Leading Pattern of Spring Drought Variability over East Asia and Associated Drivers

Drought events have become more frequent and intense over East Asia in recent decades, leading to hugesocioeconomic impacts. Although the droughts have been studied extensively by cases or for individual regions, theirleading variability and associated causes remain unclear. Based on the Standardized Precipitation Evapotranspiration Index(SPEI) and ERA5 reanalysis product from 1979 to 2020, this study evealuates the severity of spring droughts in East Asiaand investigates their variations and associated drivers. The results indicate that North China and Mongolia have ex perienced remarkable trends toward dryness during spring in recent decades, while southwestern China has witnessed anopposite trend toward wetness. The first Empirical Orthogonal Function mode of SPEI variability reveals a similarseesawing pattern, with more severe dryness in northwestern China, Mongolia, North China, South Korea, and Japan butincreased wetness in Southwestern China and southeast Asia. Further investigation reveals that the anomalously dry (wet)surface in North (Southwestern) China is significantly associated with anomalously high (low) temperature, less (more)precipitation, and reduced (increased) soil moisture during the previous winter and early spring, regulated by an anomalousanticyclone (cyclone) and thus reduced (increased) water vapor convergence. The spring dry-wet pattern in East Asia isalso linked to cold sea surface temperature anomalies in the central-eastern Pacific. The findings of this study haveimportant implications for improving the prediction of spring drought events in East Asia.     

热带平流层与对流层之间相互作用的研究进展

以热带平流层和对流层大气中的准两年振荡和ENSO现象,以及热带平流层和对流层交换（STE）过程为主轴,介绍了热带平流层和对流层大气相互作用方面的一些研究进展。众多的研究结果表明,平流层准两年振荡（QBO）和对流层准两年振荡（TBO）存在潜在联系,但二者之间相互作用的物理机制还不是很清晰。QBO对平流层大气中化学成分的空间分布具有显著的影响,但其中的化学-辐射-动力反馈作用目前还未得到充分的研究。以往的研究提供了ENSO影响热带平流层大气令人信服的证据,但影响的程度和范围还未形成一个共识,其中一个原因是平流层内的ENSO信号受到火山喷发和太阳活动的干扰。一些研究指出ENSO事件通过改变行星波的强度和传播对极地平流层大气产生影响,而平流层过程也会对ENSO的发生、发展和演变产生影响,但其可靠性需要进一步的确证。目前,一个较一致的结论是全球变暖会导致热带上涌增强,STE加快。然而,影响热带STE的大气过程较多,特别是季风系统和热带深对流系统对STE的贡献还需要深入的分析研究。      

青藏高原沙尘示踪物从对流层向平流层传输的数值模拟

利用NCEP再分析资料和卫星观测资料,结合耦合了沙尘模块的中尺度模式WRF,通过个例分析研究了青藏高原及附近地区沙尘气溶胶从近地面向对流层上部和平流层下部传输的特征和机制以及青藏高原大地形对平流层与对流层之间物质交换的影响。结果表明,深对流活动可将近地面沙尘气溶胶传输到上对流层—下平流层区域,但是下平流层区域的沙尘气溶胶浓度分布依赖于地面沙尘源的位置和对流的强度,且与对流系统内是否有降水有关。在没有穿透性对流情况下,垂直上升运动不能直接将沙尘输送到下平流层,但上对流层的沙尘可通过扩散作用和小尺度的混合过程经过数小时缓慢地进入下平流层。在没有明显系统性降水的情况下,夏季青藏高原上空旺盛的对流活动和高地形使得高原上空成为气溶胶进入下平流层的主要区域。上对流层区域的沙尘气溶胶浓度还受到平流层空气入侵的影响,在没有强的地面沙尘排放源的情况下,平流层空气的入侵对上对流层区域气溶胶浓度的分布和演变有较大的影响。