强El Niño衰减年东亚夏季风的季节内变化:1998年和2016年的对比分析

本文对比分析了1998年和2016年这两个强El Ni?o衰减年东亚夏季风的季节内变化。结果表明，在6~7月期间，由于热带印度洋海温偏高、对流偏强，造成西太平洋暖池对流偏弱，西太平洋副热带高压（副高）偏西偏强，长江流域降水偏多，华南偏少，东亚夏季风异常具有典型的El Ni?o衰减年特征。但两年的8月份有很大差异，虽然1998年8月与6~7月相似，但2016年8月份则完全不同。受乌拉尔地区异常反气旋的影响，源自西伯利亚东部的北风异常穿越东亚并直抵暖池地区，造成副高分裂并减弱东退，同时激发暖池对流发展，而对流的发展则进一步促使副高减弱。因此，2016年8月东亚夏季风异常与1998年8月相反，中国北方夏季降水异常也呈现很大差异。另外，1998年热带大西洋偏暖，并通过热带环流变化影响到东亚夏季风异常，其强迫作用与热带印度洋类似。而2016年大西洋海温异常较弱，对东亚夏季风影响也较弱。因此，El Ni?o对东亚夏季风的影响不仅与其强度有关，还与El Ni?o衰减之后造成的印度洋和大西洋海温异常有关。本文的分析结果表明，即使在强El Ni?o衰减年夏季，由于El Ni?o之间的个性差异以及其他因子的影响，东亚夏季风季节内变化仍然能呈现出显著差异，特别是在8月份。因此，在预测东亚夏季风异常时，宜将6~7月和8月分别考虑。此外，为进一步提高东亚夏季风预测水平，除传统的季度预测外，还需要进一步加强季节内尺度的预测。

Intraseasonal Variation of East Asian Summer Monsoon during Strong El Niño Decaying Summer: Comparison between 1998 and 2016

In this study, we compare the intraseasonal variation of East Asian summer monsoon (EASM) in the decaying summers of two strong El Niño years (i.e., 1998 and 2016). It is shown that during June and July, enhanced convection in the tropical Indian Ocean due to a higher sea surface temperature (SST) tends to suppress the warm pool convection in the western Pacific, resulting in more westward extension of the WPSH (the western Pacific subtropical high) with a strong intensity. There is more rainfall in the Yangtze River basin and less rainfall in southern China. This anomaly exhibits a typical feature during the decaying summer of an El Niño event. In August 2016, the EASM anomaly is totally different although the anomaly in August 1998 is similar to that in June-July 1998. Influenced by the anomalous anticyclone over the Ural region, the northerly anomaly from eastern Siberia reaches the warm pool region across East Asia, and the WPSH splits and retreats eastward. Meantime, the warm pool convection, triggered by the northern wind anomalies, begins to develop and leads to a further eastward retreat of the WPSH. Therefore, the EASM anomaly in August 2016 is contrary to that in August 1998, and rainfall in northern China also exhibits a significant discrepancy. In addition, the EASM anomaly in 1998 is influenced by the tropical Atlantic due to a higher SST through the tropical circulation, which is similar to the role in the tropical Indian Ocean. In 2016, however, the EASM is less influenced by the tropical Atlantic due to a weak SST anomaly. The influence of El Niño on the EASM is related with both the intensity and the SST anomaly in the Atlantic and Indian Ocean after the decay of an El Niño event. The result indicates that, due to different characteristics between El Niño events and the influence from some other factors, the EASM in strong El Niño decaying summers may also exhibit different intraseasonal variations especially in August. When predicting the EASM, we should consider separately the anomalies in June-July and August. In order to further promote the EASM forecast skill, we must pay more attention to the intraseasonal prediction besides the traditional seasonal prediction.