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Recent Southern Ocean warming and freshening driven by greenhouse gas emissions and ozone depletion

Abstract

The Southern Ocean has, on average, warmed and freshened over the past several decades. As a primary global sink for anthropogenic heat and carbon, to understand changes in the Southern Ocean is directly relevant to predicting the future evolution of the global climate system. However, the drivers of these changes are poorly understood, owing to sparse observational sampling, large amplitude internal variability, modelling uncertainties and the competing influence of multiple forcing agents. Here we construct an observational synthesis to quantify the temperature and salinity changes over the Southern Ocean and combine this with an ensemble of co-sampled climate model simulations. Using a detection and attribution analysis, we show that the observed changes are inconsistent with the internal variability or the response to natural forcing alone. Rather, the observed changes are primarily attributable to human-induced greenhouse gas increases, with a secondary role for stratospheric ozone depletion. Physically, the simulated changes are primarily driven by surface fluxes of heat and freshwater. The consistency between the observed changes and our simulations provides increased confidence in the ability of climate models to simulate large-scale thermohaline change in the Southern Ocean.

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Data availability

All the data used in this article are publicly available. The CanESM2 large ensembles are available at http://open.canada.ca/data/en/dataset/aa7b6823-fd1e-49ff-a6fb-68076a4a477c. The RG Argo climatology is available at http://sio-argo.ucsd.edu/RG_Climatology.html. The historical profiles from the World Ocean Database can be found at https://www.nodc.noaa.gov/OC5/WOD/pr_wod.html.

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Acknowledgements

We acknowledge the Environment and Climate Change Canada’s Canadian Centre for Climate Modelling and Analysis for executing and making available the CanESM2 Large Ensemble simulations used in this study, and the Canadian Sea Ice and Snow Evolution Network for proposing the simulations. S.T.G. acknowledges NSF awards PLR-1425989 and OCE 1658001.

Author information

N.C.S. conducted the analysis the wrote the paper. S.T.G. obtained and preprocessed the observational data. J.C.F. proposed the paper. N.P.G. advised on the detection and attribution. All the authors contributed to the scientific interpretation of the results, and helped to edit the paper.

Competing interests

The authors declare no competing interests.

Correspondence to Neil C. Swart.

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    Supplementary Text and Supplementary Figures 1–8.

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Further reading

Fig. 1: Observed and simulated changes in temperature and salinity.
Fig. 2: Detection and attribution scaling factors.
Fig. 3: Fingerprints of temperature and salinity change.
Fig. 4: Observed and simulated changes in temperature and salinity in density space.
Fig. 5: Southern Ocean heat and salt budget.