A Potential Reason for a More CP El Nino-Like SSTA Performance in CMIP6 Simulations

A systematic bias of the extremely westward zonal current (EWZC) was revealed over the equatorial Pacific in Coupled Model Intercomparison Project 6 (CMIP6) models. It tends to weaken the modeled interannual variability of equatorial zonal current anomaly (ZCA) with its maximum variability concentrated in the western Pacific. A mixed-layer heat budget analysis demonstrates that the simulation of mean circulation effect is slightly affected by the zonal current bias. However, the zonal advective feedback is closely linked to the biased equatorial ZCA variability and is overestimated (underestimated) in the Niño-4 (Niño-3) region. The magnitude of zonal advective feedback is even greater than that of thermocline feedback and becomes the most dominant process contributing to the growth of mixed-layer temperature anomaly tendency in the Niño-4 region. Such a bias is essential for a more central-Pacific El Niño-like performance in CMIP6 models.