Antarctic sea ice has paradoxically become more extensive over the past four decades despite a warming climate. The regional expression of this trend has been linked to changes in vertical redistribution of ocean heat and large-scale wind-field shifts. However, the short length of modern observations has hindered attempts to attribute this trend to anthropogenic forcing or natural variability. Here, we present two new decadal-resolution records of sea ice and sea surface temperatures that document pervasive regional climate heterogeneity in Indian Antarctic sea-ice cover over the last 2,000 years. Data assimilation of our marine records in a climate model suggests that the reconstructed dichotomous regional conditions were driven by the multi-decadal variability of the El Niño Southern Oscillation and Southern Annular Mode (SAM). For example, during an El Niño/SAM– combination, the northward sea-ice transport was reduced while heat advection from the subtropics to the Southern Ocean increased, which resulted in reduced sea-ice extent in the Indian sector as sea ice was compacted along the Antarctic coast. Our results therefore indicate that natural variability is large in the Southern Ocean and suggest that it has played a crucial role in the recent sea-ice trends and their decadal variability in this region.
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This research was funded by the ERC StG ICEPROXY project (203441), the ANR CLIMICE project, FP7 Past4Future project (243908), the RCN OCTEL project (248776/E10), the Belgian Research Action through Interdisciplinary Networks Mass2Ant project (BR/165/A2/Mass2Ant), the JSPS KAKENHI (grants 23244102 and 17H06318), the Royal Society Te Apārangi Marsden Fund (MFP-VUW1808) and the MBIE NZ Antarctic Science Platform (ANTA1801). It also benefited from the ESF PolarClimate HOLOCLIP project. D.S. benefited from the Blue-Action project (European Union’s Horizon 2020 Research and Innovation Program, grant number: 727852) and the French LEFE-IMAGO programme. Hole U1357B samples and data were provided by the International Ocean Discovery Program (IODP).
The authors declare no competing interests.
Peer review information Nature Geoscience thanks the anonymous reviewers for their contribution to the peer review of this work. Primary Handling Editor: James Super.
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a, Raw relative abundances of the Open Water Diatom (OWD) group in core DTCI2010 off Adélie Land (black line)15 and resampled relative abundances of the OWD group at a 1 year resolution (red line). b, Resampled relative abundance of the OWD group at a 1 year resolution (red line) compared to the length of the yearly sea-ice free season off Adélie Land between 1979 and 2010 (blue line)15. Grey shaded boxes represent the upper and lower end members used to scale the relative abundances of the OWD group to the yearly open water length. c, The mean upper and lower end members used to calculate the equation to scale the OWD group relative abundances to the length of the open season.
Length of the sea-ice free season (orange line) calculated by applying the equation presented in Extended Data Fig. 1c to the relative abundances of the OWD group in IODP Hole U1357B and length of the sea-ice duration calculated (blue line) by subtracting the ice-free values from 365 days.
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Crosta, X., Etourneau, J., Orme, L.C. et al. Multi-decadal trends in Antarctic sea-ice extent driven by ENSO–SAM over the last 2,000 years. Nat. Geosci. 14, 156–160 (2021). https://doi.org/10.1038/s41561-021-00697-1
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