A flexible Regional Ocean Modeling System-based hybrid coupled model for El Niño–Southern Oscillation studies – model formulation and performance evaluation

Abstract. El Niño–Southern Oscillation (ENSO) constitutes the most prominent interannual climate variation mode in the climate system that originates from ocean–atmosphere interactions in the tropical Pacific. Accurately modeling ENSO variation has consistently posed a great challenge, exhibiting strongly model-dependent representations and simulations of ENSO. This study presents a novel hybrid coupled model (HCM), denoted HCMROMS, built upon the Regional Ocean Modeling System (ROMS) that has been widely used for regional modeling studies. For basin-wide applications to the tropical Pacific, here, the ROMS is coupled with a statistical atmospheric model. The statistical atmospheric model is based on singular value decomposition (SVD), capturing interannual relationships of atmospheric perturbations such as wind stress and freshwater flux anomalies with sea surface temperature (SST) anomalies. The model is constructed in a flexible way so that various components representing atmospheric forcing and oceanic biogeochemistry can be easily included as a module in the HCMROMS. Results demonstrate that the HCMROMS can simulate a stable quasi-3-year ENSO cycle when the interannual wind stress coupling coefficient, ατ, is set to 1.5. The HCMROMS reproduces the three-dimensional (3D) evolution of ENSO-related anomalies, revealing that the most pronounced temperature anomalies occur beneath the surface at 150 m. The interannual temperature anomaly budget highlights the dominance of the advection process in simulated ENSO. Vertical mixing contributes negatively to ENSO anomalies, damping temperature anomalies from the surface due to the turbulent heat flux feedback. This newly developed HCMROMS is poised to serve as an efficient modeling tool for ENSO research in the future.