Tropical cyclones cool the sea surface and enhance the growth of algae
Snapshot from a simulation of surface wind speed and air-sea flux of carbon dioxide with the Icon model. The purple vortex idicates the position of the hurrcane. During and immediately after the hurricane the ocean releases carbon dioxide (red colour). in the the blue coloured regions the ocean takes up carbon dioxide.
© DKRZ/MPI-M/UHH
To the point
- Model simulation: A high-resolution Earth system model can be used to investigate interactions between intense tropical cyclones and the carbon cycle.
- Carbon uptake: During hurricanes, significantly more carbon dioxide is released from the ocean into the atmosphere than under normal conditions. However, as the ocean surface cools due to the hurricanes, the ocean subsequently absorbs more carbon dioxide, and this effect dominates.
- Algal bloom: Ocean mixing brings nutrients to the surface, leading to increased phytoplankton growth.
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.
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 about a week apart.
Effects on carbon and phytoplankton
Net primary production, i.e. the production of biomass, on August 28th, before the hurricanes occured in the simulation (black line; black vortex- currentposition), and on September 10th after the first and on September 17th after second hurricane. The red colour indicates, that more biomass was formed after the hurricanes.
© David Nielsen/MPI-M
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.
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.
