Suppressive MJO in April 2014 Downgraded the 2014/15 El Niño

Abstract The sudden halting of the extreme 2014/15 El Niño expected by many was attributed to the absence of westerly wind bursts (WWBs) in late spring and early summer 2014 in previous works, yet the cause of the lack of WWBs was overlooked. Using the ERA5 reanalysis and IBTrACS dataset, as well as a set of coupled model experiments, we showed that the absence of WWBs in May efficiently downgraded the intensity of the 2014/15 El Niño from a moderate to a weak event and was closely associated with a strong suppressive MJO originating from the central tropical Indian Ocean in mid-April 2014. The suppressive MJO underwent two pathways once passing through the Maritime Continent in early May. Along the eastward pathway, the strong suppressive MJO prevailed over the western–central equatorial Pacific, directly prohibiting the occurrence of WWBs at the equator via inducing equatorial easterly anomaly. Along the northeastward pathway, the downward motions with relative dry air and strong vertical zonal wind shear associated with the suppressive MJO suppressed the activity of the tropical cyclones in the northwestern tropical Pacific, another source of WWBs. Our results indicate that the contributions of MJO to the development of El Niño from both the direct and indirect ways should be taken into account for improving El Niño prediction.