Arctic heat spasm caused by stratosphere warming has a southern cousin
By Peter Hannam28 February 2018 — 8:52pm

The remarkable buckling of weather patterns in the northern hemisphere this week that has seen a relative Arctic heatwave drive frigid conditions into much of Europe has a southern counterpart that is being closely watched by Australian scientists.

A so-called sudden stratospheric warming event pushed temperatures in the high Arctic close to and above freezing even though the region remains in total darkness in the dead of winter, stunning researchers.

Snow has enveloped the UK, halting trains, closing schools and spreading a chill.

That areas off Greenland normally thick with ice at this time of year are now open sea will be little comfort to millions of Europeans coping with heavy snowfall from an event dubbed the "Beast from the East"

Scientists have been tracking the unusual Arctic winter-warmth and trying to understand how much of it is natural variability or climate change. The Arctic has been heating up at twice the average for the globe, with the most rapid warming during winter.

Research published last year in Geophysical Research Letters found that while bursts of warmth in the Arctic Ocean winters were not unprecedented, they have been increasing in frequency and duration.

Of interest is how observed changes in the stratosphere - starting about 18 kilometres above sea-level at the equator and about 8 kilometres at the poles - affect conditions at the surface.

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Eun-Pa Lim, a scientist in the Climate Research Section at Australia's Bureau of Meteorology, said the changes underway in the southern hemisphere are less clear-cut than the north.

"Although polar stratospheric warming is generally weaker and less sudden [in the south], it does occur," Dr Lim told Fairfax Media.

"It tends to warm up the surface in the Southern Ocean region, Australia, South Africa, and southern South America but makes it cold in the southern Atlantic and Indian Oceans - where no human being lives."

As yet, there is no clear trend detected in such warming events in the southern hemisphere, possibly because of the role played by the ozone hole.

The ozone hole is expected to continue to close as the ban on ozone-destroying chemicals takes effect.

"Such change in the southern hemisphere stratosphere is supposed to somehow counteract the impact of increasing greenhouse gases on changing the [hemisphere's] circulation patterns," Dr Lim said.

That tug-of-war involves the increasing greenhouse gases pushing the belt of the westerly winds in the mid-latitudes of the south poleward, whereas the mending of the ozone hole will tend to shove that belt towards the equator, she said.

The effect of stratospheric jetstream changes has been evident in the northern hemisphere this week as circulation changes propagate downward to the planet's surface. In the southen hemisphere, the impacts altering the extent of Antarctic sea-ice, marine biology in the Southern Ocean, and the weather over the continents, including Australia.

Drought effects
For Australia, the surface impact of stratospheric warming events is to bring hotter-than-usual conditions to much of the country, Pandora Hope, a senior research scientist in the bureau's climate section, said.
The last major event was in 2002, when the warming worsened a drought at the time, Dr Hope said.
Similarly, the 1982 spring-summer drought was influenced by polar stratospheric warming, Dr Lim said.
There was also a stratospheric warming event in the late spring to early summer of 2016, which "made a big contribution to the record low sea-ice around Antarctica in 2016", she said.

"Compared to the northern hemisphere, the variability and predictability of the southern hemispheric stratospheric circulation have been paid less attention," Dr Lim said.

According to Dr Lim, research priorities include the processes that trigger or amplify anomalous warming or cooling of the upper stratosphere, and whether forecast models can predict those stratospheric anomalies.