Explaining North America Winter 2024-25 Polar Vortex Events

Discussion: Climate forcing by colder than normal conditions in the stratosphere during meteorological winter 2024-25 centered on Greenland produced a mostly warmer than normal winter across the northern hemisphere, especially Canada and Eurasia. However, there was a notable exception. Two brief stratospheric warming events took place across Canada, one just after mid-January and another during mid-February. Each event caused a polar vortex episode in North America.

Beneath the (mostly) cold stratosphere during the just ending winter season (Fig. 1), the troposphere has been warm featuring a large high-pressure area across Europe stretching eastward into Russia and northwestern North America (Fig. 2). A low-pressure area has persisted near and east of New England related to a large cool pool of ocean water of the western North Atlantic. However, this persistent pattern broke twice during the winter season leading to an unusual double polar vortex event in the U.S.

The first stratospheric warming event occurred just-after mid-January centered on Canada while the Eurasia side of the stratosphere stayed cold (Fig. 3). An intense polar vortex formed in the troposphere causing an arctic outbreak for the Central and East U.S. (Fig. 4).

A second stratospheric warming event generated during mid-February in the same location (Fig. 5). Once again, a polar vortex was created in the troposphere although the strongest signature of this polar vortex was in the lower stratosphere (Fig. 6).

More than one polar vortex event in one winter season affecting the U.S. was last observed during the winter of 2013-14 and 2014-15.

Fig. 1-6: The stratospheric average temperature anomaly for winter so far and the attendant 500 MB height anomalies for the northern hemisphere (top), stratospheric warming and polar vortex event 1 from mid-January (middle), and stratospheric warming and polar vortex event 2 from just-after mid-February (bottom).