Formation mechanisms of the Central and Eastern Atlantic Niño

Abstract

Atlantic Niño is the dominant mode of interannual climate variability of the tropical Atlantic, prominently influencing climate conditions over local and remote regions. A recent study has identified two types of Atlantic Niño–central and eastern Atlantic Niño (CAN and EAN), with warm sea surface temperature (SST) anomalies centered in the central and eastern basins, respectively. Here we investigate their formation mechanisms by performing a mixed layer heat budget analysis and conducting numerical experiments. Results show that the development of both types is contributed by upper-ocean vertical processes caused by westerly wind anomalies. Furthermore, anomalous horizontal advection also plays an important role but is associated with distinct physical processes in the CAN and EAN. The difference is related to the climatological distribution of tropical Atlantic SST, exhibiting two warm centers located in the southwest and northeast tropical basins during boreal spring. Consequently, eastward current anomalies during Atlantic Niño cause warming only in the western-central equatorial Atlantic south of the equator, contributing to the formation of CAN. In contrast, Ekman convergence anomalies cause SST warming in the southwest and northeast equatorial Atlantic during CAN and EAN, respectively, favoring both types. We further analyze initiation mechanisms for the two Atlantic Niño types and find that CAN and EAN are triggered by the subtropical South Atlantic warming and oceanic Kelvin waves, respectively. These results suggest that the two Atlantic Niño types are associated with distinct physical drivers.