Quantifying the Amplifying Effect of the Winter North Pacific Oscillation on the Subsequent ENSO

Atmospheric variability associated with the winter North Pacific Oscillation (NPO), preceding El Ni & ntilde;o-Southern Oscillation (ENSO) events by about 1 year, is driven mainly by the tropical Pacific sea surface temperature (SST) anomalies but may also affect the subsequent ENSO. To isolate the latter, we conduct an idealized hindcast experiment using a climate model MIROC6, in which the NPO-related surface heat flux and wind stress anomalies are overridden over the North Pacific. The ENSO evolution can be overall reproduced without the NPO forcing, confirming the predominant intrinsic predictability arising from the tropical coupled dynamics. However, the extratropical atmospheric forcing associated with the NPO, when it reaches three standard deviations, can enhance the ENSO development by about 29.6%, contrasting with limited effects of the moderate NPO forcing. La Ni & ntilde;a events exhibit much less sensitivity to the NPO forcing compared to El Ni & ntilde;o events, possibly due to asymmetric patterns of the simulated NPO.