A Constraint of ENSO Complexity by Tropical Pacific Mean State

Transitions from one El Niño-Southern Oscillation (ENSO) event to another can occur in various ways and constitute a key element of ENSO complexity. An El Niño (La Niña) event can be preceded (1) by a La Niña (El Niño) event to become a cyclic ENSO, (2) by a neutral event to become an episodic ENSO, or (3) by another El Niño (La Niña) event to become a multi-year ENSO. In this talk, we will present observational analyses and climate model experiments to demonstrate that the tropical Pacific mean state controls the ENSO transition complexity through a series of tropical-subtropical interactions. Specifically, mean SSTs in the tropical eastern Pacific control how often cyclic ENSO events should occur whereas mean SSTs in the tropical central Pacific control how often multi-year ENSO events should occur. El Nino-La Nina asymmetries can also be affected by the tropical Pacific mean states. The mechanisms enabling these control patterns will be elaborated. CMIP5 model simulations project that the frequencies of episodic, cyclic, and multi-year ENSO events will be changed in the future warming world as mean temperatures in the Pacific rise. The projections in ENSO transition complexity will be described.