seasonmap field guide

What is the polar vortex?

"Polar vortex" refers to two distinct circulations. This page covers the stratospheric one: what it is, how it breaks down, and why the surface cold that follows arrives on a delay of weeks, not days.

Two vortices, one name

The tropospheric vortex is the ordinary ring of jet-stream westerlies around the pole in the layer where weather lives; its day-to-day wobbles are routine winter variability. The stratospheric polar vortex is a separate structure: a band of westerlies, often exceeding 100 mph, circling the pole from roughly 10 to 30 miles up. It forms each autumn as the pole loses sunlight and dissipates each spring.

A strong, circular stratospheric vortex confines the coldest air to the pole, and the eastern US tends to run mild. The forecasting interest is in what happens when that vortex is disrupted.

Sudden stratospheric warming

Tropospheric ridges — often amplified by tropical forcing and East Asian mountain-torque events — launch planetary-scale waves that can propagate upward into the stratosphere. When a large enough wave breaks there, stratospheric temperatures rise by tens of degrees within a few days, and the vortex is either displaced off the pole or split into two lobes. This is a sudden stratospheric warming (SSW); the formal criterion is a reversal of the 10 hPa zonal wind at 60°N from westerly to easterly.

Major SSWs occur in roughly six of ten Northern Hemisphere winters.

The lag to the surface

After a disruption, the easterly wind anomaly descends through the stratosphere over days to weeks. When it reaches jet-stream level, it weakens the westerlies, favors high-latitude blocking (a negative Arctic Oscillation), and allows polar air to spill into the mid-latitudes. The documented window for surface impact is two to six weeks after the SSW.

Cases on record: the January 2014 cold outbreaks, the late-winter 2018 "Beast from the East" in Europe, and February 2021, when the descending response to a January SSW coincided with −2°F in Dallas and the Texas grid failure, roughly a month after the stratospheric reversal.

Caveats

• Coupling is not guaranteed. Roughly a third of SSWs produce little surface response; the disruption has to couple downward, and some stay confined to the stratosphere.
• Location varies. Splits and displacements favor different regions; Europe and Siberia are affected about as often as North America.
• An intact vortex is also informative. A strong, undisturbed vortex shifts the odds toward a milder eastern US.