Attributing global warming to extreme weather events
Ten years ago, from a climate science perspective, it would be considered risky to attribute certain Category 3 hurricanes in the North Atlantic (such as Hurricane Sandy that hit New York City in 2012) to global warming.
As we all know, Earth’s oceans have absorbed solar energy over the past century that has not yet returned to the atmosphere. Therefore, the average temperature of the ocean is rising. Climate scientists are now investigating how global warming affects extreme weather events in certain geographic regions. Swanson (2013) summarized the idea of the relationship between global warming, global warming and extreme weather events, pointing out that the possibility of these extreme weather events is increasingly linked to man-made global warming – excessive carbon dioxide is emitted into the atmosphere. This is no longer an abstract concept. It can be felt on all levels. 
Global warming and Arctic ice melt pool
Global warming is causing the temperature of the Arctic Ocean to rise. This higher temperature extends the melting of the Arctic ice in summer into autumn and winter. The Arctic regions previously covered in white reflective snow have now been replaced by dark land and ocean regions that do not reflect sunlight. In estimating the mass of glaciers lost in Greenland, Jay Zwor of NASA’s Goddard Space Flight Center pointed out that “20%” of ice emerges from the snowfall received each year. Biello (2006)  Since this sunlight is no longer reflected, this energy is now absorbed by the ocean, further warming the ocean and amplifying the melting effect of the Arctic ice shelf.
This melting causes major changes in the jet stream (the narrow, fast-moving westerly wind that flows from west to east in the United States, Canada, Europe, and northern Asia), the main atmospheric pressure (differential) gradient that affects the northern mid-latitudes. winter weather.
Global warming causes fundamental changes in rapids
The rapids can be seen as waves in the peaks and valleys, moving around the center of the Northern Hemisphere, bending smoothly to the north and south. The temperature gradient (difference) between the Arctic latitude and the North Atlantic latitude decreases in the fall, when the Arctic Ocean releases additional solar energy that is absorbed by the increasing ice melt caused by global warming. Then the air pressure difference between the two pressure fields also decreases and the speed of the jet stream west to east wind also decreases.
In the Northern Hemisphere there are two pressure fields. Arctic Oscillation, or AO, a positive or negative pressure field extending from parallel 70° north latitude to the North Pole, North Atlantic Oscillation, or NAO, a positive or negative pressure field, extending from parallel 70° north latitude to the Arctic to the subtropical zone. Given that the NAO pressure field affected by global warming is more likely to be negative in the fall and winter, the jet stream is more likely to change in winter.
2011-2012 extreme heat in the United States and cold winter in Europe
As mentioned earlier, global warming affects the rate of polar ice melting. In summer, more solar energy is absorbed by the ocean and then heat is released by the Arctic sea in autumn, increasing the pressure (difference) gradient between the Arctic Oscillation pressure field and the North Atlantic Oscillation pressure field and the jet stream. is reduced becomes slower.. The pressure gradient between AO and NAO becomes weaker, allowing larger radius curves to extend north or south more easily.
From the winter of 2011 to 2012, there were extremely warm temperatures in the northeastern United States. The jet stream curves further north than usual over the central states of the northeastern United States, allowing warm subtropical air to reach the United States-Canada border and remain there for a long time. In addition, there is also the phenomenon of La Niña (i.e. pressure fluctuations originating from the South Pacific). This climatic phenomenon tends to deflect the jet stream north in the northeastern United States.
At the same time, the coldest winter in 25 years in Eastern Europe occurred in 2011-2012. The pressure gradient (difference) between NA and NAO is very weak.