Decadal Modulation of the Seasonal Persistence Barrier for Central Pacific El Niño by the Interdecadal Pacific Oscillation

The seasonal persistence barrier (PB) is a critical factor limiting the prediction skill of the El Niño-Southern Oscillation (ENSO). While previous studies have predominantly focused on the Niño3.4 region (Eastern Pacific El Niño), the increasing frequency of Central Pacific (CP) El Niño events since the 1990s necessitates a distinct examination of the Niño4 region, which exhibits unique predictability characteristics. Using long-term reanalysis datasets, this study investigates the PB of sea surface temperature (SST) in the Niño4 region. Climatologically, the PB for the Niño 4 index occurs during June–July, but it exhibits significant decadal variability. We identify that the Interdecadal Pacific Oscillation (IPO) plays a crucial role in modulating the timing of the PB (PB month). Our analysis reveals a robust relationship where the IPO phase regulates the seasonal locking of the prediction barrier. Specifically, during positive IPO phases, the PB tends to occur earlier, whereas distinct timing characteristics are observed during negative phases. Mechanistically, a diagnostic analysis based on the Bjerknes Stability Index (BJI) demonstrates that the IPO background state alters the seasonal cycle of the zonal advective feedback (ZA) in the equatorial central Pacific. This modulation shifts the seasonal peak of the total ENSO growth rate, thereby determining the timing of the steepest decline in prediction skill. These findings offer new insights into the decadal variability of CP-ENSO predictability and highlight the importance of background state modulation in ENSO forecasting.