GardenSMART :: Weakening Polar Vortex Could Mean More Cold Spells Ahead for North America
Weakening Polar Vortex Could Mean More Cold Spells Ahead for North America
By Brian Donegan, The Weather Channel/weather.com
Image courtesy of The Weather Channel
The position of the polar vortex has shifted over the last several decades, and that could mean more cold spells for some mid-latitude locations, including North America, according to a newly published study.
The study, published October 24 in the journal Nature, led by Dr. Jiankai Zhang and Dr. Wenshou Tian, found that over the last three decades the polar vortex in February has moved toward Europe and Asia. That means bitter-cold days could become more common in February and March – bad news for spring lovers.
This shift in the position of the polar vortex is likely related to Arctic sea-ice loss, but also may be the result of increased snow cover in the Eurasian continent, the study said.
The polar vortex is an area of low pressure in the upper atmosphere, primarily in the stratosphere, a layer of the atmosphere above which most of our sensible weather occurs (known as the troposphere).
The polar vortex is strongest in winter, thanks to an increased temperature contrast between the polar regions and the mid-latitudes, including the United States.
When the polar vortex is strongest, you're less likely to see cold air plunge deep into North America or Europe. Occasionally, though, the polar vortex is disrupted and weakens. This happens when the stratosphere warms.
When the polar vortex is weakened, a piece of the polar vortex can surge farther south, helping to push Arctic cold into portions of North America and Europe.
Example of a weakened and elongated polar vortex due to stratospheric warming over Alaska and northeast Siberia.
Based on the results of the Zhang and Tian study, the most likely timeframe for a prolonged Arctic-air outbreak in North America is late winter into early spring as the polar vortex shifts toward Europe. We experienced this in the United States in 2014, 2015, and 2016.
Noted climatologist and winter seasonal forecast expert, Dr. Judah Cohen of Atmospheric and Environmental Research (AER), a Verisk Analytics Business, told weather.com, "We have shown that increased Eurasian snow cover and decreased Arctic sea ice have forced a weakening in the polar vortex, which is consistent with their findings."
Although numerous warm-temperature records have been set in North America and Europe over the past several years, the researchers in the study found that the cold air being displaced into the mid-latitudes by the polar vortex has been able to offset some of the record-warm temperatures.
"Winter has been the most resilient season to climate change, which is the exact opposite of our expectations. I would attribute much of the severe winter weather that we have experienced over the past two decades to increasing fall snow cover and decreasing fall Arctic sea ice," Cohen said.
The Zhang and Tian study also supports using snow cover and sea ice in seasonal forecasts.
Currently, there is high snow cover over Eurasia, according to the Rutgers Global Snow Lab, and low sea ice in the Arctic, and Cohen says that could lead to more weakening of the polar vortex in the coming months, potentially leading to more severe winter weather.
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