

What's up with the wild extremes of weather?

Doyle Rice
Corn damaged by Midwest drought conditions in Brownsville, Ill., in 2012.

Recent bouts of extreme weather — including persistent warmth and drought in the western USA and cold outbreaks in the East — are linked to large fluctuations of wind patterns high above the Earth's surface, says a study published Sunday in the British journal Nature.

These strange fluctuations in wind patterns, related to jet streams at upper levels of the atmosphere, and their connection to global warming have been the subject of extensive research within the climate science community.

"Over the past three decades, there is evidence that extreme weather events are linked to changes in atmospheric air flow patterns," said study lead author James Screen of the University of Exeter in the United Kingdom.

Whether they stem from natural climate variations or global warming, changes in air flow patterns around the Northern Hemisphere — including the jet stream — are a major influence on prolonged bouts of unusual weather, be they hot, cold, wet or dry, Screen said. (Weather extremes in this context don't include severe weather, such as tornadoes and thunderstorms, or tropical weather, such as hurricanes.)

"The implication of our study is that if climate change was to make these wave patterns more frequent, this could lead to more heat waves in the western U.S., droughts in the central U.S. and cold outbreaks in the eastern U.S.," Screen said.

High-altitude winds such as the jet stream typically howl from west to east around the globe, but they don't go in a straight line. The flow meanders to the north and south, in a wave-like path. These wave patterns are responsible for sucking either warm air from the tropics, or cold air from the Arctic, to Europe, Asia or the USA. They also can influence rainfall totals by steering rain-laden storms.

Previous research has suggested that climate change may lead to more meandering jet streams.

"This is another in a recent flurry of studies about the link between climate change and extreme weather and how large-scale atmospheric circulation patterns are affected by climate change," said climatologist Michael Mann of Penn State University, who was not part of the study.

The polar jet stream is a band of fast winds high in the atmosphere that marks the boundary between cold polar air and warmer air to the south. When large waves develop in the flow, cold Arctic air moves south (as seen here over eastern US) and warmer temperate air moves north (as seen here are central US). Such "wavy" conditions increase the chance of temperature and precipitation extremes.

"The methodology and findings are very sound," agreed research scientist Jennifer Francis at Rutgers University, also not part of the study. She said its main contribution is a findingthat large north/south waves in the jet stream's path cause slow-moving weather patterns that frequently lead to extreme weather.

The weather extremes the authors analyzed in the study were month-long heat waves, cold spells, droughts and prolonged wet periods, which occurred over large areas of the world from 1979-2012.

In May, a study in the journal Science noted that this past winter's unusual chill in the central and eastern U.S., along with the dry West, was partly due to global warming, which made these "atmospheric waves" occur with greater frequency.

A second study from scientists at Stanford University, also out Sunday in Nature Climate Change, finds that climate change is expected to increase air stagnation, in which an air mass remains over an area for an extended period.

This can have serious health implications, such as cardiovascular disease and respiratory problems, because of increased pollution exposure.

According to the team's analysis, more frequent air stagnation by the late 21st century will affect areas covering approximately 55% of the current global population, with some regions potentially experiencing an increase of up to 40 stagnant days per year.

In the western USA, for example, the roughly 80 stagnant air days per year now would increase to more than 100 by the late 21st century.