Triple‐Dip La Niña in 2020–23: North Pacific Atmosphere Drives 2nd Year La Niña

La Niña persisted from 2020 to 2023, but its mechanisms are still unclear. In this study, atmosphere and ocean reanalysis and 100‐member initialized forecasts using a state‐of‐the‐art climate model were analyzed to identify factors contributing to the persistence of the first‐ to second‐year La Niña during 2020–2022. We found that North Pacific high pressure anomalies in the winter of 2020/2021 forced a negative phase of the Pacific meridional mode through the following spring, forming the broader structure of La Niña. The resultant broader La Niña pattern slowed down the recharge‐discharge process by Ekman transport, persisting La Niña. Ensemble forecast sensitivity analysis revealed that the meridional extent of La Niña explains its forecast spread, reaffirming the importance of La Niña spatial pattern. Advancing predictive understanding of 2020–2022 multi‐year La Niña can help to improve the extended seasonal forecast. Plain Language Summary Almost 3 years have passed since the sea surface temperature in the central‐eastern equatorial Pacific became a cooler‐than‐normal in 2020. This event, called La Niña, can influence our lives through causing anomalous weather conditions globally. Therefore, it is important to know why the La Niña event has lasted so long. Using the computer simulation of climate, we repeated La Niña forecasts started from November 2020 a hundred times. We found that when the shape of La Niña was broader, the forecast was successful 1 year ahead. The shape of La Niña in 2021 depended on the atmospheric condition in the North Pacific. This study suggests that checking La Niña shape informs whether La Niña continues or declines. This knowledge may improve future climate forecasts. Key Points The 100‐member ensemble forecasts initialized in November 2020 by MIROC6 captured prolonged La Niña in 2021/2022 The forecast sensitivity analysis to initial states of ensemble members revealed a critical role of the North Pacific high North Pacific high in winter 2020/2021, causing negative North Pacific Meridional Mode spring, formed the spatially broader La Niña, resulting in long persistence