CHICAGO (Reuters) - The Northern Hemisphere may be subject to more cold outbreaks than usual come January thanks to the polar vortex.
“Polar vortex” became a household phrase in North America during the bitterly cold winter of 2013-2014, but this phenomenon has always existed and can also impact both Europe and Asia.
The polar vortex is one of the factors that control how much cold Arctic air is allowed to infiltrate the mid-latitudes during the winter, when the vortex is at maximum strength.
Recent atmospheric activity in the Arctic suggests that the polar vortex is likely to come into play for the Northern Hemisphere this winter, as there may be a relationship between autumn and winter polar vortex strength.
It is impossible at this point to know exactly when and where the polar vortex anomaly will occur, though there will be some clues to watch for once winter arrives.
Should the frequency of cold episodes increase come the new year, this could provide a boost to sluggish U.S. energy markets but threaten winter grain and oilseed crops across the Northern Hemisphere, particularly in Eastern Europe where plant health is already of concern.
POLAR VORTEX 101
The polar vortex is a large, spinning mass of cold air in the upper atmosphere that sits over the poles year-round but intensifies in each respective winter.
Think of the polar vortex like a top. The faster the top spins, the more stable it becomes as it preserves shape. But when it loses speed, it also becomes unstable and the top will begin to wobble from side to side.
This is exactly what happens to the polar vortex. Left undisturbed, Arctic air is unlikely to escape the spinning “vortex” down into the mid-latitudes. If it weakens and the upper level winds slow down, the polar vortex will tilt toward North America, Europe, or Asia.
Once the weakened polar vortex begins to shift toward the continent, it can take anywhere from days to weeks for ice-cold Arctic air to arrive. The duration depends on the intensity of the event and other atmospheric factors, such as a blocking pattern.
Blocking patterns, or blocks, are large, stationary high-pressure systems that prevent the advancement of adjacent air masses. If a strong block is in place near a tilted polar vortex, this could prevent the exit of the vortex air from the region, thus prolonging the Arctic blast.
This is what happened in early 2014, when a chunk broke off from the polar vortex and descended upon North America. The mass of cold air became wedged between the northwesterly jet stream and a blocking pattern over Greenland and delivered the United States and Canada one of the coldest winters ever recorded.
Complete destabilization of the polar vortex is caused in part by Sudden Stratospheric Warming (SSW) events, which can occur when increased storm activity in the Northern Hemisphere reaches a critical threshold.
During a SSW event, the stratosphere drastically warms over a short period. The polar vortex suddenly warms as a result, causing it to break apart for several weeks, thus increasing the chance for cold outbreaks across the Northern Hemisphere.
The polar vortex seemingly operates independently of other winter-dictating phenomena, such as the North Atlantic Oscillation and the Arctic Oscillation (NAO/AO). In fact, both the NAO and AO tracked predominantly positive values throughout the winter of 2013-2014, which would generally be expected to produce warmer-than-normal temperatures in the Northern Hemisphere.
FRIGID START TO 2016?
There is good reason to believe that cold outbreaks might be commonplace in the Northern Hemisphere during January and February based on the polar vortex.
Evidence suggests that a very strong polar vortex between October and December is likely to weaken significantly during January and February (tmsnrt.rs/1j6cMdI).
During October 2015, the polar vortex was the seventh-strongest on record since 1948. If it continues its strength through the end of 2015, the chances for it to weaken in the beginning of the year are high, meaning that cold air outbreaks would be more likely in the Northern Hemisphere come January.
Also, the current El Niño state may increase these chances as some research suggests that SSW events are more likely to occur during the wintertime under El Niño conditions rather than La Niña conditions.
The exact region of impact would not be determinable at this early stage, but as we move into the winter months, we can be watching for vortex-weakening SSW events and any subsequent cold air protrusions south of the Arctic Circle.
This might be disappointing news for those in North America who have been banking on a warmer winter thanks to El Niño, but it could be good news for U.S. energy suppliers hungry for a boost to demand.
Warmer weather has dominated the United States this autumn and has sharply reduced heating demand, leading to unusually high stocks of both heating oil and natural gas for this time of year.
But in the agriculture space, this could make for unhappy farmers across the United States and Europe as it takes only one night of frigid temperatures to significantly damage or kill off dormant winter grains. Black Sea crops could be the biggest losers from this scenario as the winter wheat crop is already stressed from the recent drought.
Snow cover will be the key to protecting dormant crops from winter’s wrath. But if this year’s snowpack falls short of expectations, exposure to the elements will be farmers’ No. 1 concern.
Editing by Matthew Lewis
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