Diverse NPMM Conditions Setting the 2023/24 El Niño Apart from the 1997/98 and 2015/16 Extreme Events

The 2023/24 El Niño began with an unusually warm water volume in the tropical western Pacific, comparable to the extreme 1997/98 and 2015/16 events. However, it did not intensify into an extreme El Niño. In this study, we demonstrate that contrasting Northern Pacific Meridional Mode (NPMM) conditions were key to this divergence. During the 1997/98 and 2015/16 El Niños, a warm NPMM facilitated the propagation of the Madden-Julian Oscillation (MJO) into the tropical Central Pacific, establishing a positive zonal sea surface temperature (SST) gradient in the tropical Western-Central Pacific. This environment generated strong westerly wind bursts (WWBs), driving both events into super El Niños. In contrast, the cold NPMM during the 2023/24 El Niño created a negative zonal SST gradient, hindering MJO propagation and leading to weaker WWBs. The Central Pacific WWB in 2023/24 was instead driven by a coastal El Niño, influenced by the cold NPMM. The lack of strong MJO activity further prevented the 2023/24 event from becoming extreme. Our findings highlight the importance of considering NPMM interference when using equatorial Pacific warm water volume to predict El Niño intensity, offering potential improvements in future El Niño-Southern Oscillation (ENSO) forecasts.