Hot Summers in the Northern Hemisphere

The past two summers of 2017 and 2018 are very hot in the Northern Hemisphere, but the causes are not clear. Here we identify natural internal variabilities of seven atmospheric circulation patterns by teleconnection method that can cause the increase of summer surface air temperature: North Atlantic pattern, Europe pattern, East Atlantic pattern, West Atlantic pattern, Chukchee Sea-North America pattern, Arctic Ocean-North America pattern, and Beaufort Sea-Siberia pattern. The magnitude of increased air temperature induced by internal variabilities associated with these atmospheric circulation patterns is comparable to or even larger than that caused by global warming. A combination of natural internal variabilities and global warming explains summer surface air temperature variations in the Northern Hemisphere. The results show that accurate forecast of these atmospheric circulation patterns can help to predict summer surface air temperature in the Northern Hemisphere.