By Karen Braun
CHICAGO, Dec 8 (Reuters) – The prospect of a colder-than-normal winter is enough to keep the public on edge, but add a menacing name like “polar vortex” and a simple forecast seems like an omen of the apocalypse.
Forecasters have warned that cold outbreaks over North America linked to the polar vortex may be more likely than usual this winter, with the impending cold blast for the Eastern United States the first sign of things to come.
But blaming the polar vortex for any one or series of weather events is somewhat irresponsible as the term often carries negative perceptions despite the fact that by definition, the polar vortex is neither unusual nor extreme.
No one really knew what the polar vortex was until four years ago, when the bitterly cold start to 2014 drove up energy consumption and associated commodity prices in the United States. The media instantly fell in love with its sinister sound, and the term “polar vortex” soon gained notoriety – and misunderstanding.
Buzz-worthy though it sounds, the polar vortex is actually much less exciting. It has always existed and is not always the source of extreme weather. In some ways, it is not even a vortex at all.
But the term will likely be misused and overused in the next couple of months – especially if the cold U.S. forecast pans out, so it is important to understand the context.
The term “polar vortex” is not the easiest to understand because its usage varies even within the scientific community. It is short for circumpolar vortex and most broadly refers to the west-to-east air flow that circles the pole in middle and high latitudes.
There are two polar vortexes that meteorologists might be referring to when discussing the topic, especially as it pertains to the Northern Hemisphere. One of them describes a strong flow in the stratosphere above the Arctic Circle.
This stratospheric vortex appears in the winter months due to the sharp temperature differences between the mid-latitudes and the poles. If the stratosphere suddenly warms, the vortex weakens. This may or may not indirectly contribute to a cold weather outbreak in North America, Europe or Asia.
The other polar vortex is the one that primarily influences surface weather patterns and can often be linked to chilly winter blasts. It operates in the upper middle-latitudes at a height just below a commercial airliner´s cruising altitude.
This “tropospheric” polar vortex is a basic, year-round feature of Earth´s climate structure. It manifests in a string of high- and low-pressure anomalies – known as ridges and troughs – across the middle latitudes.
The advancement of these pressure centers is largely responsible for the rapidly changing weather we experience in the mid-latitudes of the Northern Hemisphere.
The tropospheric vortex is the one that the media likes to highlight in the wintertime as the culprit of harsh, arctic-like conditions. But this same “vortex” can also be responsible for unusually cool summer weather, though broadcasters never make that attribution.
In fact, scientists have recently suggested that using the term “vortex” in connection with surface weather disturbances is not so useful as it is highly prone to misinterpretation, especially since it may incorrectly imply that a dramatic change in global atmospheric circulation has occurred. (http://bit.ly/2iESb4Z)
It is also important to remember that the effects of the polar vortex in this form can be felt anywhere in the Northern Hemisphere, not just in the United States.
The polar vortex that has recently been in the news is simply a stronger-than-normal pressure anomaly along the edge of the tropospheric polar vortex. It could even be thought of as a lobe or appendage of the vortex.
But really it is just a deep trough of low pressure that will remain parked over the Eastern United States for at least the next 10 days. This set-up will allow cold, arctic air to travel farther south than usual, resulting in well-below-normal temperatures for the affected areas. (http://reut.rs/2AARWzL)
Strong blocking patterns on either side of this trough will keep it in place and prolong the cold weather spell for the Eastern United States. Blocking patterns are large, stationary areas of high pressure that prevent the advancement of other air masses – in this case, the trough.
There are other climate variabilities that influence winter weather every year like the North Atlantic and Arctic Oscillations (NAO/AO), both of which are very loosely connected with the tropospheric vortex.
These indexes are generally forecast to trend negatively over the next couple of weeks, which usually supports colder weather in North America, Europe, or both.
La Niña, the cool phase of the equatorial Pacific Ocean, is also known to introduce colder weather to the Northern United States and Canada during the winter, so its presence this year is likely to influence atmospheric patterns.
So the next time you see a headline hyping up the polar vortex, stop for a second, recall the proper definition, and assess whether it is mere clickbait.
Note: The latest GFS and EC weather model runs, data on the North Atlantic and Arctic Oscillations, and commodity-specific analyst commentary are available via the Cross-Asset Weather app on Thomson Reuters Eikon:
(Editing by Matthew Lewis)
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