The Weekend Wonk: Did Tropical Heat Spin up the Polar Vortex?

May 24, 2014

The real debate in atmospheric science is, of course, not whether man-caused climate change is happening, but exactly how that change is playing out – affecting atmospheric circulation, and thereby, our weather.

I’ve covered this debate from early on by interviewing proponents of two seemingly opposing viewpoints, one, made famous by Dr. Jennifer Francis of Rutgers, is that changes in Arctic ice cover may be breaking down the temperature gradient between north and south, and thereby affecting the flow of the northern hemisphere jet stream – causing weather to get “stuck”, like this past winter’s “Ridiculously Resillient Ridge”, which brought warmth and rain to the arctic while pouring frigid arctic air onto eastern north America.

Conversely, Dr. Kevin Trenberth maintains the heat flux in polar regions is not large enough to drive changes on the scale which we are observing. The answer, he feels, is in the tropics.

Tim Palmer of Oxford University, writing in the new issue of Science, adds a new puzzle piece that might help square the circle.

Science (paywalled):

Given this overall decreasing tendency in cold winters, it seems impossible to argue that the record-breaking snowy winter in the Midwest could be connected to climate change. However, there is a plausible link. To understand this link we must consider the atmospheric circulation patterns that were associated with this winter and ask whether there is evidence that climate change might have increased the likelihood of these patterns. Large-scale atmospheric circulation patterns are controlled by the position of the jet stream. The Northern Hemisphere jet stream flows from west to east at mid- latitudes; it deviates from a line of latitude through a series of ripples called Rossby waves. Regions above which the jet stream is flowing from the north are likely to expe- rience cold weather. Conversely, in regions above which it flows from the south, the weather is likely to be relatively warm. The larger the amplitude of the Rossby waves, the more anomalous the weather is likely to be at the surface.

Why does the jet stream across the United States produce cold winters in the Midwest and on the east coast? A key factor is the strength of thunderstorm activity in the tropical West Pacific, associated with above- normal sea surface temperatures (SSTs) in this region (2). Latent heat release in this region, which occurs as warm moist air condenses into water droplets in these thunderstorms, acts as a source of energy to excite particularly large-amplitude Rossby waves in the Northern Hemisphere jet stream, with just the right phase to produce cold weather in the Midwest and East Coast. In the 2013– 2014 winter, SSTs have been particularly warm in the tropical West Pacific .

Since the late 1990s, global mean temperatures 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 climate change to the deep ocean. 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 effects of man-made enhanced greenhouse gas forcing. Even though this enhanced warming may be small (it is not currently possible to estimate reliably its magnitude), its effect can be important in a region where SSTs are among the highest in the world. Any further warming in this region will lead to a relatively large increase in atmospheric water loading, leading to unusually strong latent heat release. Consistent with this, there was a very active typhoon 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. Anomalous latent heat release in the tropical West Pacific can produce a particularly strong Rossby wave response in the Northern Hemisphere. 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 season in parts of the United States may indeed have been caused at least in part by increased greenhouse gas concentrations.

NBC News:

Pinning the cold weather on interactions between global warming and natural variability in the tropical western Pacific carries more clout than a controversial hypothesis that the winter chill was driven by rapid warming in the Arctic, Trenberth noted.

According to that theory, advanced by Jennifer Francis at Rutgers University in New Brunswick, New Jersey, the Arctic is warming faster than the rest of the globe, which in turn has caused the northern polar jet stream to become wavier. Some of those bigger bends in the jet stream bring cold Arctic air further south, which could also explain the winter chill in the eastern United States.

That theory is controversial, explained Palmer, mostly because the Arctic is considered too small of an area to have a major influence on global weather patterns. In addition, what happens at the surface in the Arctic — which is definitely warming — stays at the surface unlike the western Pacific where heat from tropical storms is released to the upper atmosphere where it influences the jet stream.

Francis noted in an email to NBC News that precipitation in the tropical western Pacific has been consistently strong for the past seven years, including the winter of 2011-2012 when the eastern United States had an exceptionally warm winter.

“Because the tropical rainfall patterns in these two years were very similar but the temperature extremes in the two winters in the U.S. were opposite, the tropical pattern cannot be the only reason for the persistent cold spell this winter,” she said.

“I cannot say that rapid Arctic warming played a role in either of these unusual winters in the U.S.,” she added, “but the very amplified (wavy) jet stream during both years is consistent with the sorts of patterns we expect to occur more frequently as the Arctic continues to warm.”

Cold winters on their way out?

According to Palmer’s research, cold winters will overall become rarer as the concentration of greenhouse gases increases in the atmosphere and the planet continues to warm.

 “In fact, I’m sort of expecting in the near future they will probably become much less common because all evidence suggests we are heading into an El Niño year when all of this warm water that was previously piled up in the western Pacific will flow back across the Pacific,” he said.

If that is indeed the case, he added, the hiatus too will likely come to an end and “global mean temperatures will start to rise at the rate that they were doing back in the early 1990s.”

Bear in mind that the same pattern responsible for the “polar vortex” winter in the eastern US, also exacerbated drought conditions on the US west coast.


13 Responses to “The Weekend Wonk: Did Tropical Heat Spin up the Polar Vortex?”

  1. ubrew12 Says:

    Climatologically, I think this Western US precipitation pattern (1900-2100) expected by the IPCC models, leading to more severe droughts as this century plays out:

    can only occur if, generally, the West becomes one huge High Pressure area. Since such area’s circle clockwise, the West is going to (again, on average) push the jet stream North of it, into Canada, after which it bounces back Southward into the MidWest and East, carrying all that N Pacific moisture into those areas, and leading, in part, to this pattern of deluge frequency observed over the last half-century:

    I’m interested in what people think of this idea. If true, the expectation of harshening drought in the US West will be accompanied by greater flooding and cold snaps in the East.

    • rayduray Says:

      Re: “I’m interested in what people think of this idea. ”

      Having recently read “The West Without Water”, I can easily state that megadroughts have occurred in the U.S. Southwest before and are likely to do so in the future.

      The San Jose Merc-News has this: “The longest droughts of the 20th century, what Californians think of as severe, occurred from 1987 to 1992 and from 1928 to 1934. Both, Stine said, are minor compared to the ancient droughts of 850 to 1090 and 1140 to 1320.”

      That said, I’m guessing that California could be in for a wet winter in 2014-15 due to the return of El Nino, now about a 70% probability.

      • astrostevo Says:

        The Aussie Bureau of meteorology :

        confirms that 70% estimate for El Nino this year. I’m already dreading the likely consequences of heatwaves, drought and bushfires for us in South Australia.

        We’ve just had a really “unusually” warm and dry May too – a spell that’s just been broken with some welcome rain today.

  2. Penn prof explores interplay between climate change and Southern Ocean

  3. The link for Science
    Record-breaking winters and global climate change

  4. This may be referring to the same study:

    It seems possible that it is not an either / or situation? It may be a push / pull – both the Arctic and the tropics may be strongly affecting the jet stream?

    • dumboldguy Says:

      Since beginning to follow the Trenberth -Francis debate, I have had in the back of my brain a similar thought—-that it may not be totally either / or, and Palmer’s thoughts add to that feeling. IMO, we simply don’t know enough yet—-perhaps the effects of the oncoming El Nino will make it all clearer?

  5. redskylite Says:

    We are climate scientists, Chicago style (Starring Dr. David Archer on keyboard)

    • redskylite Says:

      Stick to the day job guys – please

    • andrewfez Says:

      I prefer Richard Alley’s music. The man’s trebled and tin-ish voice is transformed into something of a baritone as he throws down some folk poetry along topics more interesting than teenage love. Here’s one about penguins:

  6. […] Okinawa Botanical Gardens managers say that because of the excellent response from the visitors, they're planning more concerts in the future in the tropical atmosphere. […]

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