Aug 20, 2018

Summer weather is getting 'stuck' due to Arctic warming

by
Jonathan Watts
,
The Guardian
Rising arctic temperatures have slowed the circulation of the jet stream and other giant planetary winds, which means pressure fronts are getting stuck and the weather is less able to moderate itself, say researchers. Photo: Henry Nicholls, Reuters
Rising arctic temperatures have slowed the circulation of the jet stream and other giant planetary winds, which means pressure fronts are getting stuck and the weather is less able to moderate itself, say researchers. Photo: Henry Nicholls, Reuters

Summer weather patterns are increasingly likely to stall in Europe, North America and parts of Asia, according to a new climate study that explains why Arctic warming is making heatwaves elsewhere more persistent and dangerous.

Rising temperatures in the Arctic have slowed the circulation of the jet stream and other giant planetary winds, says the paper, which means high and low pressure fronts are getting stuck and weather is less able to moderate itself.

The authors of the research, published in Nature Communications on Monday, warn this could lead to “very extreme extremes”, which occur when abnormally high temperatures linger for an unusually prolonged period, turning sunny days into heat waves, tinder-dry conditions into wildfires, and rains into floods.

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Circulation stalling has long been a concern of climate scientists, though most previous studies have looked at winter patterns. The new paper reviews research on summer trends, where it says there is mounting evidence of planetary wind systems – both low-level storm tracks and higher waves in the troposphere – losing their ability to shift the weather.

One cause is a weakening of the temperature gradient between the Arctic and Equator as a result of man-made greenhouse gas emissions. The far north of the Earth is warming two to four times faster than the global average, says the paper, which means there is a declining temperature gap with the central belt of the planet. As this ramp flattens, winds struggle to build up sufficient energy and speed to push around pressure systems in the area between them.

As a result, there is less relief in the form of mild and wet air from the sea when temperatures accumulate on land, and less relief from the land when storms build up in the ocean. Last year, Hurricane Harvey had a devastating impact on Texas because it was parked an unusually long time on the coast, where it kept drawing up moisture from the sea and dumping it in the form of the greatest deluge ever recorded in the US. Scientists had previously noted that hurricanes are slowing and bringing more rain.

A separate new paper in Scientific Reports indicated that the trapping of planetary airstreams – a phenomenon known as amplified quasi-stationary waves – also contributed to the 2016 wildfires in Alberta, which took two months to extinguish and ended as the costliest disaster in Canadian history with total damages reaching 4.7bn Canadian dollars.