The Characteristics Of Extreme Low Temperature Events In China In Recent 50 Years And Preliminary Analysis On Their Formative Causes

Based on daily minimum temperature dataset from 473 stations from 1960 to 2008 in China, extreme low temperature (ELT) thresholds are determined for different stations and yearly occurrence frequency of extreme low temperature events for each station are estimated, then the spatial distribution of the ELT event was divided by it in China, and the temporal distribution of ELT is diagnosed in each sub-regions, moreover, the atmospheric circulation anomalies are analyzed when the yearly occurrence frequency of ELT events is abnormal in each sub-regions, the correlation between climatic index and the ELT event in China is diagnosed at last. The results are as follows:(1) The spatial distribution of the ELT event can be divided into six sub-regions in China. The yearly occurrence frequency of ELT event showed remarkable decreasing tendency in all of six sub-regions, though the changes of the yearly occurrence frequency of ELT event in six sub-regions are different with each other. Meanwhile, the sudden change phenomenon of the yearly occurrence frequency of ELT event is ubiquitous in China except Central China and the middle area of North China, the sudden change phenomenon occurred before 1990 in Northeast China and Northwest China, and the time is between 1990 and 1995 in other area. Besides, in all of six sub-regions the yearly occurrence frequency of ELT event showed 2-4yr period oscillation, and there is no long period in all of six sub-regions.(2) When the yearly occurrence frequency of ELT events is abnormal in one of six sub-regions, the atmospheric circulation characteristics in summer of these years are analyzed at first, the results show that:the characteristics of geopotential height fields at upper and middle levels above this sub-region are similar, at the same time, there are negative (positive) temperature anomalies at 500hPa when the yearly occurrence frequency of ELT events is positive (negative) anomaly, also, the cold air and the heat flow are easy(hard) to intersect above this sub-region. The atmospheric circulation characteristics in winter of these years are also analyzed, when the yearly occurrence frequency of ELT events is positive anomaly, there are negative geopotential height fields anomalies at 500hPa above this sub-region, and the negative anomalies of temperature at 850hPa are obvious. Besides, the cold air and the heat flow are easy to intersect or the cold air become stronger above this sub-region. The regions that through the significant test in winter are more than that in summer.(3) The effects of the Arctic Oscillation(AO), the western Pacific teleconnection Pattern (WP), the Pacific-North America teleconnection Pattern(PNA) and ENSO on the ELT in China on the interannual and interdecadal timescales are investigated by using the partial correlation analysis. The results show that the influence to ELT in China of AO mainly appears in spring on the interannual timescale, however, it mainly appears in winter on the interdecadal timescale, and its influence to North China is stable. WP has significant influence on ELT in China on the interdecadal timescale, especially in summer an in winter, furthermore, the influence scope is large, and the correlation is not influenced so much by other index. The influence to ELT in China of PNA is not obvious on the interannual timescale, it has stable effect in spring and in autumn on the interdecadal timescale, moreover, the correlation is greatly affected by WP in winter. ENSO has obvious and independent effect on ELT on the interdecadal timescale in spring and in summer, but the influence is not obvious in other seasons neither on the interannual timescale nor on the interdecadal timescale.