Mesoscale Eddies Enhance the Air-Sea CO₂ Sink in the South Atlantic Ocean

Mesoscale eddies are abundant in the global oceans and known to affect oceanic and atmospheric conditions. Understanding their cumulative impact on the air-sea carbon dioxide (CO2) flux may have significant implications for the ocean carbon sink. Observations and Lagrangian tracking were used to estimate the air-sea CO2 flux of 67 long lived (>1 year) mesoscale eddies in the South Atlantic Ocean over a 16 year period. Both anticyclonic eddies originating from the Agulhas retroflection and cyclonic eddies originating from the Benguela upwelling act as net CO2 sinks over their lifetimes. Anticyclonic eddies displayed an exponential decrease in the net CO2 sink, whereas cyclonic eddies showed a linear increase. Combined, these eddies significantly enhanced the CO2 sink into the South Atlantic Ocean by 0.08 +/- 0.04%. The studied eddies constitute a fraction of global eddies, and eddy activity is increasing; therefore, explicitly resolving eddies appears critical when assessing the ocean carbon sink. Plain Language Summary Ocean mesoscale eddies can form when part of a main current becomes separated or through internal ocean instabilities which form circular rotating currents that propagate across the oceans. These eddies last from weeks to years and can modify the ocean properties of the water captured within them, which in turn affects the net exchange of carbon between this water and the atmosphere. Little is known about how these eddies modify the absorption of carbon across the global ocean, collectively referred to as the ocean carbon sink, despite them being ubiquitous features of the global oceans. Using in situ and satellite-based observations, we show that eddies in the South Atlantic Ocean enhance the absorption of carbon from the atmosphere, thus modifying the ocean to be a stronger net sink of carbon. These results are important as they quantify how much eddies contribute to the absorption of carbon from the atmosphere to the ocean, and highlight the need to include eddies when assessing ocean carbon budgets.