Because of something that's been named the North Atlantic Oscillation, that has been known about for almost 200 years, apparently. It is a seesaw in atmospheric mass, and in recent years it has been in 'the positive phase', but it seems to have moved to 'the negative phase' in the last year or two, and is 'strongly negative' at the moment. The positive phase is associated with stronger-than-average westerlies across mid-latitudes, warm and wet winters in Northern Europe, dry winters in Southern Europe, cold and dry winters in Northern Canada and Western Greenland, and mild and wet winter weather conditions in Eastern USA. The negative phase is associated with the opposite conditions. Over a longer time period there have been roughly equal amounts of time in the positive and negative phases, and it is not obvious how much man made changes to the atmosphere are contributing to any changes that there may be in the NAO. (See also the Arctic Oscillation, or AO, where winds circulating around the North polar mass keep the cold in to a greater or lesser degree).

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