Recent severe weather experienced on both sides of the Atlantic reveal different effects but both show the influence of the Jet Stream.
Weather in North America
In the USA and Canada, severe winter weather has been experienced during the last month, with record low temperatures in a number of states.
On the Canada-US border, Niagara falls were frozen for only the third winter since 1911. In much of the MidWest of the USA, temperature plunged and reached -40C, and even -45C in places, extremes that haven’t occurred in decades.
Heavy snowstorms accompanied low temperatures with blizzard conditions across a large swathe of the country.
Weather in Europe
In the UK, severe flooding has occurred with severe coastline erosion across much of the exposed western coast, from Scotland to South West England, with winds exceeding 60 mph and record-breaking waves.
On the whole, though, in the UK, temperatures have been much milder than expected for the time of year. Portugal and France have also been inundated by storms bringing heavy rain and large coastal waves.
The Jet Stream Affects Middle Latitudes
The jet stream is a rapidly-flowing band of air at an altitude of about 10 km, which separates polar from sub-tropical air.
Usually, the jet stream is compact, leaving the polar regions controlled by the ‘polar vortex.’ This confines the cold air to high latitudes. (See the image on the right in the above diagram.) Sometimes, such as in recent weeks, the jet stream meanders (Above image, left) over a wide latitude range. This allows cold air to be transported from the Arctic to middle latitudes over certain longitude ranges.
In the last few weeks, the movement of the Jet Stream has led to the severe weather over North America and Europe. Atmospheric systems are controlled by surface temperature at the lower end, and steered by the jet stream at the upper level. Storms tend to cross the Atlantic at higher latitudes than the UK, but with the recent movements of the jet stream to the South over the British Isles, there has been a sequence of active storms which have deposited rain across the whole of the western coasts of Europe. Most of the flow has been from the South West, leading to relatively high temperatures for the time of year.
The infra-structure has struggled to survive the wind and rain, leading to widespread flooding in the West of the British Isles. It therefore appears that the weather on both sides of the Atlantic has been strongly linked by the meandering of the Jet Stream. For many years, US weather forecasters have fully recognised the impact of the Jet Stream on the weather over the medium and long range, up to about a month. Forecasters in Europe are now becoming more aware of its influence.
The Influence of Climate Change
Climate sceptics will use the recent spell of cold weather in North America to suggest that climate change is an illusion. A more balanced view will recognise that in the contiguous US, Autumn 2013 was wetter and warmer than average. Further, even though November 2013 was slightly (0.2 C) colder than average in the USA, globally it was the warmest November on record, at 0.8 C (1.4 F) above the 20th century average. Of course, climate change covers a wide range of phenomena, not just temperature.
Climate change scientists predict that the world as a whole is warming (IPCC, 2013), as indicated for the November 2013 average, and not just a small part of it, such as North America. It certainly might be argued that the movement of the jet stream in recent weeks has been triggered in part by the warming of the Arctic, which is one of the observed most serious effects of climate change.
Another possible influence on the Arctic is that the stratosphere has been showing an increased tendency towards warming (Cohen and Barlow, 2009). This is quite complex and would be relative to the overall increased cooling at those levels associated with carbon dioxide increase. In practice, the possible increase in ‘stratospheric warming’ is more likely to be a response to the increased disturbed lower atmosphere. In other words, the stratosphere is showing a possibly increased response to the lower atmosphere (the troposphere), rather than the stratosphere significantly affecting the troposphere itself.
Climate Change or Coincidence?
The official view from the UK Meteorological Office is that we can’t attribute the recent events to climate change, as reported in the Guardian newspaper. While it is certainly true that an individual sequence of events such as the frequent storms affecting Western Europe may or may not be caused by climate change, the long term view may be different.
It seems reasonable to suppose that the incidence of extreme weather may increase in the future, but it may be too early to state, as the UK prime minister, David Cameron, has done recently that recently events are likely caused by climate change.