Influence of ocean background state in Atlantic Niño diversity

Previous studies have identified diverse spatial patterns of the Atlantic Niño (AN) linked to different teleconnections. The emergence of these structures coincides with different mean conditions and driving mechanisms. Here, we explore the role of the tropical Atlantic background state in changing the effectiveness of the dynamic mechanisms that generate the AN, and in shaping the distinct AN patterns.

To this aim, we use simulations from five models of the Extratropical-Tropical Interaction Model Intercomparison Project (ETIN-MIP), where changes in the background state are induced by perturbations in incoming solar radiation across three different latitudinal bands.

Our results reveal that modifying the ocean background state could induce the reported changes in the AN pattern through the alteration of ocean wave dynamics and air-sea coupling.

Mean thermocline slope and stratification in the equatorial Atlantic have a pronounced impact on the Bjerknes Feedback (BF), shaping the AN pattern. In particular, a less tilted equatorial mean thermocline (shallower in the west) in spring could strengthen wind-thermocline coupling under strong anomalous interannual westerlies. Additionally, a tilted mean thermocline, shallower in the east and less stratified in June-August, favors the thermocline-SST coupling. 

Consequently, the stronger BF produces an eastward AN, with SST anomalies confined to the east of the basin and the coast of Africa. Conversely, when the mean state is less favorable, a weaker BF, combined with less effective wave dynamics, results in a westward AN structure.