Yes, It's Cold And Yes, It's Global Warming; Arctic Warming In Particular Driving Weakening Of Jet Stream

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It may seem counterintuitive, but the fact that global average temperatures are rising does not preclude bouts of cold, and for some Arctic-driven storms, it could exacerbate them. Scientists are examining the mechanisms connecting humanity’s insatiable appetite for fossil fuels, the far north, and winter storms. They’re revealing a complicated picture that shows some of the most profound consequences of climate change occur in the chilliest parts of the planet and during the coolest times of year. As average temperatures rise rapidly, their effects are manifesting in surprising ways, but with more observations and better forecasting tools, winter storms don’t have to be so deadly and destructive.

There are a couple key mechanisms behind chills like Winter Storm Blair. One is the polar vortex, a spinning band of strong, cold wind that is normally confined to 10 to 30 miles above the North Pole. The polar vortex picks up strength during the winter and tends to stay circular, but because of waves in the atmosphere, it can occasionally split into multiple swirls or deform into an oblong shape that reaches outside the Arctic. The other factor is a jet stream known as the polar jet. This is a band of air blowing west to east at speeds of up to 275 miles per hour at altitudes between 4 and 8 miles above the Earth’s surface.

When the polar jet is strong, it forms a well-defined ring around the Arctic and holds the icy air in place. When it weakens, it wobbles and forms a flower or clover shape when viewed from directly above the North Pole. Within those lobes, cold Arctic air spills over landmasses like Asia and North America.

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Such Arctic spillovers, though, have occurred naturally in the past. How does global warming play in? Jennifer Francis, a senior scientist at the Woodwell Climate Research Center, co-authored a review article published in December looking at the body of research trying to answer this question. The key fact is that while Earth is warming on average, the Arctic is warming up to four times faster than the planet as a whole, a phenomenon called Arctic amplification. That has important knock-on effects. One is that a warmer Arctic means there is a weaker temperature gradient between the Arctic and its surrounding regions. Francis explained that the steep difference between Arctic and sub-Arctic temperatures is what powers the polar jet in the first place, so as the north begins to feel more like the rest of the planet, the jet gets weaker, making its frigid air more likely to meander across the Northern Hemisphere — and causing more extreme winter storms like Blair.

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https://www.vox.com/climate/393566/winter-storm-blair-polar-vortex-climate-change