Climat change and the Jet Stream

Extreme Summer Weather Linked to Vanishing Ice and Snow

Loss of sea ice and snow cover gets summer going earlier, researchers say

by Ken Branson

Rutgers University,

17 December, 2013

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As summer sea-ice and snow shrink back in the Arctic, the number of summertime “extreme” weather events in the middle latitudes of the Northern Hemisphere is increasing, according to research published recently in Nature Climate Change by two Chinese scientists and their Rutgers colleague.

“It’s becoming increasingly clear, I think, that the loss of sea ice and snow cover is setting up the conditions that jump-start summer,” said Jennifer Francis, research professor at the Institute of Marine and Coastal Sciences in Rutgers’ School of Environmental and Biological Sciences. “The soil dries out earlier and that allows it to get hotter earlier. This phenomenon is also changing circulation patterns in the atmosphere.”

The jet stream – the fast-moving ribbon of air that encircles the Northern Hemisphere – has a profound impact on weather in the middle latitudes, Francis explained. Temperatures in the Arctic warm faster than in the middle latitudes because of the retreating ice and snow. Because the north-south temperature difference is the main driver of the jet stream, a smaller temperature difference means that the west-to-east winds of the jet stream are weakening.

This weakening also causes the jet stream to meander more, north and south. Because these waves in the jet stream control the movement and formation of storms, an increased meandering means that weather conditions – hot or cold – will be longer-lasting. Previous studies by Francis and her colleagues have shown that rapid sea-ice loss in fall and winter affects winter weather patterns and cold extremes in the mid-latitudes, as well.

“Just how that connection works still isn’t completely clear, but our study contributes to a growing body of evidence that the melting Arctic has wide-ranging consequences for people living in the middle latitudes,” said Quihong Tang of the Chinese Academy of Sciences’ Institute of Geographic Sciences and Natural Resources Research, who is the lead author of the study.

To reach their conclusions, Tang, Francis and Tang’s graduate student Xuejun Zhang used satellite data and atmospheric re-analysis, which are data created by using a computer weather-forecast model backward in time. The process includes data from all types of sources – satellites, ship and aircraft observations, and surface measurements – some of which may not have been available when the models were used to create weather forecasts.

“In this way the same model is used consistently, so that any changes apparent in the atmospheric fields are real, and not due to changes made in the model over time,” Francis said.