Why Antarctica's shrinking sea ice may be down to natural causes



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Sea ice cover in Antarctica rapidly accelerated to a record low in late 2016 and remained well below average.

But what behind the constant dramatic melting and low ice cover?

Research published earlier this month suggests that a combination of natural variability in the atmosphere and ocean is to blame, although human-induced climate change may also play a role.

What happened to Antarctic sea ice in 2016?

Antarctic sea ice is frozen seawater, usually less than a few meters thick. It differs from ice shelves, which are formed by glaciers, floating in the sea, and reaching up to a kilometer thick.

Antarctica

Mountains in Antarctica and ice floating in the Antarctic.

AAP

Sea ice cover in Antarctica has been critical for global climate and marine ecosystems and satellites have been monitoring it since the late 1970s. In contrast to the Arctic, the sea ice around Antarctica is slowly expanding (see Figure below).

However, in late 2016 Antarctic sea ice dramatically and rapidly dissolved reaching a record low. This picked up the interest of climate scientists because such large, unexpected and rapid changes are rare. Sea ice coverage is still below average now.

We wanted to know what caused the unprecedented decline of the Antarctic sea ice and changes in the system to those declines. We also wanted to know if this was a temporary shift or the start of a longer-term decline, as predicted by climate models. Finally, we wanted to know whether human-induced climate change contributed to the record lows.

Hunting for clues

Sea ice cover around Antarctica disappeared a lot or one decade later. In fact, the Antarctic sea ice cover has reached a record high as recently as 2014.

Antarctica

Antarctic and Arctic sea ice cover for January 1979 to May 2018.

That provided a clue. Like year-to-year and decade-to-decade sea ice cover varies so much, it can mask more-term melting of sea ice due to anthropogenic warming.

The next clue was in records far removed from Antarctica. In the spring of 2016, sea surface temperatures and rainfall in the tropical eastern Indian Ocean are high. It was in association with a highly negative Indian Ocean dipole (Iod) event that brought warmer waters to the north of Australia.

While the Iodine events involved rainfall in South East Australia, we found (using both statistical and climate model experiments) that it promoted a pattern in the winds over the Southern Ocean, which was especially conducive to decreasing sea ice.

The surface winds blowing from the north not only pushed the sea ice back to the Antarctic continent, they are also warmer, helping to melt the sea ice.

Northern winds have almost perfectly matched the major regions where sea ice declined.

Atmospheric circulation and sea ice concentration during September-October 2016. The peak figure shows September-October wind anomalies (vectors, weight at upper right, m ​​/ s) in the lower part of the atmosphere; Increasingly, red shading is increasing, airflow is more or less the same, and blue shading represents the south. The bottom figure shows sea ice gauge: Green represents more sea ice than average, and purple shows regions of a sea ice reduction.

Although the first studies connected the sea ice pattern to the sea, our studies are the first to maintain the dominant role of tropical eastern Indian Ocean driving.

But this is not the only factor.

Later in 2016, the typical westerly winds that surround Antarctica weakened to record lows. This caused the ocean surface to warm up, promoting less sea ice cover.

The Western Winds began to be at the top of Antarctica's atmosphere, known as the stratospheric polar vortex. We think the sequential occurrence of tropical and then stratospheric influences contributed to the records in 2016.

Taken together The evidence we present the idea that the rapid Antarctic sea ice decline in late 2016 is largely due to natural climate variability.

The current state of Antarctic sea ice

Since then sea ice has remained mostly healthy below average in union with warmer upper ocean temperatures around Antarctica.

We keep the products stronger than normal western winds in the previous 15 or so years around Antarctica, driven again by the tropics. The stronger westerly enjoyed a response in the ocean, with warmer subsurface water moving to the surface over time.

The combination of record tropical sea surface temperatures and weakened westerly winds in 2016 warmed the entire upper 600m of water in most regions of the Southern Ocean around Antarctica. The warm ocean temperatures have considered a reduced sea ice level.

Antarctic sea ice has seen a record low early in the New Year. It suggests the initial rapid decline seen in late 2016 is not an isolated event and, when combined with the decadal-timing of warming up the upper Southern Ocean, may mean reduced sea ice for some time.

We argue that so far, we can be understood in terms of natural variability superimposed on a long-term human-induced signal.

This is because the rain and ocean temperature records seen in the tropical East Indian Ocean that led to the initial Sea ice decline in 2016 likely to have some climate change contribution.

The warming and recovery of the Antarctic ozone hole may also affect surface wind patterns over coming decades.

Such changes can be driving climate-change effects that are starting to emerge in the Antarctic. But the limited data record and great variability indicate there is still too early to say,

We would like to acknowledge the role of our co-authors Abhic, Sicily and Beatrice, Christine Ta Chang, and Alice the Governor, Harry Hendon, Marica and Holland, the Enan-Pa Lim, Luan Thompson, and the Peter Wan Rench and Dongxia Yang. In contributing to the research discussed in this article.

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