Guest post: Deciphering the rise and fall of Antarctic sea ice extent

More powerful westerlies (black arrows) press cold, surface area water and sea ice northwards. On decadal timescales, enhanced westerlies upwell warm, deep waters to the surface area (little red arrow) leading to reduced sea ice cover.

The circumpolar flow of the westerlies is disrupted by a zonal-wave-number three pattern. As the name recommends, this is a pattern of three high-pressure ridges interspersed with 3 low-pressure troughs. The inmost of these low-pressure troughs is the Amundsen Sea Low (ASL)– shown by the blue shaded area and called after the sea off West Antarctica it usually sits above. Winds move clockwise around such low-pressure systems in the southern hemisphere, blowing cold air and sea ice northwards on the western flank. Warm air blows southwards on the eastern flank and can push sea ice back towards the coast. These winds are shown by the green arrows.

Sea ice kinds at the interface in between the environment and the ocean. The surface area winds that can blow sea ice around and bring cold or warm air to the area are controlled by a large-scale climatic flow.

Antarctic sea ice degree increased slowly over much of the satellite record (1979 to present), reaching succeeding record highs in 2012-14. 2016 saw the start of the most pronounced fall in sea ice cover ever observed. Over the 3 years that followed, the sheer drop in Antarctic sea ice was equivalent to 30 years of sea ice loss in the Arctic.

Yearly Antarctic sea ice degree from 1979 through to 2020 (average for the period is shown by dotted blue line), based on information from the National Snow and Ice Data Center (NSIDC). Based on satellite data, sea ice level is the overall location where the ice concentration is 15% or higher.

In winter, Antarctica efficiently doubles in size as 15m square kilometres (km2) of the ocean freezes. While the Arctic is hemmed in by continents that limit seasonal sea ice growth, the Antarctic is surrounded by mile-after-mile of water where sea ice can form.

While the sea ice seems to be recovering, coverage is still well listed below the 2014 record high. Sea ice cover in this area is clearly vulnerable to quick modification. If we are to forecast future change precisely, we require to pinpoint the procedures both near and far that affect the winds around Antarctica.

Maps of Antarctic sea ice extent at the most current winter season optimum on 28 September 2020 (left) and summertime minimum on 21 February 2021 (right). The pink lines reveal the 1981-2010 average level for that month. Credit: NSIDC.

The sharp decrease in Arctic sea ice over recent years has actually turned into one of the most long-lasting images of the Earths warming environment. At the same time, the sea ice modifications at the Earths south pole have been much less clear-cut.

Processes far and wide shape the ice.

Sea ice irregularity.

The chart listed below demonstrate how annual typical Antarctic sea ice degree has fluctuated through the satellite record.

Our perspective paper, published in Nature Geoscience, draws on current research to examine what the recent significant loss in Antarctic sea ice cover reveals about the processes that manage sea ice in the region..

These fluctuations led the Intergovernmental Panel on Climate Change (IPCC) to conclude in its 2019 unique report on the ocean and cryosphere that Antarctic sea ice level “overall has actually had no statistically significant pattern” due to “contrasting local signals and large interannual variability”.

Enhanced westerlies have a “two-timescale” effect on the sea ice cover. More powerful winds at first drive sea ice growth by pressing sea ice far from the Antarctic continent and bringing cold, surface area waters northwards, which then freeze. But, over years, this increased northward transport brings warm, deep ocean waters to the surface that melts sea ice, resulting in a decreased cover in the long-lasting (little red arrow).

This year-to-year variability suggests that Antarctica has less thick sea ice that lasts for several years. It likewise suggests that sea ice degree is more likely to vary significantly from one years to the next..

In addition, a maybe unexpected impact on Antarctic sea ice comes from the tropical oceans. When the ocean surface area warms, heat and wetness rise vertically in the atmosphere. The atmosphere attempts to remain in balance and a giant winding wind called a planetary wave, or Rossby wave, transfers this heat towards the poles.

The figure listed below highlights the different forces at play around Antarctica. Known as the “roaring 40s”, “furious 50s”, or “shouting 60s”– depending on their latitude– the westerly winds that encircle Antarctica (shown by the black arrows) periodically reinforce or compromise via a blood circulation pattern referred to as the Southern Annual Mode (SAM)..

However, since sea ice in Antarctica spreads to lower, warmer latitudes, less will then make it through in the summer as temperature levels rise. On average, just around 15% of Antarctic sea ice lasts from one year to the next, unlike around 40% for the Arctic. You can see this in the maps below, which reveal Antarctic sea ice extent at the 2020 winter optimum (left) and the 2021 summer minimum (right).

We find that the swift and unforeseen sea ice decrease was primarily triggered by an extraordinary weakening of the westerly winds that surround the continent of Antarctica..

The yearly fluctuation in Antarctic sea ice is one of the largest seasonal changes in surface area cover on the world. It has substantial ripple effects for the worldwide climate and marine ecosystems..

2 essential environment cycles affect Antarctic climatic flow in this way. The Interdecadal Pacific Oscillation (IPO) explains how sea surface temperatures in the western Pacific swing between warm (IPO+) and cool (IPO-) phases. The Atlantic Multidecadal Oscillation (AMO) represents warm (AMO+) versus cool (AMO-) sea surface temperatures in the North Atlantic Ocean.

Month-to-month sea-ice extent anomalies considering that 1979 with regard to the 1979-2018 climatology (blue line), based on information from the NSIDC. Trend lines (orange) are computed through the regular monthly abnormalities. Orange shading indicates when the IPO was unfavorable. Grey shading indicates when the AMO was positive. Credit: Clare Eayrs, adapted from Eayrs et al. (2021 ).

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The chart below shows the remarkable shift in Antarctic sea ice (blue line), before (orange strong line) and after (orange rushed line) August 2015.

The 2016 decrease was mainly due to coincidental, however naturally occurring variability..

Sea surface area temperature levels in the tropical oceans can influence the strength of the Antarctic westerly winds and the character of the Amundsen Sea Low in various methods.

There is still some dispute on whether deep ocean heat played a role in rapidly melting the sea ice, however we understand that, during the decade previously, IPO- enhanced the westerlies and set up the conditions to bring warm water to the subsurface of the ocean.

The IPO and the AMO act on various timescales and influence the Antarctic environment in different ways. IPO- and AMO+ cause a much deeper Amundsen Sea Low and stronger westerly winds, as displayed in the table listed below..

IPO+IPO- AMO+>> AMO-.

A strong zonal-wave-number-three pattern in the fall of 2016 resulted in southward transfer of atmospheric heat. Then, an exceptional weakening of the winds circling the Antarctic continent pushed sea ice back towards Antarctica and triggered the massive reduction in sea ice cover.

Less wind (+ a warmer ocean) = less ice.

A modification to the IPO+ and AMO- damaged the westerly winds, however lots of other explanations for the specifically deteriorated westerlies have also been proposed..

Westerliesweakerstrongerstrongerweaker.

There is still a lot of work to be done to comprehend the role of various drivers so that we can forecast how Antarctic sea ice may change in the future.

Amundsen Sea Lowshallowerdeeperdeepershallower.

A blip in the record or a harbinger of future quick decline?

Annual Antarctic sea ice degree from 1979 through to 2020 (average for the period is shown by dotted blue line), based on information from the National Snow and Ice Data Center (NSIDC). More powerful winds initially drive sea ice expansion by pushing sea ice away from the Antarctic continent and bringing cold, surface area waters northwards, which then freeze. The inmost of these low-pressure troughs is the Amundsen Sea Low (ASL)– shown by the blue shaded area and called after the sea off West Antarctica it usually sits above. Precisely determining Antarctic sea ice thickness is an active area of research and is necessary if we are to totally comprehend how the ice is changing.

The westerlies are anticipated to heighten as the climate warms. The “two-timescale” theory described above recommends that this will cause sea ice growth in the brief term but ultimately the cover will reduce. Stronger westerly winds are also anticipated to result in increased heat storage in the Southern Ocean, which also indicates a future fall in sea ice degree.

We understand that the westerly winds are key to identifying how much sea ice there remains in a given year, however we still have a lot to discover the procedures that drive these winds..

Sharelines from this story.

Over the 3 years that followed, the sheer drop in Antarctic sea ice was equivalent to 30 years of sea ice loss in the Arctic.

The authors of this visitor post would also like to acknowledge the contribution of their paper co-authors, Dr Xichen Li and Prof Marilyn Raphael.

Antarctic sea ice now appears to be going back to near to its typical state over the satellite record– although it has a long method to go to return to the record high coverage reached in 2014.

Exists a human-caused impact? The recent rapid decrease was brought on by natural irregularity, however we can not dismiss the role of human activity. In specific, the role of anthropogenic forcing in driving changes from the tropics need to be the focus of future research study.

However, it is clear that tropical variability has a function to play. Changes in Antarctic sea ice likewise have an extensive effect on the tropics and these linkages warrant further examination.

It is clear that, under certain conditions, Antarctic sea ice can be vulnerable to quick modification. Sea ice plays a substantial function in worldwide environment and it is very important that we narrow the knowledge gaps to be able to make future projections.

Eayrs, C. et al. (2021) Rapid decline in Antarctic sea ice over the last few years mean future modification, Nature Geoscience, doi:10.1038/ s41561-021-00768-3.

Sea ice degree is only part of the story. For now, we only have trusted information on sea ice cover from satellites. Accurately determining Antarctic sea ice thickness is an active area of research study and is vital if we are to fully comprehend how the ice is altering.

What next?

Our understanding of the processes that manage Antarctic sea ice cover is hindered by an absence of observations and, in the meantime, we depend on the 40-year satellite record to keep an eye on sea ice. One pushing concern is a possible two-to-five-year information space in between the anticipated decommissioning of ICESat-2 and CryoSat-2– the just 2 satellites supplying altimetry in polar regions– and the launch of the next polar altimeter, CRISTAL, in 2027.

It is clear that the procedures that influence Antarctic sea ice act upon a vast array of time and space scales. New strategies are urgently needed as we can no longer rely on linear techniques and zonally-averaged metrics to study sea ice variability.

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