The Annual Variation And Causes Of Winter Surface Air Temperature In China

Using the monthly mean temperature data of160stations by NCC, the NCEP/NCAR reanalysis data and NOAA global sea surface temperature (SST) data, the characteristics of winter temperature anomaly in the last60years are investigated. According to the results, we divided the winter temperature of60years into two periods:the cold period (1951-1985) and the warm period (1986-2010).Then we analyzed the differences of spatial-temporal distribution and related factors such as principal modes, the anomalies in atmospheric circulation,the forcing of SSTA and synoptic-scale vortex, all of which are associated with the anomalies of winter surface air temperature in China. The main conclusions can be summarized as following:(l)The two principal EOF (Empirical Orthogonal Function) modes in two periods are similar, both showed a consistent variation pattern in whole China and the pattern with north-south variation reversed. The spatial distributions of the first model for two periods are similar, but the one explains less variance in warm period. For the second model, the significant area in North China is increased while narrowed in South China during warm periods in comparison with that in cold period. At the same time, the explaining variance is also decreased in the warm period.(2)By analyzing the correlation and regression of circulation fields with winter temperature in the two periods, the results showed that the Siberian High, East Asia winter monsoon and the abnormal circulation systems around the Ural Mountains and the region South to Lake of Baikal, are the main circulation systems which have important impact on the wintertime surface air temperature in China in both cold and warm period. But the contribution to the temperature anomalies of each factor will be different in two periods. For example, the Indices of both Siberian High and winter monsoon show a trend of decrease in60years, which is the direct cause of the warming in winter.(3)The sea surface temperature anomalies (SSTA) of tropical Indian Ocean, the middle and east Pacific seem closely related to the winter temperature anomalies. The SSTA in eastern Pacific plays a more important role in the contribution of the temperature compared with the other parts of the ocean in the tropics. And the coupling relationship between SSTA in Indian Ocean and eastern Pacific has changed since the climate warming.(4)The SSTA of key regions in the tropical Indian Ocean, the middle and east equatorial Pacific seem closely related to the circulation variation in later winter linked to the winter temperature anomalies. The influences of the SSTA in these two key regions on the coming circulation are somehow independent, and the winter surface air temperature anomalies are jointly impacted by the two factors. The July and August SSTA in the tropical Indian Ocean mainly correspond to the winter circulation anomalies of Siberian High, height anomalies over Ural Mountain and the southern Lake Baikal at500hPa, while August,September and October SSTA over Middle East Pacific mainly causes anomalies of West Pacific Subtropical High and India-Burma Trough.(5)We also investigated the contribution of the synoptic eddy feedback to the large scale atmospheric circulation anomaly. The results show that synoptic eddy has positive feedback to the low-frequency circulation, strong synoptic eddy feedback will cause the Ural blocking high become stronger, the trough South to Lake of Baikal deepen, and then causes low winter temperature anomalies in China. The synoptic eddy feedback has decadal decrease in past60years, and the role on the large scale circulation variation and associated temperature anomalies of the synoptic eddy feedback also becomes weaker in warm period, which is one of the reasons for warmer winter after1986.