Biogeochemistry Physical oceanography

How important is carbon export by ocean eddies?

Source: Resplandy, L., M. Lévy, and D. J. McGillicuddy (2019), Effects of eddy-driven subduction on ocean biological carbon pump. Global Biogeochemical Cycles, 33, 1071-1084 doi:10.1029/2018GB006125.

Just as the Amazon rainforest takes up carbon dioxide (CO2) from the atmosphere, the ocean is also filled with “forests” of microalgae, called phytoplankton, that absorb CO2 through photosynthesis. When these organisms die and sink to the seafloor, the carbon they absorbed gets stored in the deep ocean—a process known as the biological pump.

In addition to the passive sinking of dead organisms, mixing processes and swirling eddies can also transport phytoplankton into the ocean interior, subsequently sequestering the carbon in their cells. However, scientists aren’t sure how much carbon is stored through phytoplankton export by these physical processes versus by sinking. Understanding the relative importance of these mechanisms is crucial for future climate predictions given the role of the biological pump in the global carbon cycle.

Ocean eddies range in size from hundreds of meters to hundreds of kilometers across. Eddies can have large vertical motions associated with them, which move water (and phytoplankton) between the ocean surface and interior. However, because eddies only last for days to months at a time, it is difficult to assess their significance to the global carbon cycle from observational data. A recent study led by Laure Resplandy at Princeton University uses an ocean model to quantify the importance of carbon storage via eddy transport of phytoplankton.

Satellite images of ocean color, a proxy for phytoplankton, show the expression of eddies (NASA Earth Observatory)

This study uses a high-resolution ocean model representing the subtropical and subpolar North Atlantic. Model resolution can be thought of like the image quality of a picture. Climate models divide the world into a three-dimensional grid. The resolution of a model describes the size of these grid boxes. Just as a high-resolution photograph has many pixels, a high-resolution climate model similarly has many grid boxes. And just as a high-resolution photograph allows you to see small details more easily, a high-resolution climate model is able to capture smaller scale processes and features.

Diagram showing cold ring and warm ring eddies (Coastal Carolina University)

The results show that eddies contribute less than 5% of the total annual export of phytoplankton to the ocean interior. This is a surprising result since the vertical motions associated with eddies can be very large. The reason that eddies account for such a small portion of the biological pump in the model has to do with the fact that there are two types of eddies. Eddies that rotate counter-clockwise (cold ring) bring deeper waters up to the surface, whereas clockwise (warm ring) eddies transport surface waters into the ocean interior. Since both types exist in the model, the downward fluxes of phytoplankton linked to warm ring eddies are nearly compensation by upward fluxes in cold ring eddies.

Observational evidence suggests that ocean eddies are important carbon sinks. But due to the limited observational data of phytoplankton in eddies, models are key in trying to understand the relative importance of eddy-driven carbon export to the total biological pump. This study suggests that eddies may not be as important as we thought due to the compensation between transport by clockwise and counter-clockwise eddies. Still, there is more work to be done to tease out the different mechanisms involved in the biological pump, which will be critical to improving our understanding of the global carbon cycle.

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