Abstract
Abstract. The scientific motivation for this study is to understand the processes in the ocean interior controlling carbon transfer across 30° S. To address this, we have developed a unified framework for understanding the interplay between physical drivers such as buoyancy fluxes and ocean mixing, and carbon-specific processes such as biology, gas exchange and carbon mixing. Given the importance of density in determining the ocean interior structure and circulation, the framework is one that is organized by density and water masses, and it makes combined use of Eulerian and Lagrangian diagnostics. This is achieved through application to a global ice-ocean circulation model and an ocean biogeochemistry model, with both components being part of the widely-used IPSL coupled ocean/atmosphere/carbon cycle model. Our main new result is the dominance of the overturning circulation (identified by water masses) in setting the vertical distribution of carbon transport from the Southern Ocean towards the global ocean. A net contrast emerges between the role of Subantarctic Mode Water (SAMW), associated with large northward transport and ingassing, and Antarctic Intermediate Water (AAIW), associated with a much smaller export and outgassing. The differences in their export rate reflects differences in their water mass formation processes. For SAMW, two-thirds of the surface waters are provided as a result of the densification of thermocline water (TW), and upon densification this water carries with it a substantial diapycnal flux of dissolved inorganic carbon (DIC). For AAIW, principal formatin processes include buoyancy forcing and mixing, with these serving to lighten CDW. An additional important formation pathway of AAIW is through the effect of interior processing (mixing, including cabelling) that serve to densify SAMW. A quantitative evaluation of the contribution of mixing, biology and gas exchange to the DIC evolution per water mass reveals that mixing and, secondarily, gas exchange, effectively nearly balance biology on annual scales (while the latter process can be dominant at seasonal scale). The distribution of DIC in the northward flowing water at 30° S is thus primarily set by the DIC values of the water masses that are involved in the formation processes.
Highlights
What is the role of the Southern Ocean carbon cycle in the global climate system? It has long been suggested that the Southern Ocean could play a critical role on global climate variations given that it serves as a window between the atmospheric reservoir and the ocean interior
To study the interplay between the cyclo-stationary overturning circulation of the Southern Ocean and the natural carbon cycle, we extended a set of diagnostic tools grounded in the methods developed and applied by Iudicone et al (2008a, b, c) for characterising the role of Southern Ocean water mass transformations in global ocean circulation
This study was organized around the following question: What are the processes that control the exchange of dissolved inorganic carbon (DIC) across 30◦ S in the ocean interior? We have found that the dominant water masses for the net export of DIC across 30◦ S are Subantarctic Mode Water (SAMW) and AABW, with Antarctic Intermediate Water (AAIW) playing a significantly smaller role
Summary
What is the role of the Southern Ocean carbon cycle in the global climate system? It has long been suggested that the Southern Ocean could play a critical role on global climate variations given that it serves as a window between the atmospheric reservoir and the ocean interior. Iudicone et al.: Water masses and the Southern Ocean Carbon Cycle the carbon cycle at global scale This has been suggested for timescales ranging from decadal (Le Quereet al., 2007) to millennial (Anderson et al, 2009). These studies drew on a wide range of inferences from model simulations to observations to paleo-proxy records and emphasize the importance of carbon/climate coupling involving the Southern Ocean. This underscores the need for a deep understanding of the ocean mechanisms controlling the exchange of carbon between the oceanic and atmospheric reservoirs. While sensitivity studies are adding insight on the issue (e.g., d’Orgeville et al, 2010), a quantitative understanding of the interplay between ocean circulation and carbon dynamics is still missing
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