Extratropical impacts on the 2020–2023 Triple-Dip La Niña event

Historically, La Niña events typically follow El Niño events, particularly in case of prolonged La Niña events that succeed strong El Niño events. However, the current triple-dip La Niña (CTD-La Niña) event during 2020–2023 occurred after a neutral event rather than a (strong) El Niño, which differs from the two historical triple-dip La Niña (HTD-La Niña; 1973–1976 and 1998–2001) events since 1950. Therefore, this paper aims to investigate the unique evolution and potential formation mechanisms of the CTD-La Niña event. Our results show that, in contrast to the HTD-La Niña, the CTD-La Niña event is not primarily influenced by tropical processes themselves. Instead, the consecutively in-phase (negative) North Pacific Meridional Mode and South Pacific Meridional Mode play a crucial role in initiating and sustaining the La Niña event of 2020–2023 through tropical-extratropical interactions. On one hand, their associated negative sea surface temperature (SST) anomalies extend toward the equatorial central Pacific each year from spring to summer, leading to negative SST and strengthening easterly surface wind anomalies in the equatorial Pacific. On the other hand, their related northeastern and southeastern wind stress can modify the wind stress curl over the equatorial Pacific, further intensifying the equatorward Ekman transport from the off-equator. These factors together provide favorable conditions for the re-occurrence of the La Niña event from 2020 to 2023. These findings may offer valuable insights into understanding the formation of long-lasting ENSO events.