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.