The Relationship Between South Pacific Atmospheric Internal Variability and ENSO in the North American Multimodel Ensemble Phase-II Models

Despite substantial progress made in the theoretical understanding and practical prediction of the El Nino-Southern Oscillation (ENSO), accurate predictions of particular ENSO characteristics (e.g., evolution, intensity, and spatial pattern) remain challenging. Using two models from the North American Multimodel Ensemble (NMME) Phase-II hindcasts, we find that the austral winter atmospheric internal variability is a key determinant of how the South Pacific atmospheric circulation responds to concurrent tropical Pacific sea surface temperature anomalies. While this internal variability may not trigger ENSO onsets, it regulates the southeasterly trades and contributes to thermodynamic feedbacks that grow into an ENSO-like structure during the following austral summer. The difference in the simulation of South Pacific atmospheric variability amongst ensemble members appears to be a significant source of the inter-member spread in ENSO predictions. Monitoring South Pacific atmospheric variability provides an opportunity to improve the prediction of ENSO intensity and flavor with about a two-season lead time.