中国东北极端持续大—暴雪事件的个例成因及可预测性

Causes and prediction skill of a case study on persistent heavy snowfall events in Northeast China

2013年11月东北大—暴雪持续日数为1982—2020年同期最多的一年。其中,2013年11月17—20日和25日先后发生两次强降雪过程,其中第一次过程降雪持续时间较长,而第二次过程日降雪强度强。基于此,从2013年11月月际异常气候背景和两次强降雪过程的角度,开展其成因和可预测性研究。研究结果表明,2013年11月北极涛动(AO)正位相异常偏强、类北太平洋涛动(NPO)负位相、巴伦支海以北的海冰月增长量(11月减9月)异常偏多和热带-南印度洋海温异常偏暖的气候背景有利于这次东北持续性大—暴雪事件的发生。其中,2013年11月巴伦支海以北的海冰月增长量偏多,意味着季节性海冰生长量增加使得向大气中释放的潜热通量增加,气温偏高,有利于AO正位相加强并激发罗斯贝(Rossby)波列,使得阿留申低压减弱;同时11月热带-南印度洋海温异常偏暖,热带印度洋对流加强和热带西太平洋对流减弱,有利于西北太平洋-阿留申地区维持“气旋-反气旋”式环流异常,呈现类NPO负位相。这样的环流形势有利于从北太平洋向东北地区持续输送水汽。第一次强降雪过程(17—20日)发生前5 d(12—16日)到降雪过程结束(20日),北大西洋涛动(NAO)维持正位相并且强度达到冬半年最强,由此激发持续东传Rossby波,使得西北太平洋-阿留申地区为持续性的南北向“气旋-反气旋”式环流异常,有利于北太平洋水汽持续输送至东北地区。在2013年11月25日第二次强降雪过程中,乌拉尔山阻塞高压显著加强,东北低涡加深,有利于热带西太平洋更为暖湿的水汽输送至东北地区,与北太平洋输送的水汽共同导致第二次强降雪过程的日降雪强度大。最后,利用CFSv2评估2013年11月异常气候背景的可预测性,结果表明CFSv2可提前1个月预测2013年11月热带-南印度洋海温异常偏暖,但对热带印度洋和西太平洋对流异常、NAO以及热带-中高纬大气遥相关预测能力较弱。在次季节尺度上,ECMWF(CMA)能提前29(12)d和13(16)d合理预测出两次过程降雪量的空间分布,这可能是由于模式能合理再现NAO和乌拉尔山阻塞高压等关键环流系统的逐日变化。因此未来还有待提升热带-中高纬大气遥相关、水汽输送以及平流层极涡的次季节-季节预测效能。

Northeast China experienced an unprecedented sequence of continuous heavy snowfall days in November 2013,the most significant event since 1982-2020.This event was characterized by two intense snowfall processes occurring from the 17th to the 20th and the 25th.The first process had a longer duration,while the second exhibited greater snowfall intensity.This study investigates the causes and predictability of these events from the perspective of the anomalous climatic background of November 2013 and the detailed dynamics of the two intense snowfall processes.The results show that the positive phase of the Arctic Oscillation(AO),the negative phase of the North Pacific Oscillation-like(NPO-like)pattern,increased sea ice growth north of the Barents Sea(November compared to September),and anomalously warm sea surface temperatures in the tropical-southern Indian Ocean during November were responsible for the persistent heavy snowfall event in Northeast China.The increased sea ice growth suggested a heightened release of latent heat flux into the atmosphere,resulting in higher temperatures that favored the strengthening of the positive phase of the AO and the excitation of Rossby wave trains,subsequently weakening the Aleutian low.Moreover,anomalously warm sea surface temperatures facilitated enhanced convection over the tropical Indian Ocean and weakened convection over the tropical western Pacific,leading to a‘cyclone-anticyclone’circulation anomaly in the Northwest Pacific-Aleutian region,presenting a negative phase of the NPO-like pattern.These atmospheric circulations favored the transport of water vapor from the North Pacific.Additionally,the causes of the two intense snowfalls were analyzed.From the 12th to the 16th,in the five days preceding the first intense snowfall(17th to 20th),the North Atlantic Oscillation(NAO)maintained a continuous positive phase,triggering the persistent eastward propagation of Rossby wave trains.This condition corresponded to a meridional‘cyclone-anticyclone’anomaly over the Northwest Pacific-Aleutian region,facilitating continuous moisture transport from the North Pacific to Northeast China.During the second intense snowfall on November 25,2013,the Ural blocking high intensified significantly,and the deepening of the Northeast low vortex promoted the transport of warmer and moister air from the tropical western Pacific to the Northeast region,augmenting daily snowfall intensity.Finally,the prediction skill of the anomalous climatic background in November 2013 was evaluated using CFSv2.While CFSv2 effectively predicted anomalously warm sea surface temperatures in the tropical-southern Indian Ocean one month in advance,its skill for predicting anomalous convection in the tropical Indian Ocean and western Pacific,the NAO,and tropical-midlatitude teleconnections was relatively limited.On the subseasonal scale,ECMWF(CMA)reasonably predicted spatial distributions of snowfall for two processes 29(12)and 13(16)days in advance,respectively.This prediction skill could be attributed to better anticipation of the daily variations of key circulation systems such as the NAO and the Ural blocking high.Therefore,future efforts should concentrate on enhancing the subseasonal-to-seasonal predictive skills of tropical-midlatitude teleconnections,moisture transport,and the stratospheric polar vortex.