变化环境下澜湄流域骤旱演变特征及驱动机制研究

骤旱已成为影响社会经济和生态环境最严重的自然灾害之一，澜沧江-湄公河流域作为骤旱发生和影响的“热点”区域，揭示变化环境下骤旱时空演变规律及驱动机制，对于保障“一带一路”区域相关沿线国家的水安全、粮食安全、生态安全具有重大战略意义。本研究以澜湄流域为研究区，基于周尺度的标准化降水蒸散指数，采用统计分析、模型模拟等方法，剖析骤旱时空变化特征，归纳骤旱发生前期大气环流的演变过程，量化骤旱期间降水和大气蒸发需求的相对贡献，对比骤旱驱动机制的季节性差异，最后从水汽循环和输送异常的角度对澜湄流域典型骤旱事件开展归因分析。

近四十年来骤旱频率、强度及影响范围在时间和空间上均显著增加。基于自组织地图聚类分析，揭示了流域雨季和旱季严重骤旱发生前期海洋和大气变量异常的一致演变模式：西北太平洋海温迅速下降，东北印度洋海温快速上升，导致大气压分布异常，在澜湄流域上空形成高压中心；高压中心引起云量减少和辐射增加使蒸发加剧，并阻碍了来自海洋的暖湿水汽进入，最终引发骤旱。但驱动机制存在差异。雨季时，骤旱可能受到大尺度天气异常的影响，而旱季时，骤旱可能主要由当地气象条件决定。

降水不足是导致骤旱的主要因素，尤其在水资源可用性更高的雨季。然而，驱动因素的长期趋势存在季节性差异：雨季骤旱期间降水的相对贡献显著增加，水汽潜在来源分析表明，印度洋、孟加拉湾和太平洋的水汽潜在来源减少导致了雨季骤旱期间降水减少；另一方面，旱季骤旱期间大气蒸发需求的相对贡献显著增加，互信息和随机森林的结果共同表明水汽压差增加是导致旱季骤旱期间大气蒸发需求增加的主要因素。

2019年典型骤旱事件可归因于外部水汽供应不足。具体而言，后向轨迹模型表明印度洋、孟加拉湾和太平洋等主要水汽来源地输送到该地区的水汽减少。水汽输送分解表明此次典型骤旱事件中动力过程比热力学过程发挥了更重要的作用。通过将2019年的大气环流模式与气候学进行比较发现，孟加拉湾异常反气旋、东北印度洋异常西风以及西太平洋异常气旋等异常大气环流条件，共同抑制了海洋地区暖湿水汽输入。

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