Is a Preceding Strong El Niño Required to Generate Multi-year La Niña?

By analyzing observational data covering the period from 1900 to 2021, we show that the known mechanism linking multi-year La Niña with a preceding strong El Niño has been overemphasized. A majority of multi-year La Niña (64%; 7 out of 11 events) do not require a preceding strong El Niño to generate their 2^nd-year La Niña. We find that the negative phase of the Pacific Meridional Mode (PMM) during 1^st-year La Niña’s decaying spring, rather than the preceding strong El Niño, offers the key mechanism to produce 2^nd-year La Niña, resulting in a multi-year La Niña. It is further found that the westward extension of the 1^st-year La Niña cold sea surface temperature anomalies, which interacts with the eastern edge of the western Pacific warm pool, is a key factor inducing the negative PMM. The negative PMM mechanism to generate multi-year La Niña is also applied to the 3^rd-year La Niña of multi-year La Niña, giving rise to a triple-dip event. The possible reason(s) how and why a multi-year La Niña can become either a double-dip or a triple-dip event will be discussed.