Processes for Occurrence of Strong Cold Events over Eastern China

A strong cold event hit eastern China around 24 January 2016 with surface air temperature reaching more than 10 degrees C below the climatological mean in most regions of eastern China south of 40 degrees N. A total of 37 strong cold events similar to the January 2016 event with temperature anomalies over eastern China exceeding -5 degrees C have been identified during the winters from 1979/80 to 2015/16. A comparative analysis of events with surface temperature anomalies of the same intensity but limited to north of 40 degrees N indicates that the southward invasion of cold air to eastern China south of 40 degrees N is related to two factors. One is the latitudinal location of the upper-level wave train, the surface Siberian high, and the midtropospheric East Asian trough over the mid-to high-latitude Eurasian continent. The other is a subtropical upper-level wave train emanating from the midlatitude North Atlantic. The emergence of the subtropical wave train is related to the positive phase of the North Atlantic Oscillation (NAO). When the mid-to high-latitude wave train is located too far northward and the subtropical wave train induces an anomalous midtropospheric high over southern China, the East Asian trough does not extend southwestward and the Siberian high does not expand southeastward. In such a case, the cold air mainly affects northeastern China and northern Japan.