![]() The polar vortex and the February 2021 cold extreme in the south-central United StatesĪccording to Butler, it’s reasonable to suppose that the polar vortex played a role in the extreme winter weather outbreak that struck the Southern Plains in late February. Screen capture from Earth.Nullschool, based on NOAA Global Forecast System data. Temperature (purple is cooler, pink is warmer) and winds (white lines) at the 250-millibar pressure level (the altitude at which the pressure is 250 millibars), showing the deeply wavy path of the polar jet stream across the United States on February 15, 2021. The Arctic Oscillation often slips into its negative phase. Under the high-pressure ridges, warm air floods north into parts of the Arctic, often driving extreme melt, while polar air fills the low-pressure troughs, bringing wintry conditions farther south than average. The exact nature of the interaction-how the polar jet “feels” the disruption in the polar vortex and why it reacts the way it does-isn’t fully understood. ![]() In the weeks following the stratospheric upheaval, the polar jet stream will often develop a wavy shape, with deep troughs and steep ridges that can become nearly stationary for days. Regardless of their “flavor,” these disruptions have one thing in common: a spike in polar stratosphere temperatures, which is why they’re called sudden stratospheric warmings. The vortex slows, and it may wobble, slide off the pole, split into several lobes, or-in the most extreme cases-temporarily reverse direction. The Arctic Oscillation, which tracks hemisphere-scale wind and air pressure patterns, is often positive.Īt the other extreme, the polar vortex is occasionally knocked off kilter when especially strong atmospheric waves in the troposphere break upward into the stratosphere. At the surface, this stable stratospheric state is often associated with an even colder than usual Arctic, and milder-than-usual weather in the mid-latitudes. When the polar vortex is especially strong, for example, the polar jet steam tends to stay farther north and to exhibit a more zonal flow, with less meandering. When it does, however, the effects can be extreme. The polar vortex doesn’t always influence winter weather in the mid-latitudes. It’s the polar jet stream that plays such a big role in our day-to-day winter weather in the mid-latitudes, not the polar vortex. It marks the boundary between surface air masses, separating warmer, mid-latitude air and colder, polar air. The polar jet stream occurs in the troposphere, at altitudes between 5-9 miles above the surface. NOAA graphic, adapted from original by .Īccording to NOAA stratosphere expert Amy Butler, people often confuse the polar vortex with the polar jet stream, but the two are in completely separate layers of the atmosphere. When the vortex weakens, shifts, or splits (right globe), the polar jet stream often becomes extremely wavy, allowing warm air to flood into the Arctic and polar air to sink down into the mid-latitudes. The coldest polar air stays in the Arctic. When the Arctic polar vortex is especially strong and stable (left globe), it encourages the polar jet stream, down in the troposphere, to shift northward.
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