Mid‐latitude leading double‐dip La Niña

Understanding the evolution asymmetry between El Niño and La Niña events is challenging. Unlike El Niño, most La Niña events are characterised by a double‐dip cooling (a.k.a. multi‐year La Niña). Herein, we examined how single‐ and multi‐year La Niña events differ by analysing observational and climate‐model data sets. Single‐year La Niña events tend to develop narrowly within the tropics from a central Pacific‐type El Niño (Niño‐4 > Niño‐3), whereas multi‐year La Niña events tend to originate from an eastern Pacific‐type El Niño (Niño‐3 > Niño‐4) and are well‐connected to mid‐latitudes through the Pacific meridional mode, which leads to a meridionally wider response of the off‐equatorial low‐level atmospheric anti‐cyclonic circulation. As the anti‐cyclonic circulation controls the amount of equatorial upper‐ocean heat recharge through Sverdrup transport, for single‐year La Niña, efficient ocean recharging due to a narrower anti‐cyclonic circulation causes a fast transition to an El Niño or a fast termination of a La Niña. In contrast, for multi‐year La Niña, a weaker recharging causes surface cooling to persist, leading to another La Niña in the following year. Time‐longitude graphs of equatorial SSTA (shading), anomalous zonal wind‐stress (vectors, black: Zonal wind‐stress >0.25 and blue: Zonal wind‐stress < −0.25, unit: 102 × N⋅m–2), and anomalous wind‐stress curl (contour, N⋅m–3 × 108) over the north off‐equator (0–15°N) for (a) single‐year La Niña events and (b) multi‐year La Niña events.