Title: The Record-breaking Mei-yu in 2020 and Associated Atmospheric Circulation and Tropical SST Anomalies
Abstract: The record-breaking mei-yu in the Yangtze-Huaihe River valley (YHRV) in 2020 was characterized by an early onset, a delayed retreat, a long duration, a wide meridional rainbelt, abundant precipitation, and frequent heavy rainstorm processes. It is noted that the East Asian monsoon circulation system presented a significant quasi-biweekly oscillation (QBWO) during the mei-yu season of 2020 that was associated with the onset and retreat of mei-yu, a northward shift and stagnation of the rainbelt, and the occurrence and persistence of heavy rainstorm processes. Correspondingly, during the mei-yu season, the monsoon circulation subsystems, including the western Pacific subtropical high (WPSH), the upper-level East Asian westerly jet, and the low-level southwesterly jet, experienced periodic oscillations linked with the QBWO. Most notably, the repeated establishment of a large southerly center, with relatively stable latitude, led to moisture convergence and ascent which was observed to develop repeatedly. This was accompanied by a long-term duration of the mei-yu rainfall in the YHRV and frequent occurrences of rainstorm processes. Moreover, two blocking highs were present in the middle to high latitudes over Eurasia, and a trough along the East Asian coast was also active, which allowed cold air intrusions to move southward through the northwestern and/or northeastern paths. The cold air frequently merged with the warm and moist air from the low latitudes resulting in low-level convergence over the YHRV. The persistent warming in the tropical Indian Ocean is found to be an important external contributor to an EAP/PJ-like teleconnection pattern over East Asia along with an intensified and southerly displaced WPSH, which was observed to be favorable for excessive rainfall over YHRV.2020年江淮流域梅雨开始早, 结束晚, 持续时间长, 雨带南北跨度大, 降水量大, 暴雨过程多, 是一次破纪录的梅雨事件. 分析发现, 东亚季风环流系统表现出明显的准双周振荡 (QBWO), 它与梅雨的开始和结束、 雨带的北移和停滞、 强降水过程的发生和维持有很好的对应关系. 梅雨期包括西太平洋副热带高压、 高层东亚西风急流和低空西南风急流在内的多个季风环流子系统均经历了与 QBWO 相关的周期性振荡, 尤其是低空西南急流的不断加强, 南风大值中心反复建立和位置的相对稳定, 使得源自热带的水汽输送一次次加强, 水汽辐合与上升运动反复发展, 从而导致梅雨在江淮流域长时间持续, 暴雨过程频频发生. 另一方面, 梅雨期欧亚中高纬度地区环流呈现出 “两脊一槽” 型, 阻塞高压活动频繁, 东亚沿岸低槽活跃, 经西北和/或东北路径的冷空气不断南侵我国, 与低层一次次加强的西南暖湿水汽在江淮区域频繁交汇, 这是造成今年梅雨异常偏强的另一重要因素. 作为一个重要的外强迫因子, 热带印度洋的持续增暖使得东亚地区环流呈现出 EAP/PJ 型遥相关型, 对应西太副高偏强偏南, 从而有利于长江中下游地区降水明显偏多.