An early winter cooling in East Asia in a global warming scenario

Opposite to the global warming, a December cooling trend after 2000 is found in East Asia region using NCEP/NCAR reanalysis data. This cooling is confined to the lower and middle troposphere (below 300 hPa) and occurs in the middle latitudes of Eurasian continent from Mediterranean Sea, Caspian Sea, and Siberia to East Asia. The East Asian winter monsoon tends to be strong in December, compared to weak winter monsoon in November, January and February. It's pointed out that this cooling is associated with the stronger semi-permanent Siberian High. Meanwhile, the trough along the east frank of Eurasian continent is stronger and shifts eastward, and the jet stream at the upper troposphere is strengthened. This may help to maintain the stronger winter monsoon in December. Note that this December cooling may origin from the stratosphere, where a stronger Aleutian High is formed, which induce a stronger sea surface pressure in Siberian and cooling in East Asia.