Winter is Coming and Staying

Snowfall in Boston
Snowfall in the Greater Boston area has been so severe this year that some public schools have extended classes until July. Source: Flickr

Extreme winters have become the new norm in northeastern states of the United States, and researchers have recently found the reason why. In March of 2018, scientists at Rutgers University published a study in Nature Communications on their findings that there is a correlation between the frequency of extreme winter weather in the North-Eastern region of the United States to changes in Arctic temperatures.

Recently warm temperatures in the Arctic cause the jet stream – a band of strong westerly air currents that encircle the globe several miles above the earth’s surface – to occasionally move farther south, causing cold air to reach all the way down to the eastern United States. The timing of this research is somewhat convenient, as it follows increasingly extreme winters, as well as record warm Arctic temperatures and low sea ice, record-breaking disruptions in the polar vortex (a large area of low pressure and cold air surrounding both of the Earth’s poles), and record-breaking disruptive snowfall in the United States and Europe.

Researchers found that severe winter weather is two to four times more likely to occur in eastern United States when the Arctic is abnormally warm than when it is colder than normal. The study also showed that colder winters in the northern latitudes of Europe and Asia are significantly related to the warming of Arctic. On the other hand, the study also showed a correlation between the likelihood of severe winter weather in the western United States when the Arctic is colder than normal.

Researchers found that when warming of the Arctic occurs on the Earth’s surface, there is only a weak connection to severe winter weather in the northeastern region of the United States. However, when warming is extended to the stratosphere, it disrupts the polar vortex and severe weather is more likely.

To make these conclusions, researchers used three metrics of Arctic variability to diagnose the relationship between severe winter weather in the Northeast and Arctic temperatures. These measurements are called the polar cap geopotential height anomaly index (PCH), polar cap air temperature anomaly index (PCT), and the Accumulated Winter Season Severity Index (AWSSI).

The PCT and PCH indices measure geopotential height (the vertical coordinate system referenced to Earth’s mean sea level) and temperature anomalies that occur between the 65th parallel north (a circle of latitude that is 65 degrees north of the Earth’s equator) and the north pole. The AWSSI identifies severe weather owning to snowfall and temperatures at individual locations across the United States. Researchers analyzed changes in AWSSI in relation to changes in PCT and PCH to explore the relationship between Arctic variability and severe winter weather.  They found that an increase in abnormalities occurring in polar cap temperatures and geopotential height are correlated with higher values of the AWSSI, meaning an increase in cold spells and heavy snowfalls.

Inevitably, there will be an increase in certain types of weather extremes due to the effects of anthropogenic global warming. Researchers at Rutgers University have presented a quantitative analysis of the link between Arctic variability and severe winter weather, suggesting that the pattern of colder and harsher winters in the Northeast are attributed to Arctic warming is no coincidence.

Judah Cohen, Karl Pfeiffer, Jennifer A. Francis. Warm Arctic episodes linked with increased frequency of extreme winter weather in the United StatesNature Communications, 2018; 9 (1) DOI: 10.1038/s41467-018-02992-9

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