Enhanced Southern Ocean CO2 outgassing as a result of stronger and poleward shifted southern hemispheric westerlies
-
Published:2023-11-06
Issue:21
Volume:20
Page:4413-4431
-
ISSN:1726-4189
-
Container-title:Biogeosciences
-
language:en
-
Short-container-title:Biogeosciences
Author:
Menviel Laurie C.ORCID, Spence Paul, Kiss Andrew E.ORCID, Chamberlain Matthew A.ORCID, Hayashida HakaseORCID, England Matthew H., Waugh DarrynORCID
Abstract
Abstract. While the Southern Ocean (SO) provides the largest oceanic sink of carbon, some observational studies have suggested that the SO total CO2 (tCO2) uptake exhibited large (∼ 0.3 GtC yr−1) decadal-scale variability over the last 30 years, with a similar SO tCO2 uptake in 2016 as in the early 1990s. Here, using an eddy-rich ocean, sea-ice, carbon cycle model, with a nominal resolution of 0.1∘, we explore the changes in total, natural and anthropogenic SO CO2 fluxes over the period 1980–2021 and the processes leading to the CO2 flux variability. The simulated tCO2 flux exhibits decadal-scale variability with an amplitude of ∼ 0.1 GtC yr−1 globally in phase with observations. Notably, two stagnations in tCO2 uptake are simulated: between 1982 and 2000, and between 2003 and 2011, while re-invigorations are simulated between 2000 and 2003, as well as since 2012. This decadal-scale variability is primarily due to changes in natural CO2 (nCO2) fluxes south of the polar front associated with variability in the Southern Annular Mode (SAM). Positive phases of the SAM, i.e. stronger and poleward shifted southern hemispheric (SH) westerlies, lead to enhanced SO nCO2 outgassing due to higher surface natural dissolved inorganic carbon (DIC) brought about by a combination of Ekman-driven vertical advection and DIC diffusion at the base of the mixed layer. The pattern of the CO2 flux anomalies indicate a dominant control of the interaction between the mean flow south of the polar front and the main topographic features. While positive phases of the SAM also lead to enhanced anthropogenic CO2 (aCO2) uptake south of the polar front, the amplitude of the changes in aCO2 fluxes is only 25 % of the changes in nCO2 fluxes. Due to the larger nCO2 outgassing compared to aCO2 uptake as the SH westerlies strengthen and shift poleward, the SO tCO2 uptake capability thus reduced since 1980 in response to the shift towards positive phases of the SAM. Our results indicate that, even in an eddy-rich ocean model, a strengthening and/or poleward shift of the SH westerlies enhance CO2 outgassing. The projected poleward strengthening of the SH westerlies over the coming century will, thus, reduce the capability of the SO to mitigate the increase in atmospheric CO2.
Funder
Australian Research Council
Publisher
Copernicus GmbH
Subject
Earth-Surface Processes,Ecology, Evolution, Behavior and Systematics
Reference85 articles.
1. Adusumilli, S., Fricker, H., Medley, B., Padman, L., and Siegfried, M.: Interannual variations in meltwater input to the Southern Ocean from Antarctic ice shelves, Nat. Geosci., 13, 616–620, https://doi.org/10.1038/s41561-020-0616-z, 2020. a 2. Arblaster, J. M. and Meehl, G. A.: Contributions of External Forcings to Southern Annular Mode Trends, J. Clim., 19, 2896–2905, https://doi.org/10.1175/JCLI3774.1, 2006. a 3. Bakker, D. C. E., Pfeil, B., Landa, C. S., Metzl, N., O'Brien, K. M., Olsen, A., Smith, K., Cosca, C., Harasawa, S., Jones, S. D., Nakaoka, S., Nojiri, Y., Schuster, U., Steinhoff, T., Sweeney, C., Takahashi, T., Tilbrook, B., Wada, C., Wanninkhof, R., Alin, S. R., Balestrini, C. F., Barbero, L., Bates, N. R., Bianchi, A. A., Bonou, F., Boutin, J., Bozec, Y., Burger, E. F., Cai, W.-J., Castle, R. D., Chen, L., Chierici, M., Currie, K., Evans, W., Featherstone, C., Feely, R. A., Fransson, A., Goyet, C., Greenwood, N., Gregor, L., Hankin, S., Hardman-Mountford, N. J., Harlay, J., Hauck, J., Hoppema, M., Humphreys, M. P., Hunt, C. W., Huss, B., Ibánhez, J. S. P., Johannessen, T., Keeling, R., Kitidis, V., Körtzinger, A., Kozyr, A., Krasakopoulou, E., Kuwata, A., Landschützer, P., Lauvset, S. K., Lefèvre, N., Lo Monaco, C., Manke, A., Mathis, J. T., Merlivat, L., Millero, F. J., Monteiro, P. M. S., Munro, D. R., Murata, A., Newberger, T., Omar, A. M., Ono, T., Paterson, K., Pearce, D., Pierrot, D., Robbins, L. L., Saito, S., Salisbury, J., Schlitzer, R., Schneider, B., Schweitzer, R., Sieger, R., Skjelvan, I., Sullivan, K. F., Sutherland, S. C., Sutton, A. J., Tadokoro, K., Telszewski, M., Tuma, M., van Heuven, S. M. A. C., Vandemark, D., Ward, B., Watson, A. J., and Xu, S.: A multi-decade record of high-quality fCO2 data in version 3 of the Surface Ocean CO2 Atlas (SOCAT), Earth Syst. Sci. Data, 8, 383–413, https://doi.org/10.5194/essd-8-383-2016, 2016. a 4. Banerjee, A., Fyfe, J., Polvani, L., Waugh, D., and Chang, K.-L.: A pause in Southern Hemisphere circulation trends due to the Montreal Protocol, Nature, 579, 544–548, https://doi.org/10.1038/s41586-020-2120-4, 2020. a 5. Bourgeois, T., Goris, N., Schwinger, J., and Tjiputra, J.: Stratification constrains future heat and carbon uptake in the Southern Ocean between 30∘ S and 55∘ S, Nat. Commun., 13, 340, https://doi.org/10.1038/s41467-022-27979-5, 2022. a
|
|