A Control of ENSO Transition Complexity by Tropical Pacific Mean SSTs Through Tropical‐Subtropical Interaction. Issue 12 (13th June 2020) - Fang, Shih‐Wei
Abstract: El Niño–Southern Oscillation (ENSO) transitions from one event to another in complex ways. Using observational analyses and forced atmospheric model experiments, we show that a preceding ENSO event can activate a subtropical Pacific forcing mechanism to trigger another ENSO event during the following year. These tropical‐subtropical Pacific interactions result in a cyclic ENSO transition if the two ENSO events are of opposite signs or a multiyear ENSO transition if they are of the same sign. The preceding ENSO event should excite deep convections in the tropical Pacific in order to activate the subtropical Pacific mechanism. This requirement enables mean temperatures in the cold tongue and warm pool to respectively control how easily the cyclic and multiyear transitions can occur. A future warmer tropical Pacific is projected to decrease the frequency of occurrence of multiyear ENSO transitions but increase the occurrence of cyclic ENSO transitions. Plain Language Summary: El Niño–Southern Oscillation (ENSO) is one of the strongest climate variation phenomena in Earth's climate system, causing regional climate extremes and massive ecosystem impacts. An ENSO event can transition from one event to another in complex ways. An El Niño (La Niña) event can be preceded by a La Niña (El Niño) event to become a cyclic ENSO, by a neutral event to become an episodic ENSO, or by another El Niño (La Niña) event to become a multiyear ENSO. The complex nature of ENSO transition challenges our understanding of ENSO dynamics and its future responses to greenhouse warming. Here we show, using observational analyses, climate model simulations, and a novel framework focusing specifically on the onset processes of ENSO, that multiyear ENSO events related to the subtropical forcing are projected to decrease and cyclic ENSO events to increase as the climate warms. These changes in ENSO transition complexity are linked to the warming of the tropical Pacific mean state, which is a key factor controlling ENSO transitions through a series of tropical‐subtropical interactions. Key Points: An ENSO event can activate a subtropical Pacific mechanism to onset another event resulting in a cyclic or multiyear ENSO transition Mean SSTs in the cold tongue and warm pool respectively control how easily cyclic and multiyear transitions can occur via this process A future warming of the tropical Pacific is projected to reduce (increase) the occurrence of the multiyear (cyclic) ENSO transitions