Improving the simulation ability of ENSO asymmetry with large sea surface temperature diurnal amplitude in climate models

ENSO asymmetry is often underestimated in climate models. Based on the Coupled Model Intercomparison Project Phase 6 (CMIP6) models, those with larger sea surface temperature (SST) diurnal amplitudes (DAs) demonstrate better skill in simulating ENSO asymmetry than models with small DAs. Furthermore, in the second version of the First Institute of Oceanography Earth System Model (FIO-ESM v2.0), experiments with large DA effectively enhance ENSO asymmetry relative to those with small DA, reducing bias by up to 44.2%. Further analysis indicates that the improvement can primarily be attributed to zonal gradient in SSTAs between the eastern Pacific and central Pacific regions, induced by interannual variations in DA anomalies during ENSO events. A larger zonal SSTA gradient, driven by a stronger zonal wind speed anomaly gradient in these regions, leads to a more pronounced positive SSTA than the negative SSTA during El Niño, thereby contributing to the improved simulation of ENSO asymmetry.