近16 a来冬季欧亚大陆中纬度地区低温事件频发及其成因

利用ECMWF的ERA-Interim再分析资料和NOAA/NCDC站点资料,通过逐日气温的季节内方差,构造了冬季冷强度指数,并研究了近16 a来欧亚大陆中纬度地区低温事件频发的原因。结果表明:近16 a来,欧亚大陆中纬度地区极端低温天气发生频率呈显著上升趋势,对应在欧洲西北部、西伯利亚以及东亚地区冬季冷强度增强趋势明显,但冬季季节内温度方差增强趋势不明显,表明新构造的冷强度指数能更好地刻画低温事件的总体变化特征。近年来欧亚大陆中纬度地区寒潮事件频发且持续时间加长,相应地,西伯利亚高压增强并向西北扩展,伴随着欧亚大陆中高纬度地区北风增强,西风减弱,对应冬季平均气温也出现显著下降趋势,导致冬季冷强度不断增强。进一步分析温度平流各项发现,近年来冬季欧亚大陆中纬度地区以冷平流为主,纬向与经向温度平流分量同等重要,除经向风及经向温度梯度变化项的作用外,纬向风与纬向温度梯度变化项也起着重要的作用。温度平流各项对各个地区的贡献不同,经向风与经向温度梯度的变化主要促使西伯利亚地区降温,纬向风变化项主要引起欧洲地区降温,纬向温度梯度变化项主要使东亚地区降温。

Intensification of winter cold events over the past 16 years in the mid-latitudes of Eurasia and their causes

In this study,we used the ECMWF ERA-Interim reanalysis datasets and NOAA/NCDC station datasets to construct a new coldness intensity(CI) index based on the amplitude of daily anomalously cold temperatures in winter,which was different from the sub-seasonal temperature variance.Next we analyzed the abnormal changes of atmospheric circulation,shortwave and longwave radiation and temperature advection,so as to reveal the change characteristics of winter cold events,as well as the causes of recent increasing winter cold events in the mid-latitudes of Eurasia during the period of 1998-2012.The results showed the following:1) Cold winters and events were frequently found in the mid-latitudes of Eurasia over the past 16 years,especially in northwestern Europe,Siberia and East Asia.The linear trend of winter coldness intensity was consistently clearly enhanced in these areas,but the sub-seasonal variance of winter temperature did not exhibit a clear intensified linear trend.The consistency of coldness intensity and cold events showed that the coldness intensity may indicate the recent intensification of cold events.2) Since 1998,an intensification and northwest expansion of the Siberian High associated with enhancing northerly wind and weakening westerly wind in the mid-latitudes of Eurasia was observed,which led to negative winter temperature anomalies and winter extreme cold events increase over the mid-latitudes.In addition,recent frequent and persistent cold surges facilitated the significant decreasing of winter temperature over the mid-latitudes of Eurasia,coupled with continuously strengthening of winter coldness intensity.3) We further decomposited the temperature advection,and observed that the temperature advection was mainly dominated by cold advection in the mid-latitudes of Eurasia during the period of 1998-2012,and this feature had been particularly significant since 2005.Cold zonal advection mainly dominated in Europe and East Asia,while meridional temperature advection mainly cooled Siberia,and both zonal and meridional advections were critical components.Aside from the meridional wind and temperature gradient components cooling effects,the zonal wind and temperature gradient components also played a particularly significant role in cold anomalies in the mid-latitudes of Eurasia.The meridional temperature gradient was one aspect that affected the temperature drop in the mid-latitudes of Eurasia,and its value was smaller than the other contributions.Abnormal zonal and meridional temperature gradients and zonal wind drove the cooling effect over Europe,among which the zonal wind value was the strongest.The changes of zonal and meridional winds played a key role cooling Siberia,while the abnormal zonal temperature gradient mainly cooled down East Asia,which may have been related to the temperature differences enhanced between sea and land there.In general,the dominant temperature advection components varied among different regions.Compared to the cooling effects of temperature advections throughout the entire Eurasian continent,shortwave and longwave radiations only cooled the local areas where autumn and winter snow cover increased.