UC Irvine

This study finds the seasonal footprinting (SF) mechanism to be a key source of El Niño–Southern Oscillation (ENSO) complexity, whereas the charged-discharged (CD) mechanism acts to reduce complexity. The CD mechanism forces El Niño and La Niña to follow each other, resulting in a more cyclic and less complex ENSO evolution, while the SF mechanism involves subtropical forcing and results in an ENSO evolution that is more episodic and irregular. The SF mechanism also has a tendency to produce multiyear La Niña events but not multiyear El Niño events, contributing to El Niño-La Niña asymmetries. The strength of CD mechanism has been steady, but SF mechanism has intensified during the past two decades, making ENSO more complicated. Most Climate Model Intercomparison Project version 5 models overestimate the strength of the CD mechanism but underestimate the strength of the SF mechanism, causing their simulated ENSOs to be too regular and symmetric.