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Weather news and views from Brian Gaze.
On balance winters in northwestern Europe have got milder in recent decades. What is the reason for this change?
In general terms cold winters in the UK happen when pressure is high to the north and low to the south. High pressure areas over Greenland, Iceland and Scandinavia and lower pressure stretching from the Azores to the Mediterranean produces a negative North Atlantic Oscillation (NAO). This set up usually leads to north or easterly winds over the UK which in the winter months pull in very cold arctic or continental air masses and a risk of snow and freezing temperatures.
The image below is produced using reanalysis data from the National Centers for Environmental Prediction (NCEP) in the USA. It shows the trend in Mean Surface Level Pressure (MSLP) in the December to February period from 1950 to 2015. The yellows and oranges to the south of the UK show a positive anomaly, in other words higher pressure than previously was typical and to the north we see lower pressure. This suggests a trend towards a stronger and flatter jet stream across the North Atlantic which leads to the UK and the rest of northwestern Europe sitting on its mild southern side for longer periods. In other words mild weather synoptics are increasingly appearing during December, January and February.
The image provides good evidence of the milder winters we've been experiencing and their cause. The next question could be what is the underlying reason for the change in the pressure patterns in our part of the world and what has been happening farther afield? Obvious possibilities are climate change and cyclical patterns. Unfortunately it's above my paygrade (and to be honest I don't want to lose half of my audience!) to authoritatively discuss this and if you want to investigate further I'd suggest you look at the evidence based science which addresses this topic.
No! The point I'm making is that in any given year the odds of a cold winter in the UK are lower than they were a few decades ago. This doesn't mean severe winters and widespread snow won't keep occurring. In recent years there have been bitterly cold spells and there will be more in the future.
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