What’s wrong with our weather?
This was the title of an enlightening BBC Horizon programme, presented by physicist Helen Czerski and meteorologist John Hammond. It explored the wider reasons for the new challenges in UK weather in recent years, which include more extremes both of prolonged rainfall and very cold winters. Their overall conclusion was that we should expect more extremes of both cold and wet winters in the coming years, and the basic reason for both is the increasing variability of the jetstream.
When there is a larger temperature difference between the arctic and the tropics, the jetstream accelerates. The winter of 2013/2014 was exceptionally cold in the US, which speeded up the jetstream. The result of this in the UK is higher winds, and weather fronts moving through very rapidly.
However, we are also seeing periods when the jetstream slows down. The Arctic is warming twice as rapidly as the Global average, which means that there are periods with a lower temperature gradient than usual: the result of this is what are called meanders, i.e. larger oscillations in the jetstream. This has caused periods of super-cold weather in the UK, when the jetstream loops south of the UK and drags colder air down over us. Another feature of these meanders is that they can stay stuck for weeks, hence the prolonged periods of one weather condition, including heavy rainfall, which we have seen in recent years.
The programme could have usefully said more about the impact of climate change: they did at least acknowledge that it is a contributing factor to these changes. For example, it is basic physics that a warmer climate will lead to more intense rainfall, as we have experienced in recent years.
Whatever the outcome of the climate change negotiations, it is unlikely that our weather will settle back to past patterns: we need to be more resilient to extremes in the weather as on other fronts.
The speed of the jetstream across the Atlantic during the severe UK storms of winter 2013-14 was double the usual speed, and hence the succession of storms which hit us was unusually rapid.