Carbon export and mesoscale eddy structure in the Southern Ocean revealed by BGC-Argo floats

The Southern Ocean, which represents 30% of the global ocean surface area, accounts for about 40% of total ocean uptake of anthropogenic carbon dioxide (CO2) by both the solubility and biological carbon pumps (Frölicher et al., 2015; Landschützer et al., 2015; Sabine et al., 2004). Quantifying annual net community production and particle flux attenuation in the Southern Ocean is important for understanding the roles of the Southern Ocean in carbon uptake and climate. Only a few studies have attempted this, mostly at individual sites based on measurements from ships or moorings. Expanding from this local, regional or process-study scale to the basin scale requires extension from traditional sampling techniques like ships and moorings to satellites without depth dimension and then towards a greater use of autonomous platforms and sensors such as Biogeochemical-Argo (BGC-Argo) floats (Johnson et al., 2017). This increased understanding from new observations can then feed into development of improved models (Lourey & Trull, 2001; McNeil & Tilbrook, 2009; Munro et al., 2015; Bender & Jönsson, 2016). Thanks to an increasing number of BGC-Argo floats deployed across the Southern Ocean, we were able to quantify annual net community production and the variability of particle flux attenuation in the Southern Ocean in Chapters 2 and 3. We also investigated the roles of mesoscale eddies in Southern Ocean biogeochemistry on a basin scale by looking at subsurface biological structure of eddies in Chapter 4.
We developed a novel approach to estimate annual net community production (ANCP) by integrating oxygen drawdown across the upper ocean from all the BGC-Argo oxygen profiles available in the Southern Ocean. We found 15% of measured dissolved oxygen drawdown occurred above 100 m, but below the euphotic zone, and 14% occurred below 500 m. Using this improved methodology, we estimated total basin-integrated ANCP in the Southern Ocean to be 3.89 GT C year-1 , higher than all previous studies.
We then partitioned backscatter data into large and small particles. We quantified the attenuation of particle concentration with depth by fitting power law functions (often called the Martin curve). We showed the variability of attenuation and investigated the controls on that by looking at the correlation between attenuation and other environmental variables such as chlorophyll and temperature.
The role of eddies in Southern Ocean biogeochemistry is starting to receive more attention, from studies based on satellites or models to debates about how mesoscale eddies might impact Southern Ocean biogeochemistry into the future (Gaube et al., 2014; Moreau et al., 2017; Patel et al., 2020; Rohr et al., 2020; Song et al., 2018). We investigated the roles of mesoscale eddies in Southern Ocean biogeochemistry by looking at their subsurface biological structure using BGC-Argo floats. We found chlorophyll anomalies (compare to outside of eddies) in the mixed layer were mostly positive for both cyclones and anticyclones in spring, summer and autumn, but anomalies in winter were close to zero. Most of the positive chlorophyll anomalies observed were due to both an increase of biomass and photo-acclimation while few cases were due to photo-acclimation only.
These results have led us to rethink the importance of the Southern Ocean in regulating oceanic carbon storage and open a bigger question of what important factors are controlling particle sinking and thus transfer efficiency. We also found interesting chlorophyll and backscatter patterns for both cyclones and anticyclones in the Southern Ocean, which expanded our understanding in the subsurface dimension. All these studies were facilitated by the expanding deployment of BGC-Argo floats across the Southern Ocean and more interesting findings are surely coming.