Winter storms like Jonas, as some are calling it, are caused when cold air from Canada collides with warm, moist air flowing up from the tropical Atlantic.
Because the waters off the east coast are much warmer than normal for this time of year, the winds blowing onshore carried more moisture than usual, which is why the snowfalls were so high – breaking records in several places.
New York’s JFK airport recorded 77 centimetres on 23 January, for instance, the most ever recorded on a single day.
Nearly 30 deaths have been blamed on the storm, from car accidents to heart attacks while shovelling snow.
The remnants of Jonas are now heading across the Atlantic to the UK, where it is feared they will cause yet more flooding.
Global warming is the obvious explanation for the unusual warmth, and computer models are likely to show that storm Jonas was made much more likely because of climate change.
The El Niño that helped push global temperatures to record-smashing levels last year may also have played a part.
But there may be more to it.
Ocean temperatures just off the east coast have been warming even faster than global temperatures, Stefan Rahmstorf of Potsdam University in Germany pointed out in a blog post on Sunday. Further north, however, the waters south of Greenland and Iceland are cooler than normal.
This is exactly the pattern expected if the circulation is slowing down: more heat staying by the east coast instead of being carried north.
We know the current has slowed over the past decade, thanks to direct measurements.
What isn’t clear, says Leon Hermanson of the UK’s Met Office, who studies how the North Atlantic Ocean influences the weather, is how much this slowdown contributed to the unusual warmth off the east coast.
Another key question is whether the observed slowdown of the current is part of an ongoing trend driven by global warming, or just a result of natural variation.
A few studies – including one published by Rahmstorf’s team last year – suggest that it is part of a long-term trend driven by global warming.
But since we have only a decade of direct measurements, it is too soon to be sure, says Hermanson.
The debate, though, is just about the timing: there is widespread agreement among researchers that climate change will eventually cause a slowdown of the Atlantic overturning circulation.
That’s because the overturning circulation is driven by cold, salty water sinking near the Arctic. As more fresh water from melting Greenland ice pours into the sea, it dilutes the seawater, making it less dense and thus less likely to sink. The faster Greenland melts, the more likely the circulation is to slow or even stop.
If Rahmstorf is right about a long-term, warming-related slowdown already being underway, North America and Europe could be in for yet more exceptional weather in the years to come, such as more severe blizzards.
The east coast will also experience very rapid sea level rise if the current continues to slow – leading to more flooding like that seen in parts of the east coast during storm Jonas.
The worse-case scenario would be an abrupt shutting down of the overturning circulation – a distinct possibility according to many models. But they suggest it isn’t likely to happen until towards the end of this century.
Read more: “Climate change: The great Atlantic shutdown“