Evolution of Subtropical Pacific‐Onset El Niño: How Its Onset Location Controls Its Decay Evolution

• Published in: Geophysical research letters 2021-03, Vol.48 (5), p.n/a
• Main Authors: Kim, Ji‐Won, Yu, Jin‐Yi
• Format: Article
• Language: English
• Subjects: CESM1 simulation; decay evolution; El Niño; observations; onset location; subtropical Pacific‐onset mechanism
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL091345

In the observations, El Niño events initiated by a subtropical Pacific mechanism (SP‐onset El Niños) show larger uncertainty in their decay evolution patterns than those initiated by a tropical Pacific mechanism. A 2,200‐year simulation of Community Earth System Model reproduces this observed feature and its SP‐onset El Niños are analyzed to understand the cause of the large uncertainty. Results show that the onset location of SP‐onset El Niño, which interacts with the eastern edge of the western Pacific warm pool, is a key factor controlling its decay evolution. When the onset is located east (west) of 155°E, the event has a strong tendency to reverse (maintain) its phase, leading to cyclic (multiyear) evolution. These two onset locations respectively activate Indo‐Pacific and tropical‐subtropical Pacific interactions to give rise to the different evolution patterns. The findings offer a potential way to predict the evolution of SP‐onset El Niños using their onset locations. Plain Language Summary Conventionally, the initiation, development, and decay of El Niño events were considered to be primarily controlled by atmosphere‐ocean coupled processes within the tropical Pacific. Recently, studies began to suggest that more and more El Niño events were initiated by subtropical Pacific processes (simply, SP‐onset El Niños). In this study, we find that this type of events tends to decay in a diverse way more than the conventional El Niños do. By analyzing a long‐term simulation from a state‐of‐the‐art climate model, we uncover the reasons why some SP‐onset El Niños transition into a La Niña phase after they peak (becoming single‐year events), while some others are able to linger or even re‐intensify after they peak (becoming multiyear events). The longitudinal location where the event starts to develop determines if the event is capable of activiating Indo‐Pacific interactions to evolve as a single‐year event or tropical‐subtropical Pacific interactions to evolve as a multiyear event. The findings offer the potential for use of the onset location of SP‐onset El Niño to predict how the event should evolve in its decaying year. Key Points The SP‐onset El Niño has a larger uncertainty than the TP‐onset El Niño in its decay evolution in both the observations and CESM1 simulation The onset location of SP‐onset El Niño plays a critical role in controlling its decay evolution pattern The eastward (westward) onset event activates Indo‐Pacific (tropical‐subtropical Pac