Possible Causes for the Sudden Drop of Air Temperature in the Northern Hemisphere from Early to MidWinter
During December 2015, the averaged surface air temperature over most regions in the midlatitude of Northern Hemisphere was above normal. Some regions in North America experienced the warmest December in the record. However, the temperature dropped suddenly in January 2016. Extreme cold weather events such as cold surges, winter storms attacked Europe, North America and East Asia. Using the NCEP reanalysis and station observation data, the possible causes for the sudden drop of air temperature during the winter were discussed. The analysis reveals that the Arctic oscillation turned from positive phase in December 2015 to negative phase in January 2016, leading to the warmer Arctic but the colder midlatitude continent. The development and decay of midlatitude atmospheric blocking also contributed to the sudden drop of air temperature. Furthermore, in the context of global warming, the sea surface temperature of tropical eastern Pacific is often above normal during the year of El Ni〖AKn~D〗o, which gives rise to larger temperature gradient between the equator and the pole, causing a more unstable climate system, and further leading to more frequent extreme cold events.