Monday, July 5, 2021

Climate change: Extremes committee validates Antarctic record heat

Image copyright NOAA
Esperanza station also held the previous record high temperature of +17.5C which occurred in 2015

From BBC by Jonathan Amos

A new record high temperature for the Antarctic continent of +18.3C has been confirmed by the World Meteorological Organization (WMO).

It occurred on 6 February last year at Argentina's Esperanza research station.

The mark was widely reported at the time but has now been validated by a WMO committee set up to check extreme weather data from around the globe.

The same group rejected an even higher Antarctic claim for 2020 of +20.75C, "recorded" on Seymour Island.

This again received international headlines, but the committee found the sensor set-up incorporated into a Brazilian permafrost experiment had not been properly protected from direct sunlight.

Thermometers are supposed to record air temperatures inside a ventilated covering, or screen.

The WMO team said that on Seymour Island this took the form of a modified length of scaffolding pipe and would likely therefore have introduced a warming bias into any data readings.

Nonetheless, temperatures on the normally frigid Antarctic continent have been rising, especially along its peninsula - the great tongue of terrain that stretches north in the direction of South America.

Over the last 50 years, the peninsula warmed almost three degrees.

And although no official temperature recording has yet gone above +20C on the continent and its close-by islands, it's just a matter of time, says Prof John King from the British Antarctic Survey (BAS).

"If you consider all the area covered by the Antarctic Treaty - that's all land south of 60 degrees South latitude - then we had a temperature of +19.8C in January 1982 on Signy Island.

"Okay, that's from the maritime Antarctic rather than the continent proper, but I wouldn't rule out seeing +20C temperatures somewhere in the northeast Antarctic Peninsula sometime within the next decade," the WMO extremes committee member told BBC News.

One of the drivers of the rise in temperatures is the strengthened westerlies that now blow around the continent.

This powerful airflow produces warming conditions on the eastern, leeward side of the peninsula's mountainous spine.

Such warm, downslope winds are well known across the Earth, and wherever they occur they tend to have a local name.

The Chinook winds that drop over the Rockies and Cascades in North America are an example of this phenomenon.
In the Antarctic, they are known as Foehn winds - a title originally used in Alpine Europe.

The incidence of these warmer conditions has been increasing on the Antarctic Peninsula.
The Foehn winds appear also to be moving poleward as well.
"The occurrence of Foehn winds varies greatly from year to year," explained Prof King.
"During the latter part of the last century, there was a strengthening and somewhat southward movement of the circumpolar westerly winds, which meant that this was probably contributing towards more intense Foehn on the eastern side of the peninsula and hence the rather rapid warming trend we saw there.
"That strengthening trend in the westerlies eased off a bit towards the end of the 20th Century and into the first part of the 21st Century, which meant the temperature trend also eased off a bit. But I think there are now indications that this phenomenon is picking up again, and is likely to be an important contributor to future warming in the region."

Image copyright BAS
The Foehn winds drop down the eastern side of the peninsula mountains

The Antarctic continent spans an area of 14 million sq km (roughly twice the size of Australia), and is covered by 26.5 million cubic km of ice.

Its weather stations are sparse but average annual temperature are seen to range from about −10C at the coast to -60C in the highest parts of the interior.

Satellites can measure the "skin temperature", or the temperature "to the touch", of the continent and have taken readings below even -90C in places.
But meteorologists are concerned with standard air temperatures and these are measured about 1.5m off the surface.

Using this definition, the coldest temperature ever recorded in the Antarctic stands at -89.2C. This occurred at the Russian Vostok station on 21 July 1983.   
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