This study aims to explore the contrasting characteristics of large-scale circulation that led to the precipitation anomalies over the northern parts of Sumatra Island. Further, the impact of varying the Asian–Australian Monsoon (AAM) was investigated for triggering the precipitation variability over the study area. The moisture budget analysis was applied to quantify the most dominant component that induces precipitation variability during the JJA (June, July, and August) period. Then, the composite analysis and statistical approach were applied to confirm the result of the moisture budget. Using the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Anaysis Interim (ERA-Interim) from 1981 to 2016, we identified 9 (nine) dry and 6 (six) wet years based on precipitation anomalies, respectively. The dry years (wet years) anomalies over the study area were mostly supported by downward (upward) vertical velocity anomaly instead of other variables such as specific humidity, horizontal velocity, and evaporation. In the dry years (wet years), there is a strengthening (weakening) of the descent motion, which triggers a reduction (increase) of convection over the study area. The overall downward (upward) motion of westerly (easterly) winds appears to suppress (support) the convection and lead to negative (positive) precipitation anomaly in the whole region but with the largest anomaly over northern parts of Sumatra. The AAM variability proven has a significant role in the precipitation variability over the study area. A teleconnection between the AAM and other global circulations implies the precipitation variability over the northern part of Sumatra Island as a regional phenomenon. The large-scale tropical circulation is possibly related to the PWC modulation (Pacific Walker Circulation).

中文翻译：

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苏门答腊岛北部夏季夏季大尺度环流影响年际降水变化的特征

本研究旨在探讨导致苏门答腊岛北部地区降水异常的大规模环流的对比特征。此外，研究了亚洲-澳大利亚季风（AAM）变化对触发研究区域降水变化的影响。应用水分预算分析来量化在JJA（六月，七月和八月）期间引起降水变化的最主要成分。然后，采用综合分析和统计方法确定水分收支的结果。使用1981年至2016年欧洲中距离天气预报中心（ECMWF）的再分析中期（ERA-Interim），我们分别根据降水异常确定了9（九）个干燥年和6（六个）湿年。研究区域的干年（湿年）异常主要是由向下（向上）的垂直速度异常支持的，而不是其他变量（例如比湿度，水平速度和蒸发量）支持的。在干旱年（湿年），下降运动会增强（减弱），从而触发研究区域对流的减少（增加）。西风（东风）的整体向下（向上）运动似乎抑制（支持）对流，并导致整个区域出现负（正）降水异常，但在苏门答腊北部地区异常最大。事实证明，AAM变异性在研究区域内的降水变异性中具有重要作用。AAM与其他全球环流之间的遥相关意味着苏门答腊岛北部地区的降水变化是一种区域现象。大规模的热带环流可能与PWC调制（太平洋沃克环流）有关。