Initiation of the Record-breaking March 2023 MJO Event: Implications for El Niño Onset and Prediction

The Madden–Julian oscillation (MJO) is the dominant intraseasonal wave phenomenon influencing extreme weather and climate worldwide. Realistic simulations and accurate predictions of MJO genesis are the cornerstones for successfully monitoring, forecasting, and managing meteorological disasters 3–4 weeks in advance. Nevertheless, the genesis processes and emerging precursor signals of an eastward-propagating MJO event remain largely uncertain. The year 2023 has witnessed the sequential genesis of a record-breaking Madden–Julian oscillation (MJO) and an unprecedented coastal El Niño in March–April, thus offering another opportunity to understand the dynamics of MJO–El Niño interactions. Here, we show that the March 2023 MJO is quite unusual as it starts from the South China Sea due to the dry intrusion of extratropical cold northerly winds and moist preconditioning effects of equatorial Rossby waves, propagates eastward fast as a double Kelvin wave system, and expands over the entire tropical Pacific largely as a Kelvin wave response to its strong suppressed convection over the Maritime Continent. Because of these unusual features, the MJO exerts widespread westerly wind forcing to the ocean surface, with two maxima over the western and far eastern tropical Pacific. Due mainly to the depressed local Ekman upwelling under MJO westerly, the upper ocean gets warmer than normal near the coast of South America, thereby helping trigger the 2023 coastal El Niño. Using an El Niño ensemble forecasting system, we quantify that the MJO westerly over the far eastern Pacific explains approximately 30% of coastal warming signals off Peru. Although only marginally increasing the end-of-year Niño-3.4 index, the March MJO can induce small-scale oceanic westward-propagating disturbances, which significantly decrease the intermember spread of the forecasted basin-scale 2023/24 El Niño. These results highlight the pivotal importance of tropical–extratropical interactions in initiating those MJOs from outside the Indian Ocean and also point out the potential roles of MJOs in dynamical El Niño evolution and prediction.