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Cessation of deep convection in the open Southern Ocean under anthropogenic climate change

Abstract

In 1974, newly available satellite observations unveiled the presence of a giant ice-free area, or polynya, within the Antarctic ice pack of the Weddell Sea, which persisted during the two following winters1. Subsequent research showed that deep convective overturning had opened a conduit between the surface and the abyssal ocean, and had maintained the polynya through the massive release of heat from the deep sea2,3. Although the polynya has aroused continued interest1,2,3,4,5,6,7,8,9, the presence of a fresh surface layer has prevented the recurrence of deep convection there since 19768, and it is now largely viewed as a naturally rare event10. Here, we present a new analysis of historical observations and model simulations that suggest deep convection in the Weddell Sea was more active in the past, and has been weakened by anthropogenic forcing. The observations show that surface freshening of the southern polar ocean since the 1950s has considerably enhanced the salinity stratification. Meanwhile, among the present generation of global climate models, deep convection is common in the Southern Ocean under pre-industrial conditions, but weakens and ceases under a climate change scenario owing to surface freshening. A decline of open-ocean convection would reduce the production rate of Antarctic Bottom Waters, with important implications for ocean heat and carbon storage, and may have played a role in recent Antarctic climate change.

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Figure 1: Spatial pattern of Southern Ocean deep convection in observations and models.
Figure 2: Southern polar ocean freshening and stratification.
Figure 3: Southern Ocean (55° S–90° S) convection area.
Figure 4: Southern polar ocean freshening and stratification in CMIP5 models.

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Acknowledgements

We thank D. Bianchi for his help with the analysis. This work was supported by the Stephen and Anastasia Mysak Graduate Fellowship in Atmospheric and Oceanic Sciences, by the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery programme, by the Canadian Institute for Advanced Research (CIFAR) and by computing infrastructure provided to E.D.G. by the Canadian Foundation for Innovation and Compute Canada. R.B. and I.M. were financially supported by grant NOAA-NA10OAR4310092.

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All authors shared responsibility for writing the manuscript. C.d.L. assembled and analysed observational data and model output. J.B.P. and E.D.G. conceived and supervised the study. I.M., R.B., E.D.G. and J.B.P. designed the CM2Mc experiments. R.B. performed the CM2Mc experiments.

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Correspondence to Casimir de Lavergne.

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The authors declare no competing financial interests.

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de Lavergne, C., Palter, J., Galbraith, E. et al. Cessation of deep convection in the open Southern Ocean under anthropogenic climate change. Nature Clim Change 4, 278–282 (2014). https://doi.org/10.1038/nclimate2132

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