Natural variability of Southern Ocean convection as a driver of observed climate trends


Observed Southern Ocean surface cooling and sea-ice expansion over the past several decades are inconsistent with many historical simulations from climate models. Here we show that natural multidecadal variability involving Southern Ocean convection may have contributed strongly to the observed temperature and sea-ice trends. These observed trends are consistent with a particular phase of natural variability of the Southern Ocean as derived from climate model simulations. Ensembles of simulations are conducted starting from differing phases of this variability. The observed spatial pattern of trends is reproduced in simulations that start from an active phase of Southern Ocean convection. Simulations starting from a neutral phase do not reproduce the observed changes, similarly to the multimodel mean results of CMIP5 models. The long timescales associated with this natural variability show potential for skilful decadal prediction.

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Fig. 1: Annual SST and sea-ice time series and trends.
Fig. 2: SO internal variability in the pre-industrial control run.
Fig. 3: Dependence of transient climate response on the initial state of SO convection.

Data availability

The HadISST data are available at; ref. 51. The NOAA’s ERSST data set is available at; ref. 52. The NSIDC NASA Team SIC and area data are available at; ref. 53. The 20CRv2 data set is available at; ref. 54. The source code of ocean component MOM6 of the SPEAR_AM2 model is available at The model experiments that support the findings of this study are available from the corresponding author on request.


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We thank L. M. Polvani and M. Bushuk for their helpful discussions on the preliminary results. We thank L. Zanna and M. Bushuk for their valuable suggestions and comments on our paper as internal reviewers. We thank A. Shao and M. Harrison for their great help in producing closed heat budget terms in SPEAR_AM2 model.

Author information

L.Z. and T.L.D. conceived the idea and wrote the paper. L.Z. wrote the first draft, performed the analysis and conducted the sensitivity experiments. T.L.D. and W.C. lead the development of the SPEAR_AM2 model. X.Y. leads the SLP assimilation based on the SPEAR_AM2 model. All authors contributed to improving the manuscript.

Correspondence to Liping Zhang.

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Supplementary Figures 1–16, Supplementary References

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Zhang, L., Delworth, T.L., Cooke, W. et al. Natural variability of Southern Ocean convection as a driver of observed climate trends. Nature Clim Change 9, 59–65 (2019).

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