How Important are the Extratropics to ENSO Diversity and Predictability?

The case-to-case variability of ENSO is particularly evident in recent unusual evolutions of ENSO events (e.g. 2012, 2014, and 2015). Several studies indicate that the extratropics may have played a role in the evolution of these events by providing a stochastic forcing in the tropics. The results from this project present evidence that extratropical variability in the atmosphere and ocean impacts the case-to-case variability of ENSO (i.e. ENSO diversity).

Model sensitivity experiments designed to eliminate extratropical variability in the atmosphere and ocean, both together and separately are performed with the NCAR/CESM-LE. Results indicate that changing the extratropical variability alters wind stress and ENSO variability in the Tropics. Namely, eliminating extratropical atmospheric variability produces westerly wind stress anomalies and ENSO events stronger than in the control simulation, while eliminating extratropical variability in the ocean reduces the duration, amplitude, and frequency of wind stress anomalies and ENSO compared to the control simulation. An investigation of the mechanisms that alter wind stress anomalies and ENSO variability when extratropical variability is eliminated will be presented.