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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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.
The authors declare no competing interests.
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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). https://doi.org/10.1038/s41558-018-0350-3
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