The Greater Mekong Subregion (GMS) is one of the world's most important agricultural regions. Over recent decades, the declining trend in precipitation has caused more frequent droughts over the northern plateau of the GMS, and this has led to a significant reduction in agricultural productivity. These drought events can also affect agriculture within the middle and lower parts of the Mekong River basin. To reveal the main causes of the decline in precipitation, this study investigates interdecadal variations in summer precipitation over the northern GMS during 1979–2022. Results indicate that summer conditions over the northern GMS entered a relatively dry period after the late 1990s. The interdecadal decrease in summer precipitation is closely associated with the warming of the northern Arabian Sea (AS), which induces a cyclonic anomaly that enhances local precipitation and associated diabatic heating. The anomalous diabatic heating induces a downstream Rossby wave train that intensifies the circumglobal teleconnection (CGT) pattern and causes the South Asian high (SAH) to extend farther southeastwards. The southeastward-displaced SAH induces easterly anomalies over the northern Bay of Bengal (BOB) that weaken the Indian summer monsoon (ISM) southwesterly flows and reduce moisture transport from the northern BOB to the northern GMS. The southeastward displacement of the SAH also causes anomalous descent over the northern GMS. Both conditions result in reduced precipitation over the northern GMS. Numerical experiments substantiate the proposed modulating role of AS warming in the interdecadal decrease in summer precipitation over the northern GMS.

中文翻译：

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阿拉伯海变暖对北部大湄公河次区域夏季降水减少的贡献


大湄公河次区域 （GMS） 是世界上最重要的农业区域之一。近几十年来，降水减少的趋势导致 GMS 北部高原的干旱更加频繁，这导致农业生产力显着下降。这些干旱事件还会影响湄公河流域中下游的农业。为了揭示降水减少的主要原因，本研究调查了 1979 年至 2022 年大湄公河次区域北部夏季降水的年代际变化。结果表明，1990 年代后期后，大湄公河次区域北部的夏季条件进入了相对干燥的时期。夏季降水的年代际减少与阿拉伯海北部 （AS） 的变暖密切相关，阿拉伯海北部 （AS） 的变暖引发了气旋异常，从而增强了局部降水和相关的非绝热加热。异常的非绝热加热诱发了下游的罗斯比波列，加强了环球遥相关 （CGT） 模式，并导致南亚高压 （SAH） 向东南方向延伸得更远。向东南移动的 SAH 在孟加拉湾北部 （BOB） 上引起东风异常，从而减弱了印度夏季季风 （ISM） 的西南气流，并减少了从北部 BOB 到北部 GMS 的水分输送。南南热源向东南的位移也导致了 GMS 北部的异常下降。这两种情况都导致 GMS 北部的降水减少。数值实验证实了 AS 变暖在大河次区域北部夏季降水年代际减少中的调节作用。