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Potential causes of the record low Antarctic sea-ice extent in winter 2023

Sea ice extent Sea ice minimum Parameter Climate Change Science Antarctic

The year 2023 marked a turning point for the Antarctic, as the Southern Hemisphere experienced a substantial decrease in sea-ice extent. In a recently released study, climatologist Dr Monica Ionita (AWI) sheds light on the drivers of this unusual event using a combination of observational, satellite and reanalysis data, with a particular focus on the macro-scale atmospheric circulation.

The sea-ice extent in the two polar regions during the winter maximum and summer minimum are indicators for the general state of ice development and the effects of climate warming in the high latitudes. While we have observed a trend of declining sea-ice extent in the Arctic for the past several years, amounting to roughly 11.9% per decade in summer and 2.3% per decade in winter, along with an “Arctic amplification” of global warming that is three to four times as intense as the global mean, the ice extent in the Antarctic has remained virtually unchanged since the beginning of continuous satellite monitoring, or even shown a slight increase. However, for roughly eight years now, we have seen a significant decline in ice extent in the Antarctic, particularly in the southern summer, compared to the long-term mean, including a record low last winter.

The year 2023 marked a turning point for the Antarctic, as the Southern Hemisphere experienced a substantial decrease in sea-ice extent and a near-record low sea-ice minimum in July 2023 that was ca. 2.4 million square kilometres below the long-term mean. But what were the drivers of this record low winter, and does it represent the beginning of a potentially fundamental change in climatic conditions (also known as a regime shift) in the Antarctic? Monica Ionita, a climate researcher and climatologist at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research in Bremerhaven, addressed this question in a recently released publication, where she takes a closer look at the potential causes. In her study, Monica Ionita sheds light on the drivers of this unusual event using a combination of observational, satellite and reanalysis data, with a particular focus on the macro-scale atmospheric circulation.

SEA ICE PORTAL: Monica, your research focuses on the analysis of climatological time series, especially those on atmospheric parameters. According to your analysis, what are the most important factors that shaped sea-ice formation in the Antarctic in 2023, and how exceptional was it compared to past years?

Monica Ionita: 2023 was a very unique year, as a number of factors contributed to the extremely low sea-ice values in the Antarctic. Compared to past years, like 2016, in 2023 we had “stress factors” not only from the atmosphere, in the form of a very “wavy” atmospheric pattern, which was conducive to the advection of heat and moisture from the lower latitudes toward the sea-ice edge, but also from the ocean. You could say the influencing factors were both top-down and bottom-up. The water in the layers below the sea ice was also extremely warm, the water at the sea-ice edge was warmer than it had ever been before, and then there was the influx of warm and moist air. Taken together, they all had the same effect: limiting ice formation.

SEA ICE PORTAL: Except for March and April, all the months up to October 2023 show a record low monthly ice extent. What triggered this, and which regions are affected the most?

Monica Ionita: The unusually low sea-ice extent from May to August 2023 was chiefly caused by the dominance of the Zonal Wave 3 (ZW3) atmospheric pattern, which was characterised by alternating high- and low-pressure systems at the surface and was conducive to the advection of heat and moisture, especially over the Ross Sea (RS), the Weddell Sea (WS) and the Indian Ocean (IO). The anomalous large-scale circulation was accompanied by near-record high temperatures on the ocean’s surface and underlying layers to a depth of 100 m in the regions with lower sea-ice extents. In addition to the air and water temperatures, extreme and sensible flows of heat and moisture were observed, particularly over the WS, RS and IO, which further intensified the decline of the sea-ice extent in these areas. Especially over the Weddell Sea, air-temperature anomalies of up to 8°C and sea-surface-temperature anomalies of up to 3°C were observed from May to July 2023. Similar temperature anomalies were recorded over the Ross Sea, particularly in July and August 2023. Generally speaking, certain regions like the Ross and Weddell Seas are especially sensitive to ZW3 fluctuations because of their geographic location and the interactions between ZW3 and other atmospheric and oceanic processes. This happened in 2023 and 2016, for example. Essentially, the ZW3 pattern shapes thermal exchanges between the ocean and atmosphere. The advection of warm air can increase thermal transfer from the atmosphere to the ocean, producing higher sea-surface temperatures and delayed sea-ice formation.

SEA ICE PORTAL: In addition to the regional atmospheric conditions, large-scale circulation patterns also affect sea-ice formation. Which patterns affect sea-ice formation in the Antarctic, and how have things changed there over the past few years?

Monica Ionita: There are different regional and large-scale atmospheric and oceanic patterns which are driving the development of the sea ice formation in the Southern Hemisphere. On one hand we have the Southern Annular Mode (SAM), which is the dominant mode of climate variability in the Southern Hemisphere, characterised by changes in the strength and position of the westerly wind belt that circles Antarctica. A positive SAM phase is associated with stronger westerly winds, which can lead to increased sea-ice extent through wind-driven ice drift and enhanced cooling of surface waters. A negative SAM phase can result in reduced sea-ice extent due to weaker winds and warmer air temperatures. Another important large-scale mode of variability is the El Niño-Southern Oscillation (ENSO), which can have teleconnections to the Antarctic region through changes in atmospheric circulation and ocean-atmosphere heat exchange. El Niño events tend to be associated with increased sea-ice extent in the western Pacific and Ross Sea sectors, while La Niña events can lead to decreased sea-ice extent in those regions. There are also more regional atmospheric patterns which can affect the sea ice in the Southern Hemisphere, like the Amundsen Low, the Pacific-South American Mode and of course ZW3. One of the reasons for the recent low sea-ice years from 2016 onwards might be related to the fact that the magnitude of some of these patterns has increased, leading to amplified negative sea-ice anomalies. The persistence and increased magnitude of these patterns led to an exceptional situation in 2023 over large parts of the Southern Hemisphere, with the 2 m mean air temperature, sea-surface temperature, sensible surface-heat flux, moisture content and sea-ice extent reaching unprecedented levels, especially over the Ross Sea and Weddell Sea.

SEA ICE PORTAL: Why do you feel the study offers a unique perspective on climatic developments in the Antarctic?

Monica Ionita: Although the paper was mainly focused on the atmospheric conditions in 2023, it is important to emphasize that such studies are absolutely necessary to understand the particular features behind such events. This paper offers a view “from above” (e.g. the large-scale atmospheric drivers) as a complement to the paper by Purich and Doddridge (2023), which mainly focused on the oceanic drivers of the low sea-ice event in 2023. Unfortunately, in the past several years there have been so many extremes, throughout the climate system, that as a scientist it’s sometimes difficult to keep up with what’s going on and find sound scientific explanations.

SEA ICE PORTAL: What can be expected for the future development of the Antarctic sea ice, and has a fundamental change, in which even the Antarctic feels the effects of global climate change, already begun?

Monica Ionita: From a climatological perspective, the year 2023 was alarming and serves as a wakeup call regarding our understanding of Earth’s polar regions and the complicated interplay between climate change and its impacts. While various studies continue to unravel the complex mix of factors that contributed to the latest, dramatic decline in Antarctic sea-ice extent, it’s up to political decision-makers and society as a whole to acknowledge these developments and recognise the pressing need to take collective action, so as to reduce the drivers of climate change as soon as possible and protect the fragile ecosystems of the polar regions. The events of the year 2023 are a stark reminder of the fundamental, far-reaching impacts of climate change on even the most remote, untouched regions of our planet.

SEA ICE PORTAL: Thank you for the interview!

 

Publication: Monica Ionita (2024): Large-scale drivers of the exceptionally low winter Antarctic sea ice extent in 2023, Front. Earth Sci., Sec. Cryospheric Sciences, Vol. 12, https://doi.org/10.3389/feart.2024.1333706.

 

Contakt

Dr Monica Ionita (AWI)

Dr Renate Treffeisen (AWI)

Dr Klaus Grosfeld (AWI)

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