Meng-Er Song, Lin Chen, Pengfei Lin, Jiuwei Zhao, Leishan Jiang, Lu Wang, Hai Zhi
Abstract Canonical El Niño (EN) events typically peak in boreal winter and then rapidly decay in the ensuing months, transitioning to a La Niña (LN) event or a neutral condition by the following winter. Strikingly, an EN event that peaked in the 1986/87 boreal winter unexpectedly persisted into the following year, generating a rare 2-yr consecutive EN event. However, the 1986/87–1987/88 2-yr EN event receives little attention. This study reveals that this 2-yr EN event encompasses some critical yet commonly overlooked processes for the formation and development of an EN event. Specifically, the high-frequency (HF) westerly wind anomalies, induced by the tropical cyclones (TCs) and Madden–Julian oscillation (MJO) events, were the pivotal drivers of the unexpected reignition in the second year. During the 1986/87 winter, the unexpected emergence of four TCs induced vigorous westerly wind anomalies over the western equatorial Pacific (WEP), disrupting the anomalous anticyclone circulation over the western North Pacific anticyclone (WNPAC) and the associated easterly wind anomalies over the WEP that were anticipated during the 1986/87 winter. Such unexpected westerly wind anomalies helped maintain the EN warming through December 1986–February 1987. Subsequently, a series of HF westerly wind anomalies, induced by TCs and MJO events in April, May, and July 1987, reinvigorated the waning warming, pulling it back into a fledged EN event by the end of 1987. Gaining insights into the formation mechanism behind this unique 2-yr EN event can deepen our understanding of El Niño–Southern Oscillation (ENSO) dynamics and provide implications for enhancing the accuracy of EN prediction. Significance Statement Considering the critical impacts of multiyear El Niño–Southern Oscillation (ENSO) events on global weather and climate, understanding the formation dynamics of multiyear ENSO events is vital for improving the accuracy of ENSO prediction. Canonical El Niño (EN) events typically peak in boreal winter and then decay rapidly; however, the 1986/87 EN event persisted unexpectedly for two consecutive years, unlike typical single-year EN events. A revisit of 1986/87–1987/88 2-yr EN event reveals critical yet commonly overlooked processes in EN formation and development. Observational analyses reveal that the high-frequency westerly wind anomalies induced by the TCs and MJOs are crucial to the formation of this 2-yr consecutive EN event. Understanding the mechanism of this unique 2-yr EN event can enhance our knowledge of ENSO dynamics.