On the Genesis of the 2021 Atlantic Nino

An extreme Atlantic Nino developed in the boreal summer of 2021 with peak-season sea surface temperature anomalies exceeding 1 & DEG;C in the eastern equatorial region for the first time since global satellite measurements began in the early 1970s. Here, we show that the development of this outlier event was preconditioned by a series of oceanic Rossby waves that reflected at the South American coast into downwelling equatorial Kelvin waves. In early May, an intense week-long westerly wind burst (WWB) event, driven by the Madden-Julian Oscillation (MJO), developed in the western and central equatorial Atlantic and greatly amplified one of the reflected Kelvin waves, directly initiating the 2021 Atlantic Nino. MJO-driven WWBs are fundamental to the development of El Nino in the Pacific but are a previously unidentified driver for Atlantic Nino. Their importance for the 2021 event suggests that they may serve as a useful predictor/precursor for future Atlantic Nino events.