Coupled conditional nonlinear optimal perturbations and their applications to ENSO ensemble forecasts

There exist limitations of existing ensemble forecasting initial perturbation methods in describing the interactions between various spheres of the Earth system. In this study, a new method is proposed, namely, the coupled conditional nonlinear optimal perturbation (C-CNOP) method, which incorporates the effect of the initial uncertainty in multisphere coupling. The El Niño-Southern Oscillation (ENSO) is a typical tropical Pacific ocean–atmosphere coupling phenomenon. The C-CNOP method is applied to ensemble forecasting of ENSO, It is demonstrated that the C-CNOP method can generate coupled initial perturbations (CPs) that appropriately consider the effect of ocean–atmosphere coupling uncertainty for ENSO. The results reveal that the generated CPs effectively improve the ability of ENSO ensemble-mean forecasts in both the temporal variability in Niño3.4 sea surface temperature anomalies (SSTAs) and the spatial variability in ENSO mature-phase SSTAs. In particular, despite the weak ocean–atmosphere coupling in the tropical Pacific occurring during the boreal spring and summer, CPs still capture the uncertainties associated with weak coupling when predicting on the basis of these seasons. This performance of CPs significantly suppresses the rapid increase in ENSO prediction errors due to the high ocean–atmosphere coupling instability during these seasons, and thus effectively extends the lead time of ENSO forecasting. Hence, the C-CNOP method is a suitable initial perturbation approach for ensemble forecasting that can consider the effect of the initial coupling uncertainties. It is expected that the C-CNOP method plays a significant role in future Earth climate system predictions.