In this last week, parts of the US were hit with incredibly cold and deadly winter weather. Places where you typically see palm trees and cacti were covered in snow and ice instead. The state of Texas saw some of its coldest temperatures in more than 30 years, with some places breaking records more than a century old. Dallas, Texas was colder than Anchorage, Alaska!
So, what is going on? It has to do with something called the “polar vortex,” a counterclockwise circulation of strong, upper-level winds that surround the northern pole. This large area of low pressure keeps cold air “locked” in the Arctic regions of the Northern Hemisphere. But sometimes the vortex becomes distorted – wavy – and it can dip far south, bringing cold air with it. This happens when the upper-level winds decrease significantly. Imagine a top – when it’s spinning fast, it’s stable. When it starts to slow down, it wobbles.
The term for the change between fast and slow winds is called the “Arctic Oscillation.” When winds are strong, the oscillation is in a positive phase and the cold air stays up in the Arctic. When the winds weaken, it’s in a negative phase and this can lead to outbreaks of very cold air to the south as the vortex “wobbles”. Serious cold snaps happen several times a year as the winds switch between fast and slow, though in different regions of the world and with different severities. This one in the U.S. right now is particularly severe!
While it seems counterintuitive that global warming could cause significant cold snaps like this one, some research shows that it could. We know that different types of extreme weather can result from the overall warming of the planet and melting of the Arctic sea ice. In fact, the Arctic is warming more than twice as quickly as the rest of the planet due to rapid loss of sea ice – the Arctic’s natural mirror that reflects energy from the Sun away from the planet. Dark open ocean water absorbs the energy instead, leading to warming. A warmer Arctic Ocean further hinders sea ice growth and generates warmer and more moist air masses over the Arctic and nearby continents. A warming Arctic reduces the temperature difference with the mid-latitudes, which has consequences for circulation patterns in the atmosphere – it can lead to a weaker and more “wobbly” conditions (more technical reading: “Evidence linking Arctic amplification to extreme weather in mid‐latitudes” https://agupubs.onlinelibrary.wiley.com/…/2012GL051000 ).
Atmospheric dynamics are complicated, as there are natural variabilities present in the climate system as well. There are multiple oscillations that occur in the ocean and atmosphere that have their own year, decadal, and multi-decadal cycles. The signal of human-caused warming in observed data for the Arctic has only just emerged out of the “noise” of year-to-year natural variability over the past couple of decades. Time will tell how the Arctic will change in the future – and how that will impact us all.
Please stay safe out there!