Oscilloscope
Britain’s cold snap is explained by the Arctic oscillation
Jan 11th 2010 | Economist.com
It is an ill wind that blows no good, as people have been remarking to each other since at least the 16th century. In the case of the bitter easterlies that have brought Britain colder, snowier weather than has been seen for a couple of decades, the proverbial benefit has been felt by the more foolish and facile of those who doubt the reality or likelihood of man-made climate change. “Snow Chaos: And still they claim it’s global warming” read the front page headline of Britain’s Daily Express on January 6th. The foolishness is not one sided. With less public prominence, those convinced of climatic doom mutter of “extreme events” being more likely in a more man-made climate, with the implication that this might in some way explain the current cold.
To try to make a climatic point either way out of a patch of unusual weather, though, is normally to be on a hiding to nothing, and so it is this winter. No one with any claim on the public’s respect has ever said that all of the natural ups and downs of climate will be ironed out onto a smooth upwards trend by greenhouse gases; their effects are expected to show up not so much in particular events, but in statistics. The reverse of the same coin is that there will still be cold snaps in a warming world. But that is not to say that the current cold might not have implications beyond the spring, or that it might not help explain more about how the climate actually works.
NOAA/ESRL
One possible implication is a change in the prospects of the current poster child for climate change—Arctic sea ice. The extent of summer ice in the Arctic Ocean has been decreasing at a rate of about 8% per decade. In 2007, as the result of prior losses, peculiar sunniness in some areas and a particular disposition of winds, the ice levels fell spectacularly. That particular alignment of circumstances did not hold sway over the following years, which accordingly saw the ice bounce back somewhat. The current cold in mid latitudes might, counterintuitively, reverse that trend and reduce the ice cover further.
The atmosphere cannot make heat, or even hold that much of it. There is more heat stored in the top four metres of the oceans than in all the Earth’s atmosphere. So when the atmosphere cools down one part of the globe, it is a good rule of thumb that it is warming some other part. In the case of the current mid-latitude chill, it is the high latitudes that are seeing the warming. In Greenland and the Arctic Ocean, December was comparatively balmy. The air above Baffin Bay and the Davis Strait was 7ºCwarmer than usual (though that still left it pretty cold).
This pole-centred roundel of warm-in-cold is symptomatic of what climatologists call the negative phase of the Arctic oscillation (AO). It is a mode of atmospheric circulation in which the stratosphere is unusually warm and westerly winds, which normally bring warmth from the oceans to northern Europe, are unusually weak. The warmth in the Arctic that this arrangement has brought may diminish the regeneration of sea ice over the course of this winter, meaning that next summer’s sun can push back the boundaries further.
The atmosphere is not just about temperature, though. Wind patterns matter too. And here the negative phase of the AO may be on the side of the ice. As Alun Anderson explains in his fascinating book “After the Ice: Life, Death and Politics in the New Arctic”, two wind-driven currents are crucial to understanding Arctic ice: the Beaufort gyre, which circulates north of Canada, and the transpolar drift, which sweeps through the waters north of Siberia towards Greenland. A positive AO weakens the Beaufort gyre with respect to the drift, allowing ice that would otherwise circulate for years to be flushed out into the North Atlantic. A lot of ice loss in the 1990s, when the AO was positive for an anomalously long time, can be put down to this effect. Conversely, a strong negative AO such as this winter’s may trap more ice in the Beaufort Sea, allowing it to thicken and refortify itself.
The two effects brought on by the extreme negative mode of the AO thus could cancel each other out. But it is also possible that one or other of them will win through, with a significant effect on this year’s summer ice. To some people, including, almost certainly, those who write the headlines in the Daily Express, the fact that the same phenomenon might explain either record low sea ice or continued recovery of sea ice will be seen as inconsistency, poor science, or something more suspect. To people actually interested in how the climate works, however, seeing what happens in a very strong negative AO may prove a boon, by allowing the roles of different processes in ice loss to be further disentangled. Weather and climate are not the same, but there are links between them in both directions—links which can usefully be understood.
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