Antarctic sea ice hits new low for second year

Antarctic sea ice extent dropped to its lowest level in 45 years of satellite observations on 21 February 2023 - the second year in a row with an area below 2 million km². This occurrence raises the question of whether the recent change in Antarctic sea ice is a brief anomaly or an early precursor to a transition to a long-term decline.

Sea ice in the Southern Ocean shows large variability, both seasonally and interannually. On 21 February 2023, the Antarctic sea ice extent reached its seasonal minimum of 1.788 million km², setting a new record low since the late 1970s [1] (Fig. 1a). It was set against the background of anomalously low ice extents since 2017, especially immediately after the previous record of 1.924 million km² in 2022 [1]. After the 2022 minimum, strong heat waves in mid-March brought large warm anomalies to East Antarctica and coastal areas, which kept the ice extent well below the climatology in March (the second lowest for the month of March on record). Since late May, the pace of seasonal ice growth had slowed dramatically, partly due to anomalous northerly/northwesterly winds in the eastern Pacific, western Atlantic, and central Indian sectors that transported warmer air and pushed the sea ice edge southward. Antarctic sea ice experienced a rare event – the lowest for three consecutive months (June, July, and August) on record. Since October, the seasonal ice melt had been at a well-above-normal pace. In response to a stronger seasonal warming anomaly, the decline in sea ice in December was greatly accelerated. This acceleration led to the second lowest December extent on record, followed by the lowest for two consecutive months (January and February 2023) on record. Large polynyas were identified in the Ross, Amundsen, and Weddell Seas.

Several atmospheric drivers associated with modes of climate variability might contribute to the 2023 new record minimum. First, at mid- and high-latitudes, 2022 experienced a persistent positive Antarctic Oscillation (AAO, except June) [2]. Moreover, a very strong AAO occurred in September, November and December 2022, and the strongest AAO for the month of January during the satellite era was set in 2023. This occurrence led to a persistent, stronger, and southwestward shift in the Amundsen Sea Low (ASL), which greatly reduced the sea ice in the Bellingshausen Sea and east of the Antarctic Peninsula through onshore warm wind advection and increased sea ice in the marginal sea ice zone between the Amundsen Sea and the eastern Ross Sea (Fig. 1b). Second, in the tropics, there was a moderate La Niña event that occurred in 2022, but it had been unusually prolonged for three consecutive years [3], making it a rare triple-dip event. Atmospheric deep convection over the southwestern tropical Pacific associated with La Niña triggered southeastward propagating Rossby waves, further deepening the ASL. Third, the joint influence of the negative phase of the Interdecadal Pacific Oscillation and the positive phase of the Atlantic Multidecadal Oscillation also favored strengthening of the ASL through stationary wave dynamics [4] but tended to have the opposite effect on atmospheric circulation in the central Indian sector.

However, unlike previous years, starting in June 2022, the Antarctic, beyond the control of the anomalous ASL, showed widespread negative sea ice concentration anomalies, particularly in spring and summer (Fig. 1b). The global ocean has absorbed most of the excess heat induced by anthropogenic forcing, and its temperature surged to a record high in 2022 [5]. The heat content of the Southern Ocean has increased faster than that of other oceans, and the subsurface south of ~55°S has been significantly warmer [6]. The circumpolar westerlies over the Southern Ocean have shown poleward intensification since satellite observations and are predicted to increase under anthropogenic forcing [7]. This can enhance Ekman suction, which facilitates warmer subsurface water being transferred upward. Recent research suggested that compared to the atmospheric circulation, the subsurface of the Southern Ocean had a smaller contribution to the extreme sea ice state before the unprecedented plunge during 2014-2017 but played a critical role in the persistent negative ice extent anomalies since 2016 [8]. Thus, human-caused global warming might act as a control valve through which subsurface ocean warming is being stirred into the surface.

More importantly, the new record low Antarctic sea ice extent in 2023 marks a reversal from the long-term positive trend to a negative trend for the time series of the minimum ice extent (Fig. 1a), indicating that Antarctic sea ice might enter a new regime. This finding further raises the question of whether changes in Antarctic sea ice in the past several years are a brief anomaly due largely to natural climate variability or early evidence of a robust transition from long-term increasing Antarctic sea ice to decreasing sea ice, in which anthropogenic forcing outweighs natural variability. Concern about a tipping point is enhanced by the fact that the latest generation of climate and earth system models projects a large decrease in Antarctic sea ice associated with increased greenhouse gases during the 21st century [9,10]. A large reduction in Antarctic sea ice would have profound impacts on the Antarctic climate and ecosystem, i.e., climate extremes, stability of ice shelves, the food chain and wildlife population, and global consequences, such as sea level rise and carbon cycle feedback. Thus, more research is needed to answer this question and improve our understanding of how future Antarctic sea ice change could interact with the broader earth system.

References

1. F. Fetterer, K. Knowles, W. Meier, M. Savoie, and A. Windnagel, Sea Ice Index, Version 3. Boulder, Colorado USA. National Snow and Ice Data Center (Accessed 22 February, 2023).
2. CPC - Antarctic Oscillation Index (Accessed 20 February, 2023).

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/aao/

1. CPC – Oceanic Nino Index (Accessed 20 February, 2023). https://origin.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ONI_v5.php
2. X. Li, et al., Tropical teleconnection impacts on Antarctic climate changes, Nat. Rev. Earth Environ., 2, 680-698, 2021.
3. L. Cheng, et al., Another year of record heat for the oceans, Adv. Atmos. Sci., https://doi.org/10.1007/s00376-023-2385-2, 2023.
4. IPCC, IPCC Special Report on the Ocean and Cryosphere in a Changing Climate. eds H.-O. Pörtner et al. Cambridge University Press, 755 pp. 2019.
5. R. Goyal, A. Gupta, M. Jucker, and M. England, Historical and projected changes in the Southern Hemisphere surface westerlies, Geophys. Res. Lett., 48, e2020GL090849, 2021.
6. L. Zhang, et al., The relative role of the subsurface Southern Ocean in driving negative Antarctic Sea ice extent anomalies in 2016-2021, Commun. Earth Environ., 3, 302, 2022.
7. J. Liu, and J. Curry, Accelerated warming of the Southern Ocean and its impacts on the hydrological cycle and sea ice, PNAS, 107, 14987-14992, 2010.
8. L. Roach, et al., Antarctic sea ice area in CMIP6, Geophys. Res. Lett., 47, e2019GL086729, 2020.

Contributions: J.L. conceived the study and wrote the manuscript, Z.Z. and J.L. prepared the figure, and all authors contributed to the manuscript preparation and discussion.

Funding: None

Data Availability: The satellite-derived Antarctic sea ice data are available at ftp://sidads.colorado.edu/DATASETS/NOAA/G02135.

Competing Interest: The authors have no conflicts of interest to declare.