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What is the Polar Vortex and what does it mean that it has split?


May 09, 2018

This winter, there has been widespread media coverage concerning a split polar vortex in the Northeast US and parts of Europe — with freezing temperatures, masses of snow and cold winds that have been dubbed the ‘Beast from the East.’ If you’re like many who have been not-so-patiently awaiting the arrival of spring, we’re here to tell you just what, exactly, this beast entails and how (if at all) it will affect the upcoming seasons.

What is the Polar Vortex?

Though it seems like a relatively new weather phenomena, the Polar Vortex, or PV, is not new at all — it’s likely been around for as long as humans have been recording the weather.

Put simply, the PV are wide expanses of swirling cold air that are parked in polar regions, meaning there’s one located in the southern hemisphere as well. A vortex consists of a large-scale, upper-level, low pressure center that is semi-permanent and rotates fairly tightly around the Earth’s poles.

At its core, the PV represents the coldest part of winter for a large part of the central and eastern United States as well as other regions of the Northern Hemisphere. Canada and northern Asia experience the lion’s share of the cold, but when the PV moves, it takes its cold air with it.

Why the unusual cold in unusual places?

The strength of the PV relies on the temperature difference between the Earth’s poles and the Equator: i.e. the colder the poles, the stronger the PV. So, although the PV is technically happening year-round, it’s during these winter months that the vortex is at its strongest (and coldest). When a PV is strong, it splits — sending its super-chilled air away from its center into unexpected places.

As a weather system that spans across continents and time, there’s no doubt as to the far-reaching effects of the PV.

world map polar vortex

Why did the Polar Vortex split — and is the split permanent?

Imagine the PV to be a large mound of air, piled up in the shape of a dome. Now imagine a giant cleaver has sliced down the middle and wedged the single dome of cold air into two. This cleaver, which is essentially a river of warm air, has now split and pushed the dome across a larger swath of geography.

A split is very difficult to forecast. This “river” of warm air is forced down from the Stratosphere and is also known as Sudden Stratospheric Warming (SSW). The origin of SSW is largely a mystery, but they are more likely to occur when stratospheric winds reverse direction while the Earth is experiencing a low period of the solar cycle, which has been the case for the past 1–2 years.

This may lead you to expect warming conditions with an SSW. However, because it acts as the “cleaver” of the PV, it effectively spreads the colder temperatures.

Once split, will the Polar Vortex re-form as a single entity?

Yes — the sudden stratospheric warming event that initially split the PV will eventually dissipate as the warm stratospheric winds retreat to their more typical altitudes. The absence of the SSW allows the PV to coalesce and return to its traditional location with its primary characteristics.

What’s the outlook for the weather this spring?

Considering that the PV is strongest when the Earth’s northern hemisphere is tilted farthest away from the sun, the PV will continuously weaken as the tilt shifts and the northern hemisphere warms.

Currently, the Earth’s tilt has positioned the Sun’s zenith just north of the Equator. This zenith will continue its northward trek until it reaches the Tropic of Cancer on the summer solstice, making the likelihood of another polar outbreak unlikely.