Tuesday, May 24

Antarctic sea ice reaches its smallest historical surface

The area of ​​sea ice (or sea ice) in the Arctic is experiencing rapid decline as a result of global warming. In contrast, at the other pole of the Earth, it had increased by around 1% per decade since the late 1970s, although with significant variation each year and substantial regional differences.

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However, against this general trend of increase in Antarctica, there was a brief but quite marked anomaly in 2017, when the southern hemisphere sea ice experienced a record low. On February 25, 2022, just five years later, the lowest values ​​of Antarctic sea ice have been recorded again, being the first time that its surface is below two million square kilometers since they began to be used. satellite observations in 1978.

The sea ice area of ​​the entire Antarctic region was 30% below the average of the three-decade reference period 1981-2010

The data shows significantly lower than normal ice cover in the Bellingshausen and Amundsen Seas, the Weddell Sea, and the western Indian Ocean. More curiously, across the region, sea ice extent was 30% lower than the average for the three-decade reference period 1981-2010.

Possible causes of the thaw

The causes of this variability are complicated. Several mechanisms have been proposed in recent years, but there is still no scientific consensus. For this reason, the appearance of this new historical minimum in such a short period of time led a team of scientists from the Sun Yat-sen University and the Guangdong South Marine Science and Engineering Laboratory (both in China) to find out what had happened and why. The study findings are published in the journal Advances in Atmospheric Sciences.

With data from the National Snow and Ice Data Center (NSIDC), the researchers made a balance of the state of the sea ice from 1979 to 2022, that is, they analyzed the amount of frozen water that was added and lost during that period of time, together with the dynamic phenomena (processes related to the movement of the fluids) and thermodynamic (such as freezing and melting).

The scientific team discovered that in summer it is thermodynamics that dominates the processes that cause melting. This occurs through anomalies in poleward heat transport in the Bellingshausen and Amundsen seas, the western Pacific Ocean, and the eastern Weddell Sea.

In summer, melting occurs mainly as a result of a positive feedback of albedo and temperature.

At this time there is also an increase in global infrared radiation and visible light as a result of a positive feedback of albedo and temperature. Albedo describes the whiteness of a surface: the whiter it is, the greater the reflection of that radiation, and the darker it is, the greater the absorption.

“Sea ice is whiter than dark unfrozen sea, therefore [en el mar] there is less heat reflection and more absorption”, explains climatologist Qinghua Yang, co-author of the study. “Which in turn melts more sea ice, producing more heat absorption in a vicious cycle.”

In spring, however, both thermodynamics and dynamics contribute to the state of the sea ice. In addition to the aforementioned thermodynamic processes, the dynamics of ice loss in the Amundsen Sea causes a northward movement of the sea that pushes more ice to the lower latitudes of the tropics, which increases melting, especially in the Amundsen Sea and Ross Sea.

Climatic phenomena behind the new historical minimum

The authors note that, according to data from the US National Oceanographic and Atmospheric Administration (NOAA), the new record low occurred at the same time as a combination of La Niña and a positive Southern Annular Mode (SAM). MAS describes a belt of strong westerly winds or low pressures that encircles the continent and moves north or south, while La Niña describes a weather pattern of powerful winds that forcefully blow warm water from the surface of the continent. ocean in the tropics, from South America to Indonesia.

All atmospheric impacts that affect the size of sea ice originate from the intensity and position of low pressures in the Amundsen Sea.

Both phenomena intensify the low atmospheric pressure of the Amundsen Sea. The variability of atmospheric conditions in this region is greater than in any other part of the southern hemisphere. In addition, the researchers found that all atmospheric impacts that affect the size of sea ice originate from the intensity and position of low pressures in the Amundsen Sea.

Through this study, scientists were able to find some explanations as to why the floating ice cap in Antarctica is decreasing. However, their findings also raised more questions, which they hope to answer in future research. “If tropical variability has such an impact, that’s the place to study next,” concludes Jinfei Wang, one of the paper’s other authors.