Tropical Atlantic forcing of increased ENSO variability since the 1970’s

The El Niño Southern Oscillation (ENSO) underwent a major shift in in the 1970’s, becoming stronger and more predictable. This shift has been attributed to changes in the tropical Pacific mean state. However, around the 1970’s, tropical Atlantic – Pacific variability became coupled, with Atlantic SST leading opposite signed changes in the Pacific by around 6-months. Here we assess the role of the Atlantic in driving the ENSO regime shift using pacemaker experiments with two climate models: ECHAM5/MPIOM and SPEEDY/RGO. In these experiments, model SST is restored to observations in the tropical Atlantic, while elsewhere the models are fully coupled. Both models capture the observed changes in inter-basin interactions and strengthening on ENSO variability after the 1970’s. The warming of the equatorial and south Atlantic and southward shift of the inter-tropical convergence zone causes inter-basin interactions to become active after the 1970’s in the models. In ECHAM5/MPIOM, this leads to Atlantic Niño variability driving increased ENSO activity. In SPEEDY/RGO, the increase in ENSO activity appears more related to induced mean state changes in the Pacific. In addition, experiments with two different versions of the Norwegian Earth System Model and two nudging approaches (anomaly and full field SST) have been performed as part of the CLIVAR RF Tropical Basin Interactions. Initial analysis reveals are rather muted impact of the tropical Atlantic on ENSO. Further analysis is being performed and results will also be presented.