Extreme El Niño events versus ENSO diversity

Over the past two decades, significant attention has been given to ENSO (El Niño Southern Oscillation) diversity, categorizing El Niño events as either central Pacific (CP) or eastern Pacific (EP) based on their dynamics, sea surface temperature (SST) and rainfall patterns, and global teleconnections. EP events include both moderate events and rare extreme events, such as those that peaked in December 1972, 1982, 1997, and 2015. Here, we will demonstrate that CP El Niño and moderate EP El Niño events are not clearly distinguishable in terms of SST, rainfall pattern, teleconnection, and driving mechanisms. In contrast, extreme EP events exhibit fundamentally different dynamics, driven by a massive reorganization of atmospheric convection across the Pacific. Drawing on our recent findings and the broader literature, we will highlight the distinctive attributes of extreme El Niño events. These events are expected to increase in frequency under global warming. They are linked to eastward-shifted teleconnection patterns, leading to specific and predictable impacts over North America. They also induce a much stronger and longer-lasting oceanic memory, resulting in a predictable transition to a two-year La Niña. Atmospheric nonlinearities, particularly those associated with the threshold for deep atmospheric convection, play a critical role in establishing those extreme El Niño events distinguishing features. In summary, CP and moderate EP events share many characteristics, while extreme El Niño events stand apart. These insights challenge the current approach to ENSO diversity and suggest that categorizing ENSO states as La Niña, neutral, moderate El Niño, and extreme El Niño is more relevant.