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The termination of the superintense 1997/1998 El Nino was extraordinarily abrupt. The May 1998 Madden-Julian Oscillation (MJO), a massive complex of stormy tropical clouds, is among possible contributors to the abrupt termination. Despite having been sensationally proposed 18 years ago, the role of the MJO remained controversial and speculative because of the difficulty of sufficiently simulating the El Nino and MJO simultaneously. An ensemble simulation series using a newly developed, fully ocean-coupled version of a global cloud system resolving numerical model replicated the specific atmosphere and ocean conditions of May 1998 in unprecedented detail, extending the prediction skill of the MJO to 46 days. Simulation ensemble members with stronger MJO activities over the Maritime Continent experienced quicker sea surface temperature drop in the eastern Pacific, confirming that the easterly winds associated with the remote MJO accelerated ocean upwelling to abruptly terminate the El Nino.

Plain Language Summary The ocean and atmosphere conditions of a extraordinarily abrupt termination of one of the most intense El Nino event ever recorded are replicated in unprecedented detail by using our newly developed global high-resolution atmosphere-ocean numerical model on one of the top-tier super-computers. Results show that a huge envelope of stormy activity centered over the Indonesian archipelago remotely enhanced easterly winds over the eastern Pacific ocean, which drove upward motion of cold water to the surface to suddenly end the El Nino condition. The newly developed model may improve predictions of weather tendencies and tropical cyclone probabilities at the sub-seasonal to seasonal time-range.