Influence of the Pacific Meridional Mode on ENSO Evolution and Predictability: Asymmetric Modulation and Ocean Preconditioning

ABSTRACT This study investigates the mechanisms behind Pacific meridional mode (PMM) in influencing development of El Niño–Southern Oscillation (ENSO) events and their seasonal predictability. To examine relative importance various factors that may modulate efficiency PMM influence, a series experiments is conducted for selected ENSO with different intensity using Community Earth System Model, which ensemble predictions are made from slightly ocean initial states but under common prescribed surface heat flux forcing. Overall, forcing matched to ENSO—that is, positive or negative prior an Niño La Niña, respectively—plays enhancing role, whereas mismatched plays damping role. For cases, event enhances more strongly than Niña. asymmetry largely originates between tropics, can be explained by nonlinearity growth equatorward propagation PMM-related anomalies sea temperature (SST) zonal wind through both wind–evaporation–SST feedback summer deep convection response. Our model results also indicate acts as modulator rather trigger occurrence event. Furthermore, response imposed modulated preconditioning upper-ocean content, provides memory coupled low-frequency evolution tropical Ocean.