Relationship between NPO and multi-year El Niño events in a 2200 years simulation of CESM1

The boreal winter North Pacific Oscillation (NPO) has been proven to be a crucial trigger in the occurrence of multi-year El Niño events. However, the limited number of multi-year El Niño events in observational data (only five times since 1950) introduces certain limitations and uncertainties to the findings. Therefore, this study reexamines the relationship between the NPO and multi-year El Niño events using a 2200-year simulation of the Community Earth System Model, version 1. Our results demonstrate that the NPO can influence multi-year El Niño events through two pathways: (1) The thermodynamic process: the boreal winter NPO event, along with its forced North Pacific Meridional mode (NPMM), can trigger central Pacific (CP) El Niño event in the following winter. Meanwhile, this CP El Niño event then induces another NPO-like SLP variability over the North Pacific, which subsequently forces the generations of the NPMM again, providing favorable conditions for another El Niño event, eventually leading to a multi-year El Niño event. (2) The dynamic process: The consecutive NPO events and their related anomalous southwesterly wind stress over the subtropical North Pacific region can modify the wind stress curl of the central Pacific. These modified curls persist and further strengthen the equatorward anomalous Ekman transports from the northern off-equatorial and suppress the off-equatorward geostrophic transports, ultimately favoring the ongoing recharge process and leading to the prolonged or intensified El Niño events. Additionally, our findings suggest that the anomalies Ekman transport at 5°N can be a good ENSO precursor for 2–3 seasons in advance. These findings may provide valuable insights for predicting the evolution of ENSO diversity.