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This study finds the seasonal footprinting (SF) mechanism to be a key source of El Nino-Southern Oscillation (ENSO) complexity, whereas the charged-discharged (CD) mechanism acts to reduce complexity. The CD mechanism forces El Nino and La Nina 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 Nina events but not multiyear El Nino events, contributing to El Nino-La Nina 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.

Plain Language Summary El Nino-Southern Oscillation (ENSO) is known to have profound climate impacts worldwide, but the causes of its complex behaviors are still not fully understood. In this study, we show that the subtropical atmosphere-ocean coupled forcing is a key source of ENSO complexity, whereas the tropical ocean heat content variation acts to reduce ENSO complexity. The subtropical forcing also has a tendency to produce multiyear La Nina events but not multiyear El Nino events, contributing to El Nino-La Nina asymmetries. In contrast to the steady strength of the tropical variation throughout the past six decades, the strength of the subtropical forcing has increased since the early 1990s. This may have made ENSO more complex recently and, if this trend does not reverse, possibly into the coming decades. Contemporary climate models overestimate the strength of the tropical ocean heat content variation but underestimate the strength of the subtropical forcing, which may be a reason why contemporary models produce ENSO behavior that is too regular.