In addition to a weakening gulf stream, another factor that contributes to colder winters (and probably warmer summers) in the northeastern United States, Europe, and especially Asia, is a weakening jet stream. The jet stream is a wind of about 180 km/h on average and up to 400 km/h in instances, that streaks around the north pole at 8km altitude. It gets its power from temperature differences between south and north. It is weakening because the arctic is warming fast. Warming in the arctic means less sea ice, which means more – dark – ocean that in turn absorbs more heat (i.e. a positive feedback loop).
A weaker jet stream meanders more, bringing warmer air to pockets in the arctic and colder air to pockets south. These ‘loops’ in the jet stream can get stuck, bringing about extreme events of cold spells and probably also heat and drought. Based on changes in the jet stream researcher predicted worsening drought in California back in 2004. The weaker jet stream does not mean that cold spells get colder though, cold extremes are getting less cold.