Program

Compared with the observed sea surface temperature in the equatorial Pacific, an excessive cold tongue that extends too far west into the western Pacific has been a common feature in several generations of climate models. Based on the historical climate simulations of 20 CGCMs from phase 5 of the Coupled Model Intercomparison Project (CMIP5), this study investigates the impacts of cold tongue intensity on the ENSO-related equatorial Pacific sea surface temperature anomalies (SSTA). During the winter of El Niño mature phase, the simulated warm SSTA extends too far west and results in the most significant SSTA bias in the equatorial western Pacific (EWP), which, as shown in previous studies, weakens the anomalous atmospheric circulations in the northwestern Pacific. The results among observations and CMIP5 models indicate that the simulation skill of ENSO-related SSTA largely depend on whether the model could reasonably reproduce the observed mean state. Warmer SSTA biases in the EWP tend to occur in the low-skill models with stronger cold tongue intensities in the central and eastern Pacific. The underlying processes are further depicted by the mixed-layer heat budget during the onset of El Niño events. The discrepancies of EWP warming amplitudes are significantly modulated by ocean thermal advections associated with the mean temperature gradient. Better understanding of the model bias may contribute to improvement of the ENSO prediction skill.