Unveiling the role of South Tropical Atlantic in winter Atlantic Niño inducing La Niña.

The boreal winter-peaked Atlantic Niño/Niña can influence La Niña/El Niño (the cold/warm phase of El Niño-Southern Oscillation, ENSO) in the following year. However, the Atlantic Niño-La Niña relationship is more uncertain than the Atlantic Niña-El Niño counterpart. Here, we show that this uncertainty arises from two distinct types of Atlantic Niño events: the Equatorial and Expanded types, which differ in their meridional sea surface temperature (SST) warming. The Equatorial type, with SST warming confined to the equator, has a weaker climate impact due to limited influence on local convective heating in spring when the intertropical convergence zone (ITCZ) shifts southward. In contrast, the Expanded type, with SST warming extending into the southern tropical Atlantic (STA), drive persistent local anomalous convection heating and strong remote atmospheric responses in the tropical Pacific from winter to spring. Our results emphasize the critical role of STA conditions in shaping the influence of winter Atlantic Niño on the Pacific.