1979-2018年东亚冬季极端低温日频次的变化特征及可能成因

Characteristics of Winter Extreme Low-Temperature Days in East Asia During 1979-2018 and Their Possible Causes

利用NCEP/NCAR再分析资料,采用EOF分析和回归分析方法,初步探讨了东亚地区1979-2018年冬季(12月至次年2月)极端低温日频次的变化特征及其与同期大气环流和前期海冰之间的联系。结果表明:东亚地区冬季极端低温日频次整体变化趋势不显著,但1985年以前极端低温日频次整体偏多,且为峰值所在时段,而1986年极端低温日频次迅速减少,之后呈现缓慢增加趋势,直至2010年达到第二峰值,此后便呈现明显的减少趋势,并在近几年趋于平缓。在东亚地区冬季极端低温日频次偏多年,海陆间气压差大、大槽加深且东亚急流偏南偏强,这有助于北极冷空气向南入侵,从而易造成东亚地区出现极端低温天气;在东亚地区冬季极端低温日频次偏少年,情况相反。影响东亚地区冬季极端低温日频次变化的关键海冰区主要位于巴伦支海-喀拉海和欧亚北部沿岸,当东亚地区冬季极端低温日频次偏多时,前期9-10月巴伦支海-喀拉海海冰密集度(SIC)偏少;反之亦然。前期SIC的异常变化是预测后期冬季东亚地区极端低温日频次变化的重要潜在预测因子。

Based on the NCEP/NCAR daily analysis data from1979 to 2018,the characteristics of the frequency of extreme low-temperature days in winter(December-February)over East Asia and their relations with the simultaneous atmospheric circulation and preceding sea ice during 1979-2018 were analyzed using EOF and regression analysis methods.The results showed that the trend of the frequency of extreme low-temperature days in East Asia was not significant over the past 40 years.However,the frequency of low-temperature days presented a peak before 1985 and then decreased rapidly in 1986.It showed a slow upward trend after 1986,with a second peak occurred in 2010,and increased slightly recent years.When the occurrence of extreme low-temperature days in East Asia was more frequent,the sea-land pressure difference enlarged,the trough deepened,and the jet stream in East Asia strengthened and moved toward the south,which were conducive to the invasion of polar cold air to the south and led to the frequent occurrence of cold weather.On the contrary,when the occurrence of extreme low-temperature days in East Asia was less frequent,the situation was opposite.The key sea ice areas that affected the frequency of extreme low-temperature days in winter over East Asia included the east of the Arctic Ocean,the northeast of the Pacific Ocean,the Barents Sea-Kara Sea and the northern coast of Eurasia.When the extreme low-temperature days in winter over East Asia occurred more frequent,the sea ice concentration(SIC)of the Barents Sea-Kara Sea in early autumn(September-October)was less than the normal situation and vice versa.The variation of the external forcing SIC factors may be of great importance to predict the inter-annual variations of the frequency of extreme low-temperature days in the later winter over East Asia.