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Long reach of El Niño to Antarctic ice

  Getz Ice Shelf. Copyright image

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The Getz ice shelf in West Antarctica feels the bond strongly

Antarctica may be thousands of kilometers from the central Pacific, but events there can have a significant effect on the ice of the White Continent.

Scientists have shown how ice shelves, the floating fronts of glaciers that end at sea, respond to the El Niño phenomenon.

The warming of the tropical marine waters of the eastern Pacific will cause a change in wind patterns in the polar south.

This promotes snowfall on the shelves and also the melting of its lower parts.

These are competitive processes, of course. One adds mass; one removes it

However, the net result is a loss, scientists say. The reason? The ice that is extracted from under the floating slabs has a greater density than the spongy snow on the surface.

Influence of pressure

It is another example of the complexity that researchers must understand as they try to measure how Antarctica will react to a warming world.

Although much of the continent is relatively static in its behavior at this time, it is losing ice in the west, especially in the Amundsen sea sector, where glaciers are thinning and accelerating.

The ice that is thrown in this region – many tens of billions of tons a year – adds to the rise in sea level worldwide.

Dr. Fernando Paolo, of the Scripps Institution of Oceanography, and his colleagues report on their work in the journal Nature Geoscience.

They analyzed more than two decades of satellite radar measurements of ice shelves. The spacecraft has continuously tracked the height of the shelves since 1994.

Once scientists eliminated the long-term negative trend, they found a variable signal that could be related to El Niño / Southern Oscillation (ENSO).

This oscillation causes the surface waters in the eastern tropical Pacific to oscillate between the warmer conditions than the average (El Niño) and the colder than the average (La Niña).

It is recognized that ENSO has a global influence, altering patterns of rain and drought, for example. And in the southern polar region, it seems to influence atmospheric pressure fields.

One in particular, known as the low sea of ​​Amundsen, governs both regional winds and ocean circulation.

During an El Niño phenomenon, it increases the snowfall rates on the shelves, but it also extracts more hot water from the depths that can go under the shelves and melt them.

The combined effect leads to a loss in the mass of the shelves. In a large El Niño phase, such as that of 1997/98, this reduction may be equivalent in scale to that derived from the long-term negative weight loss trend.

"That means for a short period of time adding the two together, and that's key information to put on computer models if you want to adequately represent the dynamics of these systems," explained Dr. Paolo, who has now moved to the US space agency.

In La Niña years, the opposite happens: less snowfall, but also less melting at the bottom of the shelves. This works briefly to stop the negative trend in progress, in the long term.

"Before this, we knew that ENSO should affect Antarctica, which should affect the ocean and atmosphere throughout the continent, but this is the first time we have detected that signal on ice shelves," said Scripps. author Prof Helen Fricker.

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Artist's impression: Cryosat, with its radar altimeter, was launched into orbit in 2010

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The study shows the value of satellite measurements in progress in the polar regions. Because there may be several years between El Niño events, a single satellite can catch only a couple of occurrences during its operational life.

Therefore, an uninterrupted series of satellites is required to capture the big picture.

The European Space Agency (Esa) has managed to do this with its radar satellites ERS-1, ERS-2, Envisat and Cryosat, although that may not be the case for much longer since Cryosat is aging and no replacement is likely to occur. it will be available until the mid-2020s.

Nasa has not done as well as Esa. It has allowed a gap to develop between its satellite laser missions, which make observations very similar to the radar on Antarctica and the Arctic.

An important Earth observation report called the "Decadal Survey" was delivered last week through the US National Academies of Science, Engineering and Medicine. UU

The authors said it was "critical" to maintain a satellite capacity to measure ice elevation in the polar regions throughout Europe or America.

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