Research On The Variation Of Extreme Cold Events In Beijingand Its Relation To Arctic Oscillation During The Winter From 1951 To 2013

Using the NCAR/NCEP reanalysis data of sea level pressure and height at 500hPa and the observational maximum and minimum temperature data collected in Beijing, the variationand the abrupt climate change of the extreme cold events’ frequency and extent during the winter from 1951 to 2013 in Beijing are studied, and the relationship between the Arctic Oscillation and the extreme cold events are further analyzed. The main conclusions are summarized as follows:1.The occurrence frequency of the extreme cold events exhibits a trend of decrease from the winter of 1951/1952, and the reduce trend of the cold nights is obvious than cold days. Abrupt climate change of the extreme cold events is observed in the mid-1980s, with the cold day events in the winter of 1982/1983 and the winter of 1984/1985 and the cold night events in the winter of 1984/1985. Extreme cold events onset frequently before the mid-1980s, and significantly decrease after the mid-1980s.2.The intensity of the extreme cold events is prone to weaken with the decrement of the extreme cold events occurrences.94.8% of cold day events’ maximum temperature range from-7℃ to 1℃,while 96.4% of cold night events’ minimum temperature range from-17℃ to-8℃ during the winter from 1951 to 2013. And over all the temperature scopes, average days of the extreme cold events during the winter from 1984 to 2013 are apparently less than that during the winter from 1951 to 1983, and also the reduction of lower temperature is most obvious. As to the time distribution, the cold day events reduced most significantly at February while the cold night events at December.3. The frequency of the extreme cold events is significantly negative related to the Arctic Oscillation index. The correlation coefficient between the AO index and the days of cold day is-0.418, and which is-0.362 with the days of coldnight.57.1% of the cold day events and 58.4% of the cold night events outbreak when the monthly AO index anomaly is negative and less than-0.5. Thus when the AO transited from the negative phase to the positive phase, the frequency of extreme cold events in Beijing reduced severely. Based on the wavelet analysis, obvious oscillationperiods of 2-6 years and quasi-16 years are discovered over the time series of the frequency of extreme cold events and the AO index, and with quasi-8 years osscilation period of the AO index also. The interannual and interdecadaloscillation periods of the cold day events and cold night events strengthened before the mid-80s, which of the AO index strengthened during the winter from 1960 to 2000. However, the quasi-2 years oscillation periods of the AO index and cold day events enhanced since the winter of 2000/2001.And we also found the quasi-8 years oscillation period of the AO index. A strong positive correlation exists between the intensity of the extreme cold eventsand AO index, which is significantly lower than that between the frequency of extreme cold events and AO index. Further analysis shows that strongly extreme cold events must be outbreak under the background of the negative AO phase.91.3% of all the extreme cold events with the maximum and minimum temperature anomalies less than-10℃ happened when the AO was in negative phase.4. Further analysis shows that during the AO negative phase, critical systems which affect the cold air outbreak in Beijing is easy to develop.It is found that the blocking over the north of Eurasia and the East Asian trough at the middle troposphere together with the Siberian High and the Aleutian low at surface develop quickly when the AO is on the negative phase. But it is should be noticed that the intensity of the Siberian High and Aleutian low reduced together with the eastward movement of the center of the Aleutian Low at sea level pressure, and also the Ural Blocking and the East Asian trough weakened at the middle troposphere since the winter of 1984/1985. However, the blocking ridge over the area in west of Lake Baikal strengthened.5. According to the synthetic analysis, the minimum of the AO index is 2 days in advance to the Ural blocking high intensity peak, and 3 days to the intensity of the Eurasian blocking ridge, and 4-5 days to the Siberian High intensity peak, and 6-7 days to the extreme cold events outbreak.Correlation coefficient analysis denotes that there is an obvious minus lag 4-days correlation between the intensity index of Siberian High and the AO index, and an significantly lag 3-days correlation between the intensity index of Eurasian blocking ridge and the AO index,So, in the intra-seasonal scale, the variation of the AO might affect the onset of the extreme cold events in Beijing. When the AO transited to the negative phase, the Siberian High and Eurasian blocking ridge would strengthen, and thus finally affect the outbreak of the extreme cold events.