北半球冬季极端低温事件变化及其与秋季海冰的联系

Variation of the Winter Extreme Cold Events in the Northern Hemisphere and its Relationship with Arctic Sea Ice in Autumn

利用欧洲中期天气预报中心逐日的地面气温数据以及美国国家冰雪数据中心逐月的海冰密集度数据,并通过气候百分位法统计了逐月的极端低温事件日数,分析了1979-2016年冬季(12月至次年2月)北半球的极端低温事件与秋季(9-11月)北极海冰变化之间的关系,并挑选了典型年份探讨了秋季北极海冰对极端低温事件可能存在的影响机理。结果表明,北半球冬季极端低温事件与秋季关键海区(波弗特海、楚科奇海、巴伦支海和喀拉海)的海冰面积变化密切相关,其中亚洲北部区域冬季的极端低温事件与秋季关键海区的海冰面积有显著负相关关系,而青藏高原区域、非洲北部区域和北美东北部区域冬季的极端低温事件与秋季关键海区的海冰面积有显著正相关关系。通过对典型年份冬季北半球极端低温事件极度异常时的大气环流进行分析后发现,当秋季关键海区海冰异常偏少时,冬季西伯利亚高压偏强,其东部偏北风加强,使得欧亚大陆北部的气温偏低,极端低温事件偏多;冬季冰岛低压偏强,其南部的西南风加强,将低纬度温暖的空气带到了非洲北部区域,使其冬季气温偏高,极端低温事件偏少;高层异常反气旋主要位于格陵兰岛附近,其下沉气流不利于形成极端低温天气而导致北美东北部区域极端低温事件偏少。

Based on daily surface temperature from European Centre for Medium-Range Weather Forecasts and monthly sea ice concentration from National Snow and Ice Data Center,the days of monthly extreme cold events were calculated by the climate percentile method,and then the correlation of winter(November to subsequent February) extreme cold events in the northern hemisphere and Arctic sea ice in autumn(form September to November) during 1979-2016 was analyzed.The possible mechanism of the influence of autumn Arctic sea ice on extreme cold events is discussed through the selected typical years.The results show that the variations of autumn sea ice area in the key region(Beaufort、Chukchi、Barents and Kara sea) has a significant relation to winter extreme cold events in the northern hemisphere.The events in northern Asia have a significant negative correlation with autumn sea ice area in the key region,while Qinghai-Tibetan Plateau、northern Africa and northeast North America have a positive correlation with autumn sea ice area in the key region.Through the analysis of the atmospheric circulation in the winter of typical example,it is found that when the sea ice in the key region is abnormally less in the autumn,the Siberian high pressure and the north winds of its east is strong in winter,resulting in low temperatures and more extreme cold events in northern Eurasia;The Icelandic low pressure is strong in winter and the abnormally southwest winds of its south bring warm air from low latitudes to the northern Africa,resulting in high temperatures and fewer extreme cold events in northern Africa;An abnormal anticyclone is mainly located near Greenland,and its downdraft is not conducive to the formation of extreme cold weather,resulting in relatively few extreme cold events in northeast North America.