The Role of Westerly Wind Bursts During Different Seasons Versus Ocean Heat Recharge in the Development of Extreme El Niño in Climate Models

Abstract

AbstractThe mechanisms and predictability of extreme El Niño events, as in 1982, 1997, and 2015, remain unclear. Here we investigate the relative roles of the ocean initial heat recharge and westerly wind bursts (WWBs) that occur during different seasons in the generation of extreme El Niño events in a climate model (CESM1.2). Several sets of 100‐member ensemble simulations, along with CMIP5 and atmospheric reanalysis data, are used. We show that while the probability of extreme El Niño increases progressively with ocean heat recharge, tropical conditions by year‐end are least predictable (the largest ensemble spread) for moderately‐to‐strongly recharged ocean initial states, rather than neutrally charged. Even after strong early‐year WWBs and recharged ocean the occurrence of extreme El Niño has limited predictability. While the early‐year WWBs are still important for strong El Niño, it is to a lesser extent than suggested previously, as mid‐year WWBs play a critical role in El Niño progression.