Climate Cast: Is climate change changing the jet stream?

Those of us who look at jet streams everyday for the past 30 years have noticed something seems to be changing.

This may not your grandfather's jet stream roaring overhead.

It's a young and evolving subset of climate science, but several researchers have latched onto a theory and supporting observations that climate changes may be changing our jet stream. More specifically, so called "Rossby Waves" or planetary scale waves seems to be slowing down, and getting stuck more often.

Why should we care?

Because depending on which part of the wave you get stuck under, you may be pumping out your basement or watching your corn crop drown from the wettest year on record, as some Minnesota farmers have.

Meanwhile, California farmers have watched their crops die in the field from the deepest drought in a century.

Our frequent Climate Cast contributor University of St. Thomas Professor John Abraham penned a good piece this week for the Guardian on how climate changes may be affecting the upper winds and waves that drive the weather over our heads.

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Here are a few snippets from John's excellent piece in The Guardian.

Very recently, a paper Amplified mid-latitude planetary waves favor particular regional weather extremes was published in the journal Nature Climate Change. The authors, James Screen and Ian Simmonds, investigated the role that changes to upper level winds in the atmosphere have on the occurrence of extreme weather. What they found was very interesting.

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The authors went back into our weather records (1979–2012) and found the 40 months with the most extreme weather (most extreme precipitation and most extreme temperatures). They then evaluated how “wavy” the jet stream was during those extreme months. They found that,

months of extreme weather over mid-latitudes are commonly accompanied by significantly amplified quasi-stationary mid-tropospheric planetary waves. Conversely, months of near average weather over mid-latitudes are often accompanied by significantly attenuated waves.

In common parlance, this means that when the jet stream undulates and travels very slowly, we see more extreme weather. Conversely, when the jet stream travels in a straighter path, the weather is less extreme.

This association itself is not new but it brings the connection of large-scale climatic variations and our local weather to the fore of attention. Perhaps more important, however, are the follow-on observations from the authors. In particular, they report,

Depending on geographical region, certain types of extreme weather (for example hot, cold, wet, dry) are more strongly related to wave amplitude changes than others. The findings suggest that amplification of quasi-stationary waves preferentially increases the probabilities of heat waves in western North America and Central Asia, cold outbreaks in eastern North America, droughts in central North America, Europe, and central Asia, and wet spells in western Asia.

Dr James Screen, a Research Fellow at the University of Exeter and lead author of the study told me,

The impacts of large and slow moving atmospheric waves are different in different places. In some places amplified waves increase the chance of unusually hot conditions, and in others the risk of cold, wet or dry conditions.

To my knowledge, previous investigations have not elucidated the geographical locations of extreme weather nor have they identified the location-dependent weather extremes. It will be interesting to watch present and future weather to determine if these relationships between waves and weather continue.

The observations seem to fit with the theory that as the Arctic warms twice as fast as the equator, the smaller temperature contrasts between north and south may trigger changes in upper air patterns, and thus our daily weather.