Intensifying circumpolar winds contributes to reducing winter Antarctic sea ice growth

During the austral autumn/winter of 2023 Antarctic sea ice exhibited a pan-Antarctic wide delay in refreezing of roughly a month (peaked in July 2023, hereafter referred to as W23 event). As such it is unprecedented over the satellite era and may point to a start of a transitioning to a new state of Antarctic sea ice. However, the relatively short observational record obscures our understanding how natural variability in Antarctic sea ice can act together with anthropogenic climate change in creating favorable conditions for extreme Antarctic sea ice changes. Here we show that an anomalous atmospheric circulation pattern prior to W23 (May-to-July 2023) is part of a longer-term (1979-2022) trend in observed mid-to-upper tropospheric winds around the Antarctic continent towards a wavier manner that favors anomalous moisture transport to the Weddell Sea. We further show that this circulation pattern is associated with winter sea ice anomalies on both year-to-year and interdecadal timescales in preindustrial control simulations of CMIP6 climate models as well as in future projections of large ensemble simulations under greenhouse gas emission scenarios. By conducting standalone simulations with the global ocean-sea ice model NEMO4-SI³ (forced by the atmospheric reanalysis ERA5) at two horizontal resolutions (1º & 0.25º), we also study the influence of the recently observed acceleration of ocean warming around the Antarctic continent and the effect of model horizontal resolution on the simulation of sea ice extremes. Our results overall suggest that internal atmospheric-sea ice coupling could be an important contributor to future winter Antarctic sea ice changes, enhancing the forced Antarctic sea ice changes that are primarily driven by ocean warming.