The unveiling role of South Tropical Atlantic in winter Atlantic Niño inducing La Niña

The relationship between the Atlantic Zonal Mode (AZM) and El Niño-Southern Oscillation (ENSO) has been a topic of controversy in previous studies. This paper focuses on the winter-peaked (AZM) due to its strong correlation with ENSO one year in advance. Two distinct types of winter-peaked Atlantic Niño (positive phase of AZM) events are identified based on the range of warm sea surface temperature (SST) anomaly: the Equatorial type and Expanded type. SST anomalies are confined to equator and extending to south tropical Atlantic (STA) for these two events respectively, which can induce different diabatic heating. For the Equatorial event, local precipitation responses only happen in winter. In contrast, due to the seasonal southward migration of the intertropical convergence zone (ITCZ), the Expanded event can persistently induce strong rainfall change in the spring season despite of the weakening SST anomalies, leading robust atmospheric responses over the western tropical Pacific through adjustments in the Walker Cell and ultimately favoring the development of La Niña. Given the stronger impacts of the Expanded type, it is imperative to consider the STA status when identifying winter Atlantic Niño events and their inter-basin boosting role to the Pacific.