Observations show that reduced regional sea-ice cover is coincident with cold mid-latitude winters on interannual timescales. However, it remains unclear whether these observed links are causal, and model experiments suggest that they might not be. Here we apply two independent approaches to infer causality from observations and climate models and to reconcile these sources of data. Models capture the observed correlations between reduced sea ice and cold mid-latitude winters, but only when reduced sea ice coincides with anomalous heat transfer from the atmosphere to the ocean, implying that the atmosphere is driving the loss. Causal inference from the physics-based approach is corroborated by a lead–lag analysis, showing that circulation-driven temperature anomalies precede, but do not follow, reduced sea ice. Furthermore, no mid-latitude cooling is found in modelling experiments with imposed future sea-ice loss. Our results show robust support for anomalous atmospheric circulation simultaneously driving cold mid-latitude winters and mild Arctic conditions, and reduced sea ice having a minimal influence on severe mid-latitude winters.
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The model output is available on reasonable request from the corresponding author. ERA-Interim reanalysis data were obtained from the ECMWF data server (https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era-interim).
The code used to create the figures is available on request from the corresponding author.
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We thank the ECMWF for making the ERA-Interim reanalysis data available for use. The HadGEM2 model simulations were performed on the ARCHER UK national computing service. R.Blackport and J.A.S. were supported by Natural Environment Research Council grant number NE/P006760/1. For the creation of maps included in all figures, the authors used Python package ‘basemap’ (https://matplotlib.org/basemap/), copyright Jeffrey Whitaker 2011.
The authors declare no competing interests.
Peer review information: Nature Climate Change thanks John Fyfe, Qiuhong Tang and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Supplementary Figs. 1–12, Notes 1–5 and references.
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Nature Climate Change (2019)