In winter 2012/2013,China's average temperature was -3.8 ℃,which was lower than the same period in a normal year by 0.4 ℃.The temperature of Northeast China,North China,Huang Huai,Jiang Huai and northern Xinjiang was lower than in normal years.Using monthly mean temperature data from 160 observation stations for the period 1951-2013,supplied by the National Climate Center,along with global SST data from the UK Hadley Center,NCEP/NCAR reanalysis data,and sea-ice satellite remote sensing data from the University of Bremen,Germany,we employed EOF analysis,regression analysis,composite analysis,and correlation analysis,to study the mid-high latitude atmospheric circulation anomalies in East Asia that resulted in the 2012/2013 winter temperature anomalies in China.The causes of the abnormal circulation were also analyzed,from the perspective of anomalies in the oceanic environment.
The results showed that the winter temperature in China has two modes:the first mode shows the change all over China is warmer or colder,and the second mode shows an adverse change between Northeast China,North China,northern Xinjiang and other areas of China.The 2012/2013 winter temperature anomaly distribution was very similar to the second mode,and the 500 hPa height anomaly field of the Northern Hemisphere in winter 2012/2013 was also similar to that of the second mode.Through regression analysis of the time coefficient of the second mode and 500 hPa height field of East Asia,the high latitudes of the Northern Hemisphere,form the North Atlantic subpolar along the waveguide,from west to east,showed a "+-+-" abnormal distribution of the wave train.The Atlantic area,Urals and Siberia was higher than normal,and the East Asian trough was significantly deeper.
The East Asian winter monsoon index and Pacific Decadal Oscillation(PDO) both showed characteristics of decadal changes,but their phases were opposite.The PDO in positive phase corresponds to a weak winter monsoon index,and vice versa.From 2002,the PDO entered into a negative phase,while the East Asian winter monsoon became stronger.A strong winter monsoon often causes low temperatures,especially in the north of China.Composite analysis showed that when the PDO is in its cold phase,the Aleutian low weakens,the Siberian high enhances,and the East Asian trough deepens in the winter season.The 500 hPa circulation over Easte Asia in winter 2012/2013 was similar to this case.Such a circulation configuration usually leads to cold air behind the trough moving towards the south from Siberia,resulting in a wide range of low temperatures in East China in winter.
Finally,analysis revealed that the Arctic sea-ice extent in early September in the autumn season may have implications for better prediction of late-winter Arctic Oscillation(AO) index.If the early September Arctic sea-ice extent is smaller,high pressure usually controls the high latitudes to the polar regions of the Northern Hemisphere,and the pressure in midlatitude areas is lower than in normal years..This kind of pressure distribution shows a typically negative AO phase.A negative phase of the AO indicates that high pressure is controlling the Arctic area,which is the source of cold air.The cold air of the polar regions is usually squeezed and breaks out to the south,leading to the emergence of cold air and causing low temperatures in the high latitudes of the Northern Hemisphere.In autumn 2012,the Arctic sea-ice extent was smaller than in normal years and the AO remained in its negative phase in late winter.Thus,the winter season in China demonstrated abnormally low temperature characteristics,particularly in Northeast China and northern Xinjiang.