News•May 22, 2014

Cold U.S. Winter Caused By Warm Tropical Waters?

The polar vortex got all the blame for the frigid winter that held much of the eastern U.S. in its icy grip this year, but the wild kinks in the jet stream that sent that cold air southward may be due to thunderstorm activity half a world away, one scientist says.

The first four months of 2014 in the U.S. were the coldest such period the country has seen since 1993, due to the large dips in the jet stream that pulled Arctic air down over the central and eastern portions of the country. The same extreme dips in the jet stream also sent storm after storm over the UK, causing them to have their wettest winter on record.

Visualization of surface winds (lines and black arrows) and temperatures (shaded colors) in January, as an Arctic blast swept from the Midwest to the South and East.
Click image to enlarge. Credit: earth.nullschool.net.

“Waves in the jet stream are what make the weather we feel here on the surface,” climate scientist Jennifer Francis, of Rutgers University, said in an email. “And when the jet has big kinks — like this winter and in 2011-12 — those waves tend to get ‘stuck’ and so do the weather patterns they create.”

Francis and other scientists have been looking at the ways that global warming may be causing changes in the atmosphere and at Earth’s surface that could influence the likelihood of these “big kinks” occurring. While Francis’ work has linked the jet stream’s wild wanderings to warming-induced changes in the Arctic’s sea ice cover, climate scientist Tim Palmer draws a tropical connection in an article published in the May 24 issue of the journal Science.

Palmer, of the University of Oxford, argues that heat released by unusually strong thunderstorm activity in the western tropical Pacific — helped along by heat absorbed from Earth’s warming atmosphere — set in motion the meanderings of the jet stream. The scenario is an example of how changes in atmospheric patterns aren’t caused just by natural variability or the forcing of man-made warming as greenhouse gases are increasingly added to the atmosphere, Palmer said.RELATEDPolar Vortex in U.S. May be Example of Global Warming
April Was 2nd Warmest Globally; Average for U.S.
Arctic Warming is Altering Weather Patterns, Study Shows
Arctic Warming, Extreme Weather Link Far From Certain

“There is actually kind of a gray area in the middle,” he told Climate Central.

Palmer is quick to caution that the link he is proposing doesn’t mean that the U.S. will keep seeing these colder winters in the future — in fact, the opposite is expected, as global warming stacks the deck for ever milder, warmer winters. And in the short term, the El Niño expected to develop this summer could put an end to this pattern, Palmer says, and could bring snow-weary Americans a milder winter.

“The point really is that the climate system is a subtle one and one shouldn’t be too simplistic,” Palmer said. “That it’s not a simple matter of everywhere getting warmer.”

A Tropical Heat Engine

Palmer first noticed a link between cold U.S. winters and a warmer tropical Pacific in the 1980s, with one region influencing the other through the generation of those kinks in the jet stream, technically known as Rossby waves.

Areas to the north of the jet stream tend to experience cold weather, while those to the south tend to see warm weather. The bigger the Rossby waves in the jet stream, the more pronounced that cold and warm can be.

The conditions in the western tropical Pacific can influence the size of those Rossby waves, Palmer says. When sea surface temperatures there are warmer than average, as they were during the winter of 2013-14, they amp up thunderstorm activity. As water droplets condense in the thunderclouds, they release more latent heat, providing a source of energy that can jolt the jet stream and create those kinks. And the waves begin in just the right spot so that the eastern U.S. is on the cold side of the jet stream, Palmer writes in his article.

“Winter 2013-14 shows a clear visible connection in the atmospheric circulation from the tropical West Pacific and out to the Northeast Pacific,” said Adam Scaife, a climate scientist with the UK's Met Office, which put out a report linking their wet winter with the phenomenon Palmer describes.

Added to that is the extra juice added by the “hiatus” in global warming, or the slower rise in global temperatures that has been observed since the 1990s. Over that period of time, trade winds have piled up warm surface waters in the western tropical Pacific, and some scientists think this pattern is causing ocean currents there to draw down all the “missing” heat, warming the waters even more.

This extra-warm water boosted the latent heat released in thunderstorm activity even more, Palmer suggests, producing a particularly strong response from the jet stream that “made the likelihood of this particular winter being cold (in the U.S.) more likely,” he said.

(Palmer cites the flurry of tropical cyclone activity, including Typhoon Haiyan, over the western tropical Pacific during the winter as evidence of the extreme energy available in the system.)

El Nino’s Influence

Francis points out that other recent winters, particularly that of 2011-2012, also saw amped up thunderstorm activity in the western tropical Pacific, but had unusually warm conditions in the U.S.

A new set of imagery provides an unmistakable view of the El Niño conditions that appear to be developing. Click image to enlarge.
Credit: NASA Earth Observatory.

“The tropical pattern cannot be the only reason for the persistent cold spell this winter,” Francis said in an email.

Palmer agrees that the pattern is likely only one of many factors influencing the jet stream. “The atmosphere is chaotic and given a run of years where the West Pacific forcing was more or less the same, one wouldn't expect to necessarily get a cold winter on every such occasion,” he said in an email.

“Hence the sort of winter we had in 2013-14, could perhaps have occurred in 2012-13 (when the tropical forcing was similar), but, due to flaps of (the) butterfly's wings, didn't,” he wrote.

Palmer expects the impending El Niño to set winters back on the warming path, as the trade winds that keep the warm water piled up in the western Pacific weaken and release the water, like a ball at the top of a slope, sending it “flooding back again toward the Eastern Pacific and that will lead us into a very different type of circulation pattern,” Palmer said.

And not only could that disrupt the tropical signal amplifying the Rossby waves, but it could see the return of quickly rising global temperatures again.

“If we are coming out of this kind of regime into an El Niño this year, the question is, ‘Is that going to trigger the end of this hiatus period?’,” Palmer said.