ST Explains: What is causing the bitter winter weather in the Northern Hemisphere?

SINGAPORE – Dozens of people died over the Christmas holiday period as winter storms and intense cold swept across much of the United States, Canada and Japan, knocking out power and causing travel chaos.

While winter storms are not unusual, it was the ferocity and huge amount of snow triggered by the latest storms that alarmed meteorologists and local officials, especially in the US, where the arctic blast affected much of the nation all the way towards the border with Mexico.

The Straits Times looks at some of the likely reasons for the intensity of the storms and whether climate change could have played a role.

1. What happened in the United States and Canada?

At its simplest, intense cold air from the Arctic moved south due to weakening of atmospheric circulation patterns.

Normally, the Arctic’s freezing weather is locked in by the polar vortex, a large rotating band of cold air that swirls around the far northern latitudes – a bit like a spinning top rotating counterclockwise. It forms a boundary between the colder air over the North Pole and the warmer air to the south.

But sometimes, the vortex can be disrupted. It is accompanied by changes to the jet stream – high-speed winds that run west to east – which develops a wavy, snakelike pattern as it circles the globe.

Sometimes, the vortex splits into several fragments that move south, and it can become stretched, like a rubber band.

This is what seems to have happened over the past week, allowing the freezing air to move out of the Arctic across Canada and into the US.

But this can also occur in parts of Asia and Europe.

The large mass of cold air is combined with a rapidly intensifying low-pressure system called a “bomb cyclone” to trigger howling winds and intense blizzards over the Great Lakes and into south-eastern Canada.

2. Is there a link to climate change?

Scientists are still debating different theories.

But one that is gaining support is that a rapidly warming Arctic is disrupting weather patterns. The Arctic is warming about four times faster than most other places in the world, meaning the temperature difference between the North Pole and the tropics is less than before.

Some scientists say the rapid warming is causing disruptions in the polar vortex, through the changes in the polar jet stream. Others say that computer modelling suggests naturally variable factors are driving disruptions.

Dr Judah Cohen, a climate scientist at Atmospheric and Environmental Research, a weather-risk assessment firm in Lexington, Massachusetts, told The New York Times that the warmer conditions create larger and more energetic atmospheric waves that make the jet stream wavier, with greater peaks and troughs.

That affects the polar vortex circulation.

Scientists say they need more data and more evidence to be certain.

But there is far more certainty on the link to climate change and the intensity of rain and snowfall. That is because a warmer atmosphere holds more moisture, and despite the intense cold of the latest winter storms, overall, data shows winter temperatures are becoming milder globally.

In general, climate change has made cold extremes warmer and shorter, Dr Patrick Brown at The Breakthrough Institute, a California think-tank, told New Scientist magazine in December. “In the metrics that look at cold extremes, it’s not getting colder,” he said.

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