Climate Change, Waves, and Winters

June 24, 2014

Above, my video on the changes in atmospheric circulation that have exacerbated California’s drought, as well as leading, paradoxically, to brutal cold in Eastern North America.  Takeaway: the creeping realization that slow, incremental increases in global heat content can trigger abrupt changes in circulation patterns that we have built agriculture and civilization around.

Below, if you have not seen the SkyNews piece on the role of western Pacific warmth on this past winter’s “Ridiculously Resilient Ridge”, do take a look and consider the context of the current discussion.

Climate Central:

The pattern of a wavy jet stream was a recurring theme in U.S. weather forecasts this winter as a particularly jagged one essentially split the country in two. While there is a debate over whether climate change causes that pattern, new research shows that the waviness does exacerbate extreme weather.

The research, published in Nature Climate Change on Sunday, looked at planetary waves on a monthly timescale. Waves are essentially the ridges and troughs left as the jet stream, a fast-moving river of air, cuts it way across the middle of the northern hemisphere. The jet stream essentially helps drive weather patterns around the northern half of the globe by pushing around storm systems and sometimes impeding their progress.

James Screen, a climate scientist at the University of Exeter who co-authored the study, said he wanted to examine how planetary waves influenced persistent weather patterns, such as drought or extreme heat or cold.

In the U.S., Screen found that increased waviness made the western part of the country more susceptible to heat waves and the eastern part more likely to experience extreme cold. Droughts in the central part of the country as well in as Europe and central Asia, and wet spells in western Asia were also much more likely when waviness increased.He examined the timeframe from 1979-2010, looking for 40 months that exhibited the most extreme precipitation, and for 40 months that showed the most extreme temperature departures from the norm. And the data showed that more wavy waves overwhelmingly accompanied months with temperature or precipitation extremes. Only a small percentage of months with extreme weather corresponded with a more relaxed series of waves.

The regional differences largely stem from geographical features on the ground that influence planetary waves.

“The locations are tied to things like mountain ranges and the temperature contrast between the land and ocean. These are factors that don’t change,” Screen said.

They ensure that while there might be some month-to-month differences, the waves generally follow a similar pattern. And because they’re separated by the jet stream, that helps determine what impacts each region will see.

Kevin Trenberth, a senior scientist at the National Corporation for Atmospheric Research, said the study quantified a fairly well known pattern, though one he said climate scientists often take for granted. Climate researchers have started to look at these waves more closely, from how to use them to predict heat waves to how climate change could alter them.

commentary in Science last month argued that climate change was at least in part to blame for the pattern that set up over the U.S. this past winter by making waves more common. That commentary is based on research published in 2012 that made the case for why rapid Arctic warming is increasing the odds of wilder planetary waves.

Science (paywalled):

Since the late 1990s, global mean tem- peratures have been rising quite slowly. This pause or “hiatus” in global warming is linked to an increase in the westward trade winds across the tropical Pacific (4). The ocean currents, forced by these intensified trade winds, have been drawing down much of the excess heat associated with human cli- mate change to the deep ocean (5, 6). How- ever, one region that has not been cooling is the tropical West Pacific. Over the period of the hiatus, warm surface waters have been piled up in the tropical West Pacific by the intensified trade winds.

The surface waters in the tropical West Pacific will have been warmed further during this hiatus period through the local ef- fects of man-made enhanced greenhouse gas forcing. Even though this enhanced warming may be small (it is not currently possible to estimate reliably its magni- tude), its effect can be important in a re- gion where SSTs are among the highest in the world. Any further warming in this re- gion will lead to a relatively large increase in atmospheric water loading, leading to unusually strong latent heat release. Con- sistent with this, there was a very active ty- phoon season over the tropical West Pacific in 2013, including typhoon Haiyan, which devastated parts of the Philippines. These intense tropical weather systems continued into the 2013−14 winter season (7). Anoma- lous latent heat release in the tropical West Pacific can produce a particularly strong Rossby wave response in the Northern Hemisphere (8, 9). The phase of this Rossby wave response is consistent with the cold and snowy season seen in the U.S. Midwest and East Coast. If this line of argument is correct, the extremely cold and snowy sea- son in parts of the United States may in- deed have been caused at least in part by increased greenhouse gas concentrations

The Arctic is warming twice as fast as areas around the equator because of unique feedbacks involving ice cover in the region. The research argues that as the temperature gradient between the poles and the equator decreases, planetary waves are getting out of whack and becoming even more extreme, though other research has challenged those findings.

Screen’s study, however, only looked at the relationship between waves and extreme events rather than any long-term shift in trends.

“It’s still hotly debated whether we see any change in these waves,” Screen said. “I’m currently sitting in the middle, thinking it’s a plausible hypothesis, but currently the evidence is inconclusive at this point.”

Jennifer Francis, a researcher at Rutgers University who proposed the hypothesis, said there’s a ways to go toward understanding how climate change could affect planetary waves, and the meanderings of the jet stream.

“This is a complicated problem, and finding answers is further challenged by the short time period over which those regional temperature changes have emerged as clear signals from the highly variable atmosphere,” she said in an email. “New approaches to this question are underway, however, and I’m confident that a clearer picture will come to light in the next few years.”

Francis also stressed that understanding waves is just one component of understanding the larger category of extreme weather. Natural fluctuations in ocean temperatures, such as El Niño, and human-caused deforestation and air pollution, can all have an impact. Smaller fluctuations in the atmosphere can also lead to sudden, shorter-scale extreme events.

Trenberth said that putting aside the impact climate change could have on waves, it can also alter the water cycle because a warmer atmosphere can hold more water, increasing the odds of heavy precipitation and extreme dryness.

Below, contrasting views from Trenberth and Francis:




2 Responses to “Climate Change, Waves, and Winters”

  1. […] Above, my video on the changes in atmospheric circulation that have exacerbated California's drought, as well as leading, paradoxically, to brutal cold in Eastern North America. Takeaway: the creeping realization that slow, incremental increases in global heat content can trigger abrupt changes in circulation patterns that we have built agriculture and civilization around.  […]

  2. rayduray Says:

    I think we’re entering a new geologic age.

    We’re leaving the Holocene and entering the Holy Cow!

    One might even be induced to believe that this humble scribbler agrees:

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