Polar Vortex or Not?

This week Texas has seen rolling power outages, some for as long as 10 hours, as unseasonably cold weather blankets the state and demand for power for heating surges. Here on the West Coast, we’ve had below zero temperatures and over 45 cm of snow, while the cold weather has hammered the Prairie provinces with temperatures in the -40⁰C range.

A strong cold front has dropped down the centre of North America, reaching as far south as the beaches of Galveston and parts of Louisiana still recovering from two back-to-back hurricanes only six months ago. This cold snap is predicted to have a major impact on agriculture, as it brings the coldest temperatures the southern states have experienced since 1989.

While it may seem strange to think of colder temperatures as harbingers of climate change, the climate change signal is not in the temperatures themselves but in the structure of the specific weather systems spanning the globe that cause this weather.

Climate warming impacts the jet stream, which has subsequent effects on weather systems in North America. During the winter season, a strong jet stream contains cold air – the polar vortex – to Arctic regions. However, if the jet stream is weakened due to a decrease in temperature differences between the poles and the equator, it gets more wobbly, drawing down the polar vortex from the Arctic to the centre of the North American continent and bringing extremely cold temperatures.

Judah Cohen, who studies the jet stream and its impacts on climate, noted that “It’s getting increasingly difficult to get severe winter weather into the mid-latitudes without a polar vortex disruption…And amplified Arctic warming is favorable for disrupting the polar vortex.”

However, while some are calling this latest cold snap a polar vortex, I can’t figure out how that’s possible as the Canadian Arctic is warm – as much as 10-14⁰C above the 1979-2000 normal – while the States are up to 18-24⁰C below normal (see that first picture above). How can it be a polar vortex if there are no cold Arctic temperatures to draw down south?

It could instead be due to the North Atlantic Oscillation (NAO), which is a climate signal derived from the pressure difference between the Azores and Iceland that also affects the jet stream. During a negative NAO, the Canadian Arctic is warmer and the continental US is colder. The argument is that the Arctic has become warmer over the decades because of sea ice loss, meaning that the NAO is becoming more negative and so there are colder winters over the US. But the impacts of a negative NAO are usually focused on the eastern part of North America, not the centre (see image below).

Another option is a sudden stratospheric warming event (SSW), which is associated with a breakdown in the polar vortex. This seems most likely to me as, during an SSW, “the tropospheric jet stream [can] weaken, which allows cold air bottled up near the polar cap to escape and expand into the middle latitudes, such as the United States.” Scientists observed an SSW in early January of 2021, which they predicted would lead to cold weather over the continental US and warm weather in the Canadian Arctic. This seems to match what we’re seeing with this cold spell. And if you look at the image below, which is an average of the weather pattern after all SSWs from 1958-2013, you can see that the temperatures are similar to what we saw in the first image: warm over the Arctic and cold in the interior of North America.

Scientists disagree over the precise Arctic mechanisms driving mid-latitude climate, saying that models don’t reproduce mid-latitude weather changes as a function of Arctic warming. But other scientists note that Arctic warming has to impact mid-latitude weather because of the atmospheric physics involved, and argue that perhaps the models aren’t accurately representing the baseline relationships between the Arctic and mid-latitudes.

Regardless, we can likely say that the current extreme cold weather in the interior of North America is not a typical polar vortex event, given the anomalously high temperatures in the Arctic. It is instead a function of an SSW event that destabilized the polar atmosphere in early January of 2021, and subsequent atmospheric reorganization, including the movement of cold polar air south across the North American continent.  

Note (23 Feb 2021): The Washington Post has a good explainer of the cold weather here. They bring in the Arctic Oscillation, which affects the waviness of the jet stream and can pull cold air south.