Variable and robust East Asian monsoon rainfall response to El Niño over the past 60 years (1957–2016)
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Severe flooding occurred in southern and northern China during the summer of 2016 when the 2015 super El Niño decayed to a normal condition. However, the mean precipitation during summer (June–July-August) 2016 does not show significant anomalies, suggesting that—over East Asia (EA)—seasonal mean anomalies have limited value in representing hydrological hazards. Scrutinizing season-evolving precipitation anomalies associated with 16 El Niño episodes during 1957–2016 reveals that, over EA, the spatiotemporal patterns among the four categories of El Niño events are quite variable, due to a large range of variability in the intensity and evolution of El Niño events and remarkable subseasonal migration of the rainfall anomalies. The only robust seasonal signal is the dry anomalies over central North China during the El Niño developing summer. Distinguishing strong and weak El Niño impacts is important. Only strong El Niño events can persistently enhance EA subtropical frontal precipitation from the peak season of El Niño to the ensuing summer, by stimulating intense interaction between the anomalous western Pacific anticyclone (WPAC) and underlying dipolar sea surface temperature anomalies in the Indo-Pacific warm pool, thereby maintaining the WPAC and leading to a prolonged El Niño impact on EA. A weak El Niño may also enhance the post-El Niño summer rainfall over EA, but through a different physical process: the WPAC re-emerges as a forced response to the rapid cooling in the eastern Pacific. The results suggest that the skillful prediction of rainfall over continental EA requires the accurate prediction of not only the strength and evolution of El Niño, but also the subseasonal migration of EA rainfall anomalies.
KeywordsEl Niño impact monsoon rainfall East Asian monsoon Asian monsoon precipitation variability monsoon–ocean interaction western Pacific subtropical high
2016年夏季, 当2015 超强厄尔尼诺事件减弱到正常状态时, 中国南北方均发生了严重的洪涝灾害. 然而, 2016年夏季(六-八月)平均降水并没有呈现明显异常.这表明东亚夏季平均降水对于洪涝灾害的表征具有局限性.通过进一步分析最近60年间16次厄尔尼诺事件, 我们发现四种强度不同类型的厄尔尼诺事件引起的东亚降水异常的空间分布与季节演变相差甚大, 导致共同信号不显著, 给预报带来困难. 这是由于厄尔尼诺的强度和演变过程差异显著, 加之东亚夏季降水异常所独具的季节内北进使得夏季平均降水量失去代表性.唯一共同显著的信号表现在厄尔尼诺发展阶段的夏季中国北方中部地区的降水偏少. 在厄尔尼诺衰退的夏季, 区分强弱厄尔尼诺事件对于东亚降水的影响非常重要. 只有强厄尔尼诺事件能激发异常西太反气旋, 它通过与印度-西太暖池异常海温的相互作用得以维持, 增强自厄尔尼诺冬季至次年夏季东亚副热带锋区降水.弱厄尔尼诺事件也会加强东亚次年夏季降水, 但其物理过程不同, 那是由于东太平洋迅速降温而导致西太异常反气旋重新生成.结果表明, 准确的东亚夏季陆地降水预报要求动力模式不仅能对厄尔尼诺强度和演变做出准确预报, 还需要模式有能力预报出东亚异常雨带的季节内北进.
关键词厄尔尼诺影响 季风降水 东亚季风 亚洲季风 降水变率 季风海洋相互作用 西太平洋副热带高压
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This study was supported by the National Natural Science Foundation of China (Grant No. 41420104002) and the National Research Foundation of Korea through a Global Research Laboratory grant of the Korean Ministry of Education, Science and Technology (Grant No. 2011-0021927) and the Atmosphere–Ocean Research Center (AORC). The AORC is partially funded by Nanjing University of Information Science and Technology (NUIST). This paper is NUIST-Earth System Modeling Center publication number 163, School of Ocean and Earth Science and Technology publication number 10027, and International Pacific Research Center publication number 1259.
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