Modulation of cloud radiative effect on the strength asymmetry in two types of El Niño events

Corresponding to the pronounced amplitude asymmetry for the central Pacific (CP) and eastern Pacific (EP) types of El Niño, an asymmetry in the strength of the Bjerknes positive feedback is found between these two types of El Niño, which is manifested as a weaker relationship between the zonal wind anomaly and the zonal gradient of sea surface temperature (SST) anomaly in the CP El Niño. Detailed analyses have indicated that this strength asymmetry is mainly derived from the weaker sensitivity of the zonal sea level pressure (SLP) anomaly to that of the diabatic heating anomaly during the development phase of CP El Niño events, which mainly results from the large cancelation induced by the negative SST–cloud thermodynamic feedback that negates the positive dynamical feedback. This study also validates these conclusions by using historical runs of 20 models involved in the Coupled Model Intercomparison Project Phase 5 (CMIP5). It suggests that the CMIP5 models generally depict the asymmetry in amplitude between the two types of El Niño events well, which is consistent with successfully simulating the strength asymmetry of the Bjerknes feedback. As observed during both types of El Niño events, variations in the total cloud amount and short wave radiation also indicated that the cloud radiative effect is an important factor that causes amplitude asymmetry between CP and EP El Niño events. However, the CMIP5 models are severely biased when capturing realistic CP El Niño structures, namely few models can simulate the significantly weaker warming anomalies in the EP relative to the CP.