By Steff Gaulter
The first real cold snap of the season has blasted North America, triggered by an old tropical cyclone. Normally the remains of a tropical system will bring stormy, but mild weather, but this storm had other ideas. On Tuesday night, temperatures in Calgary dropped to an icy -23C (-9F), and the cold weather spread rapidly southeast causing temperatures to plummet quickly.
For a tropical cyclone to bring cold weather is quite unusual, after all they form over warm water near the equator. This means that they usually still retain some warmth even when they move northwards into cooler climates. Even in this case, the region which was hit by the storm stayed relatively mild, but it triggered a dramatic drop in temperatures several hundred kilometres away.
The storm itself was the remains of a tropical cyclone called Nuri. Tropical cyclones are given different names depending on where in the world they form, much like the green summer squash is called a courgette in the UK, but a zuchinni in the US. If a tropical cyclone develops in the waters around the Americas, they are called hurricanes, but if they form west of the dateline in the middle of the Pacific Ocean, then they are called typhoons. This storm developed to the south of Japan and was therefore called a typhoon.
Typhoon Nuri was a very powerful system. In fact, it was one of the most intense storms seen on the earth in 2014, but there’s a chance that you didn’t even hear about it, for the simple reason that it didn’t make landfall. It formed in the western Pacific, and headed northwards, then northeastwards, carefully sidestepping Japan as it did so.
As it ran northwards over the open waters of the Pacific, the waters beneath the system became cooler. This is a problem for most tropical cyclones — the warmth of the sea is their energy source, so when they move over cooler waters, they usually start to lose power and disintegrate. Typhoon Nuri also started to lose its intensity, but then merged with a vigorous weather system which was also crossing the Pacific.
At this point, the remains of Typhoon Nuri metamorphosed into an extratropical storm. This rather fancy sounding description simply means that Nuri no longer needed the warm sea to supply its energy, but instead it was free to gain or lose strength like any other non-tropical weather system anywhere in the world. In Nuri’s case, this allowed the storm to intensify rapidly as it hurtled eastwards towards Alaska.
As the storm barrelled over the Bering Sea, the northernmost part of the Pacific Ocean between Russia and North America, the pressure dropped to 924mb. This was one of the deepest low pressure systems ever seen in the region and it was the tiny Aleutian Islands which felt the brunt of the storm. The Aleutian Islands belong to the US, and are a narrow string of islands that extend 1,900km (1237 miles) west from mainland Alaska. With thousands of miles of ocean to their south, the islands are very exposed to the elements.
Thankfully the worst of the storm was seen over the open water of the Bering Sea and the largely uninhabited islands of the western Aleutians. The Air Force base on the western island of Shemya reported a gust of wind of 154 kph (96mph) and waves of up to 15 metres (50 feet), but fortunately there were no reports of injury or serious damage from the storm.
After battering the Aleutian Islands, the storm weakened significantly. By the time it hit the coast of mainland Alaska, it just resembled a typical winter storm. However, the story doesn’t end there, as Nuri was such a powerful storm it also disrupted the jet stream.
A jet stream is a fast flowing ribbon of air high up in the atmosphere. There are a number of them that encircle the globe, gently meandering their path, shifting a little to the north then a little to the south again. Most of the time we can ignore the fact they exist, but we do notice that flights from the US to Europe are usually quicker than flights from Europe to the US; this is because we are blown along by one of these jet streams. The other times we may notice a jet stream is when they deviate from their usual path or if they get stuck, because these jets also direct weather systems.
If a jet stream stops meandering, then a succession of rainbelts will follow the same track, and the region underneath the jet will see persistent rain, which can lead to flooding. On the other hand, if the jet stream shifts to the south of its usual path, cold air will be able to flow in from the north.
The remains of Typhoon Nuri were so powerful that they actually disrupted the jet stream that flows over North America. As the storm barrelled northwards, the jet stream was also forced to buckle northwards. That triggered the jet further east to swing much further south than usual, allowing cold air to surge through western Canada and into the USA.
As the temperatures dropped to their lowest of the season, people in North America were given a stark reminder of the ‘polar vortex’ which made the headlines last year. The warmth of spring suddenly seems like a very long way away.
