Joint effects of three oceans on the 2020 super mei‐yu

Abstract

An unexpected super mei-yu struck in 2020 in the Yangtze–Huaihe River basin, southern Korea, and southern Japan (hereafter referred to as the mei-yu regions), causing many casualties and huge economic losses. The super mei-yu was characterized by a remarkably early onset (around 1 June), late withdrawal (around 1 August), and intense rainfall during the mei-yu season. The precipitation in the early onset and late withdrawal stages contributed more than half of the total mei-yu-period precipitation over the mei-yu regions in 2020. In this study, the authors explored the dominant remote forcing of the mei-yu early onset and late withdrawal to understand the mechanisms of this super mei-yu. The early onset can mainly be attributed to an early northward-shifted East Asian jet stream (EAJS). The late withdrawal mainly resulted from the stagnant EAJS and the western North Pacific subtropical high (WPSH) during 10 July to 1 August. Specifically, North Atlantic sea surface temperature anomalies (SSTAs) excited a Rossby wave, which was steered by atmospheric anomalies related to the western North Pacific SSTAs, causing the early northward-shifted EAJS and generating an early onset. The record-breaking warm SSTAs over the North Indian Ocean to South China Sea and the reduced sea-ice concentration (SIC) over the Laptev–East Siberian Sea played important roles in causing the stagnant WPSH and EAJS during July, which led to the late withdrawal. Meanwhile, the SIC anomalies may have caused the inhomogeneous rainfall distribution in the mei-yu regions. Furthermore, projection results suggest that the probability of a late mei-yu withdrawal similar to the 2020 case will increase in the future. Finally, potential predictors of an extreme mei-yu are discussed.摘要2020年发生在江淮流域, 朝鲜半岛和日本南部 (简称梅雨区) 的暴力梅造成了巨大的人员伤亡和经济损失. 此次暴力梅的主要特征为: 入梅早 (6月1日) , 出梅晚 (8月1日) 以及较强的梅雨期降水. 2020年异常早入梅和晚出梅时期的降水占梅雨期总降水的一半以上. 因此, 为了深入解析2020暴力梅的机制, 本文将分析2020异常早入梅和晚出梅的主导遥强迫机制. 研究表明, 东亚急流提前北跳是造成入梅偏早的主要原因, 而西北太平洋副热带高压 (西太副高) 的作用并不显著. 具体来说, 5月北大西洋海表温度 (SST) 异常激发出东传的罗斯贝波, 同时西北太平洋的暖SST异常改变该罗斯贝波的传播. 进而在两者共同的作用下造成东亚急流的提前北跳, 并导致入梅偏早. 2020年7月破纪录的北印度洋-南海SST暖异常和拉普特夫-东西伯利亚海的海冰密集度 (SIC) 的偏少阻止西太副高和东亚急流的北跳.推迟北跳的西太副高和东亚急流有利于梅雨降水增多, 最终导致出梅偏晚. 同时SIC异常可能是梅雨雨带偏北的原因. 此外, 预估结果表明, 类似于2020出梅偏晚的概率在未来将会增加. 最后, 本文也探讨了暴力梅的潜在预测因子.