Increased multi-year La Niña since 1960s driven by internal climate variability

Multi-year La Niña (ML) has occurred more frequently since the 1960s, but whether and to what extent it is due to external forcing remains unknown. Here, using 15 large ensemble (LENS) experiments under the same external forcing with ~500 realizations, we find that external forcings contribute only 18% of the post-1960 ML increase. The observed ML increase is mainly attributed to the higher transition rate of strong El Niño (SE) into ML because of increased SE amplitude, with a relatively small contribution from its increased frequency. However, the SE amplitude remains unchanged in the experiments under external forcing. It suggests that internal variability plays a dominant role in the increased SE amplitude in observations, hence increases its transition and the ML frequency. Associated with stronger SE, some factors outside the tropical Pacific become more active, also favoring ML development. The essential role of internal variability is confirmed by comparing the SE amplitude and ML frequency changes in members with the highest increased transition rate across LENS with observations.