanes also cooled the ocean surface, increasing the uptake of carbon dioxide for several weeks after the storm had passed. In combination, these two opposing effects—immediate release and long-term uptake—resulted in a small net uptake.
Another striking effect of the hurricanes was how they induced mixing in the upper ocean, bringing nutrients to the surface. Phytoplankton growth increased tenfold in response. The bloom lasted for a few weeks after the passage of the hurricanes and was not restricted to their wake: Local currents, partially intensified by the storms, distributed the biomass across large parts of the western North Atlantic. “It was exciting to learn that as a consequence, the hurricanes also increased the amount of organic carbon sinking down in the ocean, contributing to the long-term storage of carbon in deeper ocean layers,” said Tatiana Ilyina, group leader and co-author of the study.
Previously, scientists had observations of some of these processes. “However, this simulation allows us to study them in detail and link them to the global scale, which is important if we want to understand how tropical cyclones might respond to and impact our climate under global warming”, said Nielsen. As a next step, the team will also look at other km-scale processes and their impact on the ocean carbon cycle, such as interactions between storms and ocean eddies, not only in the tropics but also in polar regions.
Dr. David Nielsen, Max Planck Institute for Meteorology/CLICCS Cluster of Excellence: david.nielsen@mpimet.mpg.de
Prof. Dr. Tatiana Ilyina, University of Hamburg/Helmholtz-Zentrum Hereon/Max Planck Institute for Meteorology: tatiana.ilyina@uni-hamburg.de
Nielsen, D., Chegini, F., Serra, N., Kumar, A., Brüggemann, N., Hohenegger, C., and Ilyina, T. (2025) Resolved tropical cyclones trigger CO2 uptake and phytoplankton bloom in an Earth system model simulation. PNAS, Vol. 120, No. 0, e2506103122, https://doi.org/10.1073/pnas.2506103122
?si=_YJjOfa4EIQnecNL - animation of the model simulation showing air-sea CO2 flux and surface wind speed
Net primary production, i.e. the production of biomass, before the hurricanes and after the first an ...
Source: David Nielsen
Copyright: David Nielsen, MPI-M
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