Ocean Dynamics are Key to Extratropical Forcing of El Nino

El Nino-Southern Oscillation (ENSO) has been recently linked with extratropical Pacific Ocean atmospheric variability. The two key mechanisms connecting the atmospheric variability of the extratropical Pacific with ENSO are the heat flux-driven seasonal footprinting mechanism (SFM) and the ocean dynamics-driven trade wind charging (TWC) mechanism. However, their relative contributions to ENSO are still unknown. Here we present modeling evidence that the positive phase of the SFM generates a weaker, short-lived central Pacific El Nifio-like warming pattern in the autumn, whereas the TWC positive phase leads to a wintertime eastern Pacific El Nino-like warming. When both mechanisms are active, a strong, persistent El Nino develops. While both mechanisms can trigger equatorial wind anomalies that generate an El Nino, the strength and persistence of the warming depends on the subsurface heat content buildup by the TWC mechanism. These results suggest that while dynamical coupling associated with extratropical forcing is crucial to maintain an El Nino, thermodynamical coupling is an extratropical source of El Nino diversity.

Automated summary

The study suggests that both the heat flux-driven seasonal footprinting mechanism and the ocean dynamics-driven trade wind charging mechanism play crucial roles in connecting extratropical Pacific Ocean atmospheric variability with El Nino. When both mechanisms are active, a strong and persistent El Nino develops.