The tropical Atlantic as a negative feedback on ENSO

The influence of the tropical Atlantic on El Nino-Southern Oscillation (ENSO) is examined using dedicated climate model experiments with sea-surface temperature (SST) restoring. Partial SST restoring to climatology in the tropical Atlantic leads to slower decay of ENSO events and to a shift of the power spectrum to longer periods. Perfect model hindcast experiments with and without restoring tropical Atlantic SST to climatology indicate that both the northern tropical and equatorial Atlantic have a very small influence on ENSO development. During decaying ENSO events, on the other hand, northern tropical Atlantic SST anomalies strongly accelerate the decay. Key to the Atlantic influence on ENSO decay are Atlantic SST anomalies just north of the equator (similar to 5 degrees N). These lead to local convection anomalies that change the Walker circulation so as to accelerate ENSO decay. Importantly, anomalous events in either the northern tropical or equatorial Atlantic fail to develop in the hindcast ensemble mean, when tropical Pacific SSTs are restored to climatology. This indicates that anomalous tropical Atlantic events in boreal spring and summer are strongly dependent on preceding ENSO events in boreal winter. Thus, the role of the tropical Atlantic is to mediate a negative feedback of ENSO on itself. Despite this passive role of the tropical Atlantic in the Pacific-Atlantic interaction, accurate simulation of the Atlantic feedback should play some role in ENSO prediction. Further model experiments will be required to evaluate model dependence of these findings and to quantify the impact of the Atlantic on ENSO prediction skill.