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El nino and la nina simple diagrams2/8/2024 Specifically, a strong El Niño amplitude in the year prior to an LN can greatly deplete the equatorial band of upper-ocean heat content, leading to a significant heat deficit (or discharge) in the equatorial Pacific. This OHC mechanism creates long-term oceanic memory across ENSO phases, carrying the effect of a strong El Niño from the preceding year into subsequent LN events. One of the primary mechanisms is based on the slow recharge-discharge process of ocean heat content (OHC), which is reflected by sea surface height or thermocline depth anomalies in the equatorial Pacific 32, 33, 34. Therefore, researchers have focused on gaining a better understanding of the dynamics that govern the temporal evolution of LN, with a greater emphasis on studying the mechanisms behind the formation of multi-year LN events. For instance, they cause consecutive droughts in the southern United States during boreal winter 27, extreme warm temperatures over East Asian countries during boreal summer 28, 29, torrential rain and floods in eastern Australia during austral winter and spring 30, and pronounced zonal shifts in Antarctic sea ice concentration anomalies during austral winter 31. Studies have shown that these multi-year LN events can be particularly impactful, as they exert more severe climate impacts or shifts in anomaly patterns than single-year LN events. There have also been increased occurrences of LN events that persist for multiple years since the 1990s, including not only double-dip LN (events that last 2 years) but also triple-dip LN (events that last 3 years), like the latest event that began in September 2020 17, 26 (Supplementary Table 1). Unlike most El Niño events, which typically have a short lifetime within a year, roughly more than half of LN events (43‒70%) 16, 17, 18 persist for 2 years or longer, becoming a multi-year LN 20, 21, 22, 23, 24, 25. Understanding the diverse ENSO evolution patterns and their underlying mechanisms has been a central topic for ENSO research in the past decade, as the ability to precisely predict whether an ENSO event will terminate shortly after 1 year or last 2 years or longer is of global importance. The multi-year event can be further divided into a lingering or re-intensified type according to its decaying evolution pattern 19. Others, however, do not decay after their peak but linger or re-intensify in the following year, producing a successive multi-year event. For example, some ENSO events rapidly decay after their peak and transition to the opposite phase in the following year, resulting in a transitional single-year event. One of the complex ENSO behaviors arises from the diverse temporal evolutions, which can be characterized as transitional single-year or successive multi-year ENSO events 9, 10, 11, 12, 13, 14, 15, 16, 17, 18. El Niño and La Niña (LN) events, the warm and cold phases of the El Niño/Southern Oscillation (ENSO), respectively, exhibit considerable inter-event differences with respect to their amplitudes, spatial structures, and temporal evolutions (known as complex ENSO behaviors) 1, 2, 3, 4, 5, 6, 7, 8.
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