Summary
An unprecedented decline in Antarctic sea ice extent (SIE) in late 2016 ended a weak mutidecadal expansion. Multiple mechanisms have been hypothesized to explain this decline, including large-scale atmospheric influences—a strong zonal wave 3 (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 hypothesized drivers and several years of anomalously low SIE post-2016 potentially signifying a state change in Antarctic sea ice, we reevaluate likely drivers of the 2016 event. Here, we show that the decline occurred in distinct submonthly 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, an 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 reemerge in subsequent seasons. Significance Statement Antarctic sea ice extent (SIE) declined dramatically in spring 2016. Multiple explanations of its causes include large-scale atmospheric drivers, El Niño–related upper-ocean warming, and long-term subsurface warming from stronger Southern Ocean winds. Our study shows that SIE decline occurred during several submonthly episodes with varying atmospheric circulations. We show that the transition in the Southern Annular Mode from strongly positive in September to strongly negative in November was important in explaining the magnitude of SIE decline. Interactions between ice cover, overlying atmosphere, and upper ocean likely contributed to the speed and extent of the overall decline in SIE. Significantly reduced sea ice cover by summer may have led to ongoing upper-ocean warming, affecting subsequent sea ice growth.