When the North Gets a Chill: The Stratosphere's Role in US Winter Woes

The US has been experiencing some serious cold snaps lately, even as winters warm up overall. Scientists have been digging into why this happens and found some interesting connections to the stratosphere, the layer of the atmosphere above where planes fly. Two key patterns have been identified. The first involves a displaced upper-level vortex hovering over western Canada, which tends to bring harsh winter conditions to the northwestern US. The second pattern features a weakened upper-level vortex shifting toward the North Atlantic, leading to severe winter weather in the central and eastern parts of the US.Both patterns share some common traits. They involve a stretched lower-level vortex and something called stratospheric wave reflection. Since 2015, there's been a noticeable shift in where these severe winter weather events happen. They've been moving northwestward across the US. This shift coincides with a rise in the frequency of the westward-focused pattern and a change in the El Niño-Southern Oscillation phases. So, what's driving these changes? It's a complex mix of factors, including shifts in the stratosphere and ocean patterns. Understanding these connections can help improve winter weather predictions and prepare better for extreme cold events.