The atmospheric conditions have been very unfavourable lately
The UK is too small to have its own weather, we participate in the weather of the North Atlantic region, or indeed the whole world. So to understand why the last UK winter was record-breakingly wet, we need to look at atmospheric behaviour on a large scale. I’ve turned to MERRA – an hour-by-hour reconstruction of the whole atmosphere – and made the video of northern hemisphere weather above. (There’s a lot going on, I recommend watching it in full-screen, press the ‘X’ on the control bar).
The key feature is the sequence of storms that spin off North America, and then head out into the North Atlantic in the Prevailing Westerly Circulation (anti-clockwise in the projection of the video). In November these storms mostly follow the standard path northwest to Greenland or Iceland and Scandinavia, but in December the weather changes: North America becomes much colder and the path of the storms moves south, driving the bad weather straight at the UK. This persistent pattern of a cold North America and southerly Atlantic Storm Track is the outstanding feature of the winter, and it shows up even more clearly a bit higher in the atmosphere – the Upper-Level Winds have a simpler structure, as they are not complicated by contact with the Earth’s surface.
The temperature difference between cold polar air and warmer southerly air stirs up an overturning circulation, and the rotation of the Earth turns this into a strong anti-clockwise (westerly) rotating wind – the Polar Vortex. As early as 1939, Carl-Gustaf Rossby realised that this circulation would not be smooth and stable, and the characteristic undulations (Rossby waves) have a major impact on our weather. It’s a series of these waves that push cold polar air much further south than usual over eastern North America, producing a Very Cold Winter in those parts, shifting the storm tracks south and causing the wet, stormy weather in the UK.
But of course I’m not really interested in modern weather – that’s too easy, with ample satellite observations and tremendous tools like MERRA to show us what’s going on. The challenge is in providing the long-term context needed to understand these modern events – is there a consistent pattern, if not, what’s changed. And it just happens that a previous Markedly Wet UK Winter occurred 99 years earlier, in 1914/5, and we’ve been rescuing logbook observations for that time so we can use them to make improved studies of that winter.
This time we use the Twentieth Century Reanalysis (more precisely a test version of 20CR updated to benefit from oldWeather-rescued observations). In some areas (most obviously the high Arctic) there are no observations so the analysis is too uncertain to be useful, but over the US, UK, and Atlantic storm-track region we can reconstruct the weather of that year.
Again, the picture is clearer if we look at the upper-level circulation:
Do we see the same picture in 1914/5 as in 2013/4? Reality tends to be somewhat messier than the simple explanations that scientists treasure – but I think we do see the same pattern: a persistent tendency for cold, polar air to extend south over North America, and a North Atlantic storm track shifted to the south.
We can say quite precisely what happened last winter, and (thanks, in part, to oldWeather) how last winter compared to previous Exceptional Winters. However the obvious follow-on question is ‘Why did the polar vortex behave like that, and can we predict when it’s going to do it again? We’re still working on that one.