2015/2016冬季北极世纪之暖与超级厄尔尼诺对东亚气候异常的影响

Impact of Arctic warming and the super El Nio in winter 2015/2016 on the East Asian climate anomaly

利用1980—2016年美国国家海洋与大气管理局气候预测中心的ENSO指数和NCEP/NCAR再分析资料,研究了2015/2016年冬季北极增暖和超级厄尔尼诺对东亚气候的影响。2015/2016年冬季热带中东太平洋爆发了超级厄尔尼诺事件,尽管大气环流出现了对ENSO的响应特征（如西北太平洋反气旋异常,东亚南部南风异常）,但东亚（尤其是我国东北、华北地区）1月的气温却明显偏低。分析表明,此次东亚气温偏低现象可能与2016年1月北极显著增暖有关。1980—2016年1月再分析资料的统计诊断分析结果显示,巴伦支海—喀拉海气温的升高会引起局地大气的上升运动异常,之后在下游（70-90°E附近）向南运动,并在西伯利亚地区（60-100°E,50-70°N）下沉,使得西伯利亚高压增强,其东侧的北风异常导致东亚气温偏低。基于Nino3.4指数、北极温度指数,采用多元线性拟合所得到的2016年1月东亚气温的回报结果与观测气温之间的空间系数为0.71,表明2016年1月北极增暖以及热带中东太平洋的厄尔尼诺事件能够从一定程度上解释东亚气温偏低的现象。

The influence of El Nino on the East Asian climate has been studied extensively. Generally,East Asia is warmer than normal in winter when El Nino occurs. During winter 2015/2016,an extreme El Nino event took place—one that was almost the strongest since records began. However,the winter of 2015/2016 was not much warmer than usual. By contrast,East Asia was even colder than normal in January 2016. At the same time,the Arctic experienced extreme warming in winter 2015/2016,which might have had an influence on the winter climate of East Asia. Therefore,in this study,the influences of Arctic warming and the super El Nino during winter2015/2016 on the East Asian winter climate was investigated using the ENSO index of the Climate Prediction Center of NOAA,along with NCEP/ NCAR reanalysis data,for the period 1980—2016. In winter 2015/2016,a super El Nino emerged in the central tropical Pacific. In January 2016,there was a dominant anomalous anticyclone over the western North Pacific and an anomalous cyclone over the Northeast Pacific. M eanwhile,a southerly anomaly was observed over East Asia,which was confined to the area south of 30°N. This meant that there was a particularly evident response of atmospheric circulation in January 2016 to the forcing of ENSO. However,significantly warmer conditions were not observed over East Asia as expected. On the contrary,it was much colder than normal in January. It is interesting to find that the northerly wind in the domain（ 30—50° N,110—125° E） was stronger in January 2016,which could explain the colder conditions over East Asia. The Rossby wave activity in January 2016 indicates a wave source over the Barents-Kara seas,suggesting that the colder conditions over East Asia might have been related to the Arctic warming that occurred in January 2016. To address this issue,an Arctictemperature index was defined as the area-weighted mean in the Barents-Kara seas（ 70—80° N,30—70° E）. The statistical results from the reanalysis data during 1980—2016 revealed that the warming in the Barents-Kara seas was concurrent with a significant negative temperature anomaly over East Asia. The teleconnection between the Arctic and East Asian temperature was well supported by an obvious southeastward propagation of Rossby waves.Further analysis suggested that the warming in the Barents-Kara seas could induce anomalous ascending motion in situ. The anomalous ascending motion extended eastwards（ around 70—90°E） and turned southwards due to the Coriolis force,and finally descended in Siberia（ 50—70°N,60—100°E）. As the air mass accumulated over Siberia,the Siberian high was intensified,leading to anomalous northerly flowin its east,which favored the cold conditions over East Asia. To verify the combined effect of the Arctic warming and El Nino,a statistical prediction model was established via the multivariate linear regression of surface air temperature（ SAT） onto the Arctic temperature index and Nino3. 4 index. It was found that the hindcasted SAT anomaly over East Asia in January 2016 resembled the observed counterpart well,with a spatial correlation coefficient of 0. 71. This implied that the Arctic warming and super El Nino during winter 2015/2016 can to some extent explain the colder than expected conditions in East Asia.