The 2016 Abrupt Antarctic Sea Ice Decline: a re-evaluation with new perspectives

Abstract An unprecedented decline in Antarctic sea ice extent (SIE) in late 2016 ended a weak mutidecadal expansion. Multiple mechanisms have been hypothesised to explain this decline, including: large-scale atmospheric influences – a strong Zonal Wave Three (ZW3) in September 2016 and negative Southern Annular Mode (SAM) in November; oceanic contributions attributed to El Niño-associated surface warming and sustained warm-water upwelling due to intensification of the westerly winds. Given the diversity in hypothesised drivers and several years of anomalously low SIE post-2016 potentially signifying a state-change in Antarctic sea ice, we re-evaluate likely drivers of the 2016 event. Here we show that the decline occurred in distinct sub-monthly episodes from late-August to mid-December, with significant variation in synoptic conditions and that SAM phase transitions, rather than a strong ZW3 preceding a negative SAM, likely explain the magnitude of decline. Oceanic and ice-cover preconditioning may also have compounded the sea ice response to atmospheric drivers. Anomalously warm upper-ocean during 2016 was associated with early sea ice retreat in the Amundsen-Bellingshausen Seas and Indian Ocean sectors. Moreover, unusually large open water area developing in the Weddell Sea from autumn preconditioned that region for rapid heat uptake and SIE decline in warmer months. Finally, we show that record low sea ice-cover and broadscale upper-ocean warming during spring/summer point to enhanced radiative surface fluxes and oceanic heat uptake via ice-albedo feedbacks. Lingering effects from upper-ocean warming likely delayed the 2017 growth season onset, with heat potentially being subducted into the subsurface to re-emerge in subsequent seasons.