Ocean eddies are spinning parcels of water about 100km across and 1500m deep. They occur everywhere in the ocean.
In the Southern Hemisphere, eddies that spin clockwise are cooler than the surrounding ocean because their rotation causes cold, deep water to move upwards. This upwelling brings nutrients essential for photosynthesis to the surface and makes clockwise-rotating eddies more productive. Satellites can measure this productivity by sensing differences in ocean colour, which result from the increased plankton.
By analysing thousands of Southern Ocean eddies, we found that in summer and autumn, eddies behave opposite to our expectations. That is, clockwise rotating eddies have lower plankton concentrations compared to neighbouring waters, and counter-clockwise rotating eddies have higher concentrations.
To explain this we examined how deep these eddies mix the ocean in the preceding months. We found that counter-clockwise rotating eddies mix the ocean deeper in winter, allowing more nutrients to enter their interiors, leading to higher productivity. This work is important because eddy productivity plays a significant role in the exchange of carbon between the ocean and the atmosphere. Carbon exchange in the Southern Ocean is thought to be changing, and this work helps explain an important piece of that process.
Paper: Dawson, H. R. S., Strutton, P. G., & Gaube, P. (2018). The unusual surface chlorophyll signatures of Southern Ocean eddies. Journal of Geophysical Research: Oceans, 123. https://doi.org/10.1029/2017JC013628