The Southern Ocean plays a key role in the balance of the climate. Its effectiveness in absorbing CO₂ depends in particular on powerful underwater eddies that modify the exchange of carbon and oxygen, with effects that are still poorly understood. Better studying them is an essential element in refining forecasts and being able to anticipate adaptation to changes.
by Laurie Henry
The Southern Ocean captures almost 50% of the CO₂ absorbed by the oceans and plays a key role in the circulation of oxygen to the deep waters. However, this huge reservoir is not uniform, as it is crossed by eddies known as mesoscale eddies, which are a kind of powerful mixing current that rotates over a diameter of 40 to 150 km and modifies the distribution of carbon and nutrients between the surface and the seabed.
These eddies are not distributed evenly, either in space or in time. Their distribution varies by region and season, and their effects vary just as much, without being fully understood. A team of researchers from the Scripps Institution of Oceanography and the College of Marine Science has quantified their impact on the biogeochemistry of the Southern Ocean by cross-referencing satellite data and measurements from Argo floats. Their conclusions, published in the journal AGU Advances, highlight key mechanisms that are still underestimated in climate models.
Understanding the effect of eddies
In the southern hemisphere, eddies can be cyclonic (CE), i.e. rotating clockwise, or anticyclonic (AE) in the other direction.
CE eddies carry carbon and nutrients from deep water up towards the surface. While this upwelling stimulates biological production at the surface, it also leads to an increase in the partial pressure of CO₂, which encourages its outgassing into the atmosphere. Conversely, AE eddies cause the waters to sink from the surface to the bottom, increasing the ocean’s absorption of atmospheric CO₂.
Diagram of eddy processes in the Southern Hemisphere: (i) Vertical movement of water in the AE (red, downwards) and CE (blue, upwards). (ii) Effect of the wind on water circulation, influencing the upwelling and downwelling of water. (iii) Lateral mixing of water masses. (iv) Trapping and transport of water by the AEs. © Keppler et al 2024
To identify and track these eddies, the study by Keppler et al uses the Meta3.2DT database, which lists the eddy structures detected by satellite altimetry between 1993 and 2022. The scientists then cross-referenced this information with measurements from BGC-Argo floats, which are capable of recording the concentration of dissolved inorganic carbon (DIC), nitrates and other biogeochemical parameters in the water column down to a depth of 1,500 metres. This approach has made it possible to precisely locate the eddies and analyse how they influence the vertical distribution of key elements of the carbon cycle.
These data are then compared with reference climatologies in order to evaluate the biogeochemical anomalies induced by the eddies. For this purpose, the Argo profiles used are classified into three categories: those inside cyclonic eddies (CE), those inside anticyclonic eddies (AE), and those outside eddies which serve as a reference for measuring the impact of eddy structures on the biogeochemical properties of the water in the area.
Thanks to this method, it has been possible to quantify the effect of ECs and EAs on vertical flows of carbon and nutrients. A marked seasonal variability has been demonstrated. In autumn and winter, due to strong winds, ECs dominate in the powerful Antarctic Circumpolar Current (ACC) that animates the Southern Ocean. They then increase the degassing of CO₂ by 0.06 Pg C/year, or 19% of the average flow in this region. In spring and summer, AEs become more frequent, especially north of the ACC, due to the stratification of surface waters caused by seasonal warming. These eddies increase the absorption of CO₂ by the ocean.
Location of BGC-Argo floats equipped with pH (a), nitrate (b) and dissolved oxygen (c) sensors that emerged in cyclonic (blue), anticyclonic (red) or non-turbulent (grey) eddies between 2014 and 2022. The maps (d–f) show air-sea flow anomalies. The light blue area indicates seasonal sea ice cover. © Keppler et al, 2024By directly influencing carbon exchanges between the ocean and the atmosphere, these eddy structures play a decisive role in the Southern Ocean’s capacity to capture CO2 and mitigate climate change.
The complex impact of eddies on oxygenation and biological productivity
Mesoscale eddies modify the distribution of phytoplankton in the Southern Ocean by influencing nutrient supply and growth conditions through upwelling and mixing. Chlorophyll-a, a key indicator of phytoplankton biomass, varies greatly depending on the type of eddy and the region considered.
In the Antarctic Circumpolar Current (ACC), cyclonic eddies (CE) bring up deep water that is locally less rich in nutrients compared to that located further north. This phenomenon, combined with intense vertical mixing, disperses phytoplankton at greater depths, depriving it of sufficient light to grow effectively. This results in a decrease in chlorophyll-a in these eddies.
On the other hand, to the north of the ACC, the EC rise from deeper waters that are more heavily loaded with nitrates. This input stimulates the growth of phytoplankton at the surface, despite the mixing, and results in a net increase in chlorophyll-a. The researchers attribute this difference to the contrast in the composition of the water masses between the two regions.
Finally, the eddies do not just act vertically: they also transport pockets of water laterally, modifying the regional distribution of nutrients. The EAs, in particular, can trap and move waters rich in chlorophyll-a, while certain EEs disperse depleted water masses, accentuating local heterogeneities in productivity in the Southern Ocean.
Average anomalies of DIC, nitrates and dissolved oxygen in the ACC (green) and north of the ACC (purple) for AEs (red), CEs (blue) and outside eddies (black). Calculated according to depth, isopycnals and their difference. Figures: number of profiles used. © Keppler et al 2024
A challenge for climate models and ocean monitoring
Current climate and oceanographic models struggle to accurately integrate the influence of mesoscale eddies on carbon and nutrient fluxes. Most simulations treat these structures as simple diffusive mixing agents, ignoring their specific effects on the upward or downward movement of CO₂, nitrates and oxygen. However, the data from the study by Keppler et al show that mesoscale eddies cover on average 22% of the surface of the Southern Ocean and directly modify about 5% of the carbon fluxes in this region. These structures therefore have a much greater influence on climate regulation than is currently estimated by the models.
To address these shortcomings, researchers are using two complementary types of data: satellite observations, which make it possible to detect and monitor eddies in real time, and biogeochemical Argo floats, deployed throughout the Southern Ocean. The latter offer unprecedented spatial and temporal coverage, reducing uncertainty about the biogeochemical processes at play.
However, despite these advances, major gaps remain in the monitoring of the Southern Ocean. The number of floats equipped with biogeochemical sensors is still insufficient for detailed analysis at the scale of each region and season. Current satellites also do not allow observation of the depth penetration of eddies, which limits a detailed understanding of vertical carbon and nutrient exchanges.
* Isopycnals are surfaces of constant density in the ocean. They allow the vertical and horizontal movements of the water to be analysed without being influenced by variations in temperature and salinity. They are particularly useful for studying the exchange of nutrients, oxygen and carbon in stable bodies of water.