Jet Stream changes and extreme weather

Jet Stream Changes Driving Extreme Weather Linked Again To Global Warming, Arctic Ice Loss

Think Progress,

19 August, 2014

California is suffering through its worst drought on record, while the East Coast sees off-the-charts flooding. Both types of extremes are worsened by global warming as scientists have explained for decades.

But in recent years you may have noticed a disproportionate increase in record-smashing extreme weather and suspected that’s also linked to global warming. A new study from a team of scientists from the Potsdam Institute for Climate Impact Research (PIK) says you’re right. The PIK release explains:

Weather extremes in the summer — such as the record heat wave in the United States that hit corn farmers and worsened wildfires in 2012 — have reached an exceptional number in the last ten years. Man-made global warming can explain a gradual increase in periods of severe heat, but the observed change in the magnitude and duration of some events is not so easily explained. It has been linked to a recently discovered mechanism: the trapping of giant waves in the atmosphere. A new data analysis now shows that such wave-trapping events are indeed on the rise.

A number of studies in recent years have linked this quantum jump in extreme weather to global warming and the warming-driven loss of Arctic ice (see hereand here).

Jennifer Francis of Rutgers University’s Institute of Marine and Coastal Sciences has been at the forefront of this research. She explains her findings in this video.

A key point is that the path of the jet stream “typically has a meandering shape, and these meanders themselves propagate east, at lower speeds than that of the actual wind within the flow. Each large meander, or wave, within the jet stream is known as a Rossby wave.”

This new PIK study offers a specific mechanism for why we’re seeing this quantum leap in extreme weather — some Rossby waves are stalling out for extended periods of time: “the study shows that in periods with extreme weather, some of these waves become virtually stalled and greatly amplified.”

Why is this happening? Here things get a little technical, as befits a study titled, “Quasi-resonant circulation regimes and hemispheric synchronization of extreme weather in boreal summer.” But read on — our emerging understanding of why extreme weather has begun running amok may be one of the most important and consequential scientific findings in recent years:

We show that high-amplitude quasi-stationary Rossby waves, associated with resonance circulation regimes, lead to persistent surface weather conditions and therefore to midlatitude synchronization of extreme heat and rainfall events. Since the onset of rapid Arctic amplification around 2000, a cluster of resonance circulation regimes is observed involving wave numbers 7 and 8. This has resulted in a statistically significant increase in the frequency of high-amplitude quasi-stationary waves with these wave numbers.

Note that the study doesn’t merely find that stalling Rossby waves lead to an increase in extreme weather events. It also leads to extreme heat events and extreme rainfall events becoming synchronized (as, for instance, has happened just last week).

Here’s what that increase since 2000 looks like:

The number of planetary wave resonance events is shown as grey bars for each 4-year interval. While there used to be one or two events in a 4-year period, 2004-2007 saw three such events and 2008-2011 even five events. For comparison the red curve shows the change in Arctic temperature relative to that in the remainder of the Northern Hemisphere. Since 2000, the Arctic has warmed much faster than other latitudes [aka Arctic amplification]. Graph: PIK

Arctic amplification is the accelerated warming that occurs in the Arctic relative to the rest of the globe’s human-driven warming. A key reason it occurs is that as the more reflective snow and ice melt in the Arctic, darker land and ocean are exposed — and they absorb more solar energy. Other elements of Arctic amplification are discussed here.

Resonance regimes are associated with standing waves, which under the right condition can have a very large amplitude. Quasi-resonant means “almost resonant,” as scientist-blogger Greg Laden writes in his detailed explanation of the study, “and resonant means that instead of the meanders meandering around, they sit in one place (almost).”

What is the specific link between stalling Rossby waves and Arctic amplification? The study concludes, “We argue that recent rapid warming in the Arctic and associated changes in the zonal mean zonal wind have created favorable conditions for double jet formation in the extratropics, which promotes the development of resonant flow regimes.”

What’s a double jet stream formation? Wikipedia notes, “Jet streams can split into two due to the formation of an upper-level closed low, that diverts a portion of the jet stream under its base, while the remainder of the jet moves by to its north.” Last year, Popular Mechanics had a good discussion in its article, “How the Dual Jet Stream Sparks This Weird Summer Weather.”

Here is the jet stream from May/June 2012:

And here is the double jet stream from May/June 2013, a period of very unusual weather in Europe and the U.S. — “McGrath, Alaska, hit 94 degrees on June 17, four degrees warmer than Miami, which sits 4200 miles closer to the equator.” In the graphic, the two jet-streams are the (small) green band of wind surrounding the Arctic and the (larger) one over the United States.

We have much more to learn about “Recent Arctic amplification and extreme mid-latitude weather,” as made clear in a recent Nature Geoscience paper (with that title) written by several of the leading researchers in the field, including Francis. But the evidence is mounting that we have entered a new regime of extreme weather thanks to our as-yet unrestricted emissions of greenhouse gas