Decadal Variations of Winter Extreme Cold Days in Northern China

Weather and climate extremes often lead to dramatic losses in our society and warrant improvement of their understanding. In this study, the decadal variations in the first two dominant empirical orthogonal function (EOF) modes of winter extreme cold days (WECDs) in northern China (NC) have been investigated. Results show that both EOF modes show distinct decadal variations that together explain around 24% of total variances. At the decadal time scale, the EOF1 is closely related to the decadal Arctic Oscillation (AO);the negative AO can lead to spatially consistent increase of WECDs in NC. On the other hand, the decadal EOF2 can be influenced by the decadal El Niño-Southern Oscillation (ENSO). The decadal El Niño can result in the large-scale negative sea level pressure (SLP) anomalies in the Eurasian continent west of the western NC and the positive ones over western China. The anomalous southwesterlies between the contrasted SLP anomalies can advect the warmer air from the lower latitudes to the western NC, decrease the WECDs there, and contribute to the east-west asymmetric WECD anomalies in NC. The impacts of El Niño are confirmed by the numerical simulations in the Atmospheric Model 2.1 (AM2.1) when forced by the El Niño-related sea surface temperature (SST) anomalies in the tropical Pacific.

Study on the extremely cold winter of 1670 over the middle and lower reaches of the Yangtze River

The snow-cover days over the middle and lower reaches of the Yangtze River （MLRYR） in the winter of 1670 were extracted from Chinese historical documents. By these records, the winter temperature anomalies （compared to the mean of 1961-1990） recorded at seven meteorological stations and the regional mean winter temperature were estimated. The results show that： （1） There was an average of about 30 snow-cover days over the MLRYR region in 1670, ranging from 11-20 days in Shanghai and eastern Zhejiang to 5140 days in eastern Hunan Province. The snow-cover days averaged about 40 days in Anqing and Nan- cheng, and ranged from 30 to 40 days in Quzhou, Jingdezhen, and Nanchang; and （2） the regional mean winter temperature in 1670 was estimated to be approximately 4.0 ℃ lower than that of 1961-1990. The maximum negative anomaly of 5.6℃ occurred in Nanchang and the minimum anomaly of-2.8 ℃ was detected in Quzhou. Both of these were lower than that of the coldest winter during the instrumental observation period of 1951-2010. This research could not only provide a method to es- timate historical climate extremes, but also provide a background to understand the recent instrumentally climate extremes.