Impacts of Tropical Cyclones and MJO on El Niño Evolution: Revisiting the Formation Mechanism of the 1986/87~1987/88 Two-year El Niño

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 1986/87 boreal winter, unexpectedly persisted into the following year, generating a rare two-year consecutive EN event. However, the 1986/87~1987/88 two-year EN event receives few attention. This study reveals that this two-year EN event encompasses some critical yet commonly overlooked processes for the formation and development of 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 re-ignition 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 (WNPAC) and the associated easterly wind anomalies over WEP that were anticipated during the 1986/87 winter. Such unexpected westerly wind anomalies helped maintain the EN warming through December 1986 to 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 two-year EN event can deepen our understanding of ENSO dynamics and provide implications for enhancing the accuracy of EN prediction.