Since the start of December the UK has seen a prolonged period of particularly unsettled weather, with a series of storms tracking in off the Atlantic bringing strong winds and heavy rain.
The windiest month since 1993
In order to compare the recent spell with the numerous stormy periods of weather in the past the Met Office’s National Climate Information Centre has done an analysis of the number of weather stations in the UK which have registered winds over certain thresholds since the start of December.
This measure suggests that December 2013 is the stormiest December in records dating back to 1969 and is one of the windiest calendar months for the UK since January 1993.
December was also a very wet month across the UK, particularly in Scotland where it was the wettest December and wettest month overall in the records dating back to 1910.
But why has this been the case?
Storms are expected in winter
First of all, we do generally expect to see stormy conditions in winter months. This is because we see a particularly big difference in temperature between the cold air in the Arctic and the warm air in the tropics at this time of year.
This contrast in temperatures means we see a strong jet stream, which is a narrow band of fast moving winds high up in the atmosphere.
The jet stream can guide storms as they come across the Atlantic, and it has been sitting in the right place to bring those storms to the UK over the past few weeks.
There’s also a close link between the jet stream and storms. The jet stream can add to the strength of storms, but then storms can also increase the strength of the jet stream. This positive feedback means storms can often cluster together over a period of time.
But why has it been particularly stormy?
Even accounting for the fact that it’s winter, the jet stream has been particularly strong over the past few weeks – but why is this the case?
It’s partly due to particularly warm and cold air being squeezed together in the mid-latitudes, where the UK sits. This could be due to nothing more than the natural variability which governs Atlantic weather.
However, looking at the broader picture, there is one factor which could increase the risk of a stormy start to winter and this is called the quasi-biennial oscillation (QBO for short).
This is a cycle, discovered by the Met Office in 1959, which involves a narrow band of fast moving winds (much like our jet stream) which sits about 15 miles up over the equator. The cycle sees these winds flip from easterly to westerly roughly every 14 months.
In 1975 Met Office researchers discovered that when the QBO is in its westerly phase, it tends to increase the westerlies in our own jet stream – meaning there’s a higher risk of a stronger, more persistent jet stream with more vigorous Atlantic storms. It has been in its westerly phase since early 2013 and we expect it to decline over the next few months.
This is just one factor among many, however, which needs to be considered – so it doesn’t mean that the westerly phase of the QBO will always bring us stormy winters.
What about climate change?
Climate models provide a broad range of projections about changes in storm track and frequency of storms. While there’s currently no evidence to suggest that the UK is increasing in storminess, this is an active area of research under the national climate capability.